FU HUA CHEN (Eds.) Foundations on Expansive Soils 1975

Further titles in this series: 1. G. SANGLER AT THE PENETROMETER AND SOIL EXPLORATION 2. Q. ZARUBA AND V. MENCL LANDSLIDES AND THEIR CONTROL 3. E.E. WAHLSTROM TUNNELING IN ROCK 4A.R. SILVESTER COASTAL ENGINEERING, I Generation,

Propagation

and Influence

of Waves

4B. R. SILVESTER COASTAL ENGINEERING, II Sedimentation,

Estuaries,

Tides,

Effluents

and

Modelling

5. R.N. YOUNG AND B.P. WARKENTIN SOIL PROPERTIES AND BEHAVIOUR 6. E.E. WAHLSTROM DAMS, DAM FOUNDATIONS, AND RESERVOIR SITES 7. W.F. CHEN LIMIT ANALYSIS AND SOIL PLASTICITY 8. L.N. PERSEN ROCK DYNAMICS AND GEOPHYSICAL EXPLORATION Introduction

to Stress Waves in

Rocks

9. M.D. GIDIGASU LATERITE SOIL ENGINEERING 10. Q. ZARUBA AND V. MENCL ENGINEERING GEOLOGY 11. H.K. GUPTA AND B.K. RASTOGI DAMS AND EARTHQUAKES

Developments

in Geotechnical Engineering

12

FOUNDATIONS ON EXPANSIVE SOILS by

FU HUA CHEN President, Chen and Associates, Consulting Soil Engineers, Denver, Colo., U.S.A.

Inc.,

ELSEVIER SCIENTIFIC PUBLISHING COMPANY Amsterdam — Oxford — New York 1975

ELSEVIER SCIENTIFIC PUBLISHING COMPANY 3 3 5 J a n van Galenstraat P.O. Box 2 1 1 , Amsterdam, T h e Netherlands

A M E R I C A N E L S E V I E R PUBLISHING COMPANY, INC. 52 Vanderbilt A v e n u e New York, New York 10017

ISBN

0-444-41393-6

C o p y r i g h t © 1 9 7 5 b y Elsevier Scientific Publishing C o m p a n y , A m s t e r d a m All r i g h t s r e s e r v e d . N o p a r t o f t h i s p u b l i c a t i o n m a y b e r e p r o d u c e d , s t o r e d in a retrieval s y s t e m , o r t r a n s m i t t e d in a n y f o r m o r b y a n y m e a n s , e l e c t r o n i c , mechanical p h o t o c o p y i n g , recording, or otherwise, w i t h o u t the prior written permission of t h e publisher. Elsevier Scientific Publishing C o m p a n y , J a n van G a l e n s t r a a t 3 3 5 , A m s t e r d a m P r i n t e d in T h e N e t h e r l a n d s

To my wife Edna with love and

appreciation.

vi

PREFACE T h e p r o b l e m s associated w i t h expansive soils are n o t widely appreciated o u t s i d e areas of their o c c u r r e n c e . T h e a m o u n t of damage caused b y expansive soils is alarming. It has b e e n e s t i m a t e d t h a t t h e damage t o buildings, r o a d s , and o t h e r s t r u c t u r e s f o u n d e d o n expansive soils exceeds t w o billion dollars annually. In t h e past 2 0 years, considerable progress has b e e n m a d e in u n d e r s t a n d i n g t h e n a t u r e of expansive soils. This new k n o w l e d g e can be separated i n t o t w o categories. T h e first emphasizes t h e theoretical a p p r o a c h and is t h e result of studies m o s t l y b y a c a d e m i c i n s t i t u t i o n s . I n s t i t u t i o n a l research involves soil mineralogy, s t r u c t u r e , and m o d i f i c a t i o n . Academicians have also advanced n e w theories such as effective stress, soil s u c t i o n , and o s m o t i c pressure w h i c h reveal p r o p e r t i e s of swelling soils previously little k n o w n t o engineers. T h e second category is c o n c e r n e d w i t h t h e field

performance

of

expansive

soils w i t h

emphasis

on

design

criteria

and

construction

p r e c a u t i o n s for structures f o u n d e d o n expansive soil. Practical a p p r o a c h e s of c o m b a t i n g t h e swelling soils p r o b l e m are m o s t l y u n d e r t a k e n b y soils engineers; t h e r e f o r e , t h e y m u s t offer practical and e c o n o m i c a l solutions t o their clients, so t h a t t h e s t r u c t u r e will be free

from

damaging f o u n d a t i o n m o v e m e n t . U n f o r t u n a t e l y , p r e s e n t d a y k n o w l e d g e of expansive soils has n o t reached a stage w h e r e rational solutions can be assigned t o t h e p r o b l e m . It is difficult for t h e public t o u n d e r s t a n d w h y the soils engineer is n o t capable of offering easy solutions. When t h e first crack appears in a s t r u c t u r e , a lawsuit is t h r e a t e n e d . This b o o k provides t h e practicing engineer w i t h a s u m m a r y of t h e state-of-the-art

of

expansive soils and practical solutions based u p o n t h e a u t h o r ' s e x p e r i e n c e . Part I discusses t h e o r y and practice, and summarizes s o m e of t h e theoretical physical p r o p e r t i e s of expansive soils. It also discusses various t e c h n i q u e s e m p l o y e d t o found s t r u c t u r e s on expansive soils such as drilled pier f o u n d a t i o n , m a t f o u n d a t i o n , m o i s t u r e c o n t r o l , soil r e p l a c e m e n t , and chemical stabilization. Part II presents typical case studies. T h e a u t h o r has found t h a t few records are available on t h e cause of structural distress, their remedial measures, and m o r e i m p o r t a n t , t h e degree of success after t h o s e measures have b e e n c o m p l e t e d . I n . t h e last 15 years, t h e a u t h o r has investigated m a n y t h o u s a n d s of building sites*in expansive soil areas in t h e R o c k y M o u n t a i n region. He has also investigated over 1,000 cracked buildings and has suggested remedial measures. It is t h e a u t h o r ' s h o p e t h a t b y sharing his k n o w l e d g e and t h e k n o w l e d g e of o t h e r practicing engineers, a b e t t e r u n d e r s t a n d i n g of expansive soil p r o b l e m s can b e achieved. T h e a u t h o r wishes t o t h a n k t h e entire staff of C h e n and Associates for sharing t h e w o r k load while t h e a u t h o r was devoting his t i m e t o writing this b o o k and also t h e assistance given b y t h e m in t h e p r e p a r a t i o n of t h e m a n u s c r i p t . Many t h a n k s t o t h e various consulting firms, especially Woodward-Clyde and Associates, Jorgensen and H e n d r i c k s o n ,

Ketchum-Konkel-Barrett-Nickel-

Austin, and Ε. H. Tippets C o m p a n y for allowing t h e publication of their valuable findings. Mr. B y r o n Eskesen has c o n d u c t e d m o s t of t h e field investigation and l a b o r a t o r y testing presented in this b o o k . Denver, C o l o r a d o August, 1975

Chapter

1

NATURE OF EXPANSIVE SOILS

INTRODUCTION T h e p r o b l e m of expansive soils was n o t recognized b y soil engineers until t h e latter p a r t of 1930. Prior t o 1 9 2 0 , m o s t of t h e lightly loaded buildings in t h e United States consisted of frame dwellings. Such structures could w i t h s t a n d considerable m o v e m e n t w i t h o u t exhibiting noticeable cracks. By 1 9 3 0 , brick veneer residences b e c a m e widely used. It was t h e n t h a t t h e o w n e r found cracks developing in t h e brick course. T h e damages were a t t r i b u t e d t o s h o d d y c o n s t r u c t i o n and s e t t l e m e n t of the f o u n d a t i o n at o n e corner, w i t h o u t recognition of t h e role of expansive soils. T h e U.S. Bureau of R e c l a m a t i o n [ 1 ] * first recognized t h e swelling soil p r o b l e m in 1938 in c o n n e c t i o n w i t h a f o u n d a t i o n for a steel siphon at their O w y h e e Project in Oregon. Since t h a t t i m e , engineers

realized t h e cause of damage was s o m e t i m e s o t h e r t h a n s e t t l e m e n t .

increasingly extensive use of c o n c r e t e slab-on-ground c o n s t r u c t i o n , after

1 9 4 0 , has

The

further

increased t h e damage to s t r u c t u r e s caused b y expansive soils. T o d a y , t h e r e is a world-wide interest in expansive clays and shales. Engineers from C a n a d a , Australia, S o u t h Africa,

Israel, and the United

States have c o n t r i b u t e d i m m e n s e l y t o the

knowledge and t h e p r o p e r design for s t r u c t u r e s on expansive soils. T h e first significant n a t i o n a l conference on expansive clay p r o b a b l y was o n e held at t h e C o l o r a d o School of Mines in G o l d e n , C o l o r a d o in 1 9 5 9 . T h e I n t e r n a t i o n a l Research and Engineering Conference on Expansive Soils held their first and second conferences at T e x a s A & M University in 1965 and 1 9 6 9 , and their third conference in Haifa, Israel, in 1 9 7 3 .

ORIGIN O F EXPANSIVE SOILS G. W. D o n a l d s o n [ 2 ] classified the p a r e n t materials t h a t can be associated w i t h expansive soil i n t o t w o groups. T h e first g r o u p comprises t h e basic igneous r o c k s , such as t h e basalts of t h e Deccan Plateau in India, t h e dolerite sills and d y k e s in the central region of S o u t h Africa and the g a b b r o s and norities west of Pretoria N o r t h , Transvaal. In these soils, t h e feldspar and p y r o x e n e minerals of the p a r e n t rocks have d e c o m p o s e d to form m o n t m o r i l l o n i t e and o t h e r secondary minerals. T h e second g r o u p comprises t h e s e d i m e n t a r y r o c k s t h a t c o n t a i n m o n t m o r i l l o n i t e as a c o n s t i t u e n t w h i c h breaks d o w n physically to form expansive soils. In N o r t h America, b e d r o c k shale found in the Pierre F o r m a t i o n and the m o r e r e c e n t Laramie and Denver F o r m a t i o n s are examples of this t y p e of rock. In Israel, there are t h e marls and limestones and in S o u t h Africa, the shale of t h e Ecca Series. *Numbers in brackets refer to items in the references at the end of each chapter.

2

FOUNDATIONS ON EXPANSIVE SOILS

Γ

/V

/

Λ

)

Marine mud j and clay in I ocean basin

m

i

i

— —ι— .. Ά

I

r

. Highland source of. sediments S volcan*: ; materials I

V

or Figure 1. Geographic setting of deposition of Pierre and Bearpaw Shales and related rocks in Late Cretaceous time in Rocky Mountain and Great Plains region. (After Tourtelot, 1973)

T o u r t e l o t [ 3 ] r e c o n s t r u c t e d the paleogeographic c o n d i t i o n in t h e R o c k y M o u n t a i n and Great Plains regions as s h o w n on figure 1. In t h e Late Cretaceous t i m e , t o t h e west of t h e R o c k y M o u n t a i n s were high-to-moderate u p l a n d s , and t o t h e east t h e Great Plains regions were once ocean basins w h e r e the Pierre and Bearpaw shales and their equivalent were deposited. T h e source of the sediments consists of volcanic r o c k in t h e n o r t h e r n part ( M o n t a n a ) and a range of rock t y p e s in t h e s o u t h e r n part. Separating t h e coastal plain from t h e ocean basin is a belt of sandy deposit. T h e shale is sandier and siltier adjacent t o the coast and possesses a lower swelling p o t e n t i a l . Inland, the shale consists almost entirely of clay-size material with high swell p o t e n t i a l .


3

T h e m o n t m o r i l l o n i t e was p r o b a b l y f o r m e d from t w o separate origins. T h e p r o d u c t s of weathering and erosion of the rocks in t h e highlands were carried b y streams t o the coastal plains. T h e fine grained soils eventually b e c a m e shale a c c u m u l a t i n g in t h e ocean basin. Meanwhile, volcanic e r u p t i o n s , sending u p clouds of ash, fell on t h e plains and t h e seas. These ashes were altered t o m o n t m o r i l l o n i t e . Figure 2 illustrates, in a general w a y , the a b u n d a n c e of m o n t m o r i l l o n i t e in b e d r o c k geologic formations in t h e United States. M o n t m o r i l l o n i t e is regionally a b u n d a n t in c o n t i n u o u s geologic formations t h r o u g h o u t the R o c k y M o u n t a i n s , m o s t of t h e Great Plains, large parts of t h e Gulf Coastal Plains, a n d t h e Mississippi E m b a y m e n t as well as California and t h e Pacific N o r t h w e s t .

DISTRIBUTION O F EXPANSIVE SOILS G. W. D o n a l d s o n [21 s u m m a r i z e d t h e d i s t r i b u t i o n of r e p o r t e d instances of expansive soils a r o u n d the world (fig. 3 ) . T h e c o u n t r i e s in w h i c h expansive soils have been r e p o r t e d are as follows: Argentina

Iran

Australia

Mexico

Burma

Morocco

Canada

Rhodesia

Cuba

S o u t h Africa

Ethiopia

Spain

Ghana

Turkey

India

U.S.A.

Israel

Venezuela

Figure 3 indicates t h a t t h e p o t e n t i a l l y expansive soils are confined t o t h e semi-arid regions of the tropical and t e m p e r a t e climate z o n e s . Expansive soils are in a b u n d a n c e where t h e a n n u a l é v a p o t r a n s p i r a t i o n e x c e e d s t h e p r e c i p i t a t i o n . This follows t h e t h e o r y t h a t in semi-arid zones, t h e lack of leaching has aided t h e f o r m a t i o n of m o n t m o r i l l o n i t e . Potentially

expansive

soils

can

be

found

almost

anywhere

in

the

world.

In

the

u n d e r d e v e l o p e d n a t i o n s , m u c h of the expansive soil p r o b l e m s m a y n o t have been recognized. It is t o be e x p e c t e d t h a t m o r e expansive soil regions will be discovered each y e a r as the a m o u n t of c o n s t r u c t i o n increases r World problem

of expansive

soils

T h e status of t h e art of dealing with world p r o b l e m s on expansive clay soils was s u m m a r i z e d in t h e I n t e r n a t i o n a l Panel Review during t h e first conference o n expansive clay soils at T e x a s A & M, 1 9 6 5 . T h e following are t h e typical findings in each c o u n t r y : Australia — T h e major city t h a t experienced expansive soil p r o b l e m s is Adelaide in S o u t h Australia. Even t h o u g h t h e damage caused b y expansive soils is m o d e r a t e , in a city of

4 FOUNDATIONS ON EXPANSIVE SOILS

Figure 2. General abundance of montmorillonite in near-outcrop bedrock formations. (Modified from Tourtelot, 1973)


5

Figure 3. Distribution of reported instances of heaving. (After G. W. Donaldson)

s o m e 6 0 0 , 0 0 0 i n h a b i t a n t s , t h e aggregate damage associated w i t h f o u n d a t i o n cracks is a substantial a m o u n t . Canada - T h e wide range of climate and geology in Canada p r o d u c e s a great variety of foundation

problems.

In Western

Canada,

including

Saskatchewan

and

Alberta,

expansive clay p r o b l e m s are strongly evident. T h e soils in this region are generally desiccated. Also, in this area of C a n a d a , shallow b a s e m e n t s placed on shallow footings are c o m m o n l y used. T h e r e have been very m a n y cases w h e r e pressures of t h e expansive clays have caused lateral deflections of b a s e m e n t walls. Basement floors have been k n o w n t o heave as m u c h as 6 inches in 18 m o n t h s . India - T h e so-called black c o t t o n soils cover a large area of a p p r o x i m a t e l y 2 0 0 , 0 0 0 square miles, in t h e heart of India. This soil is characterized b y its e x t r e m e hardness w h e n d r y and with high swelling p o t e n t i a l during the process of wetting. Israel — Expansive soil p r o b l e m s exist t h r o u g h o u t Israel. Israel has a rainy w i n t e r season and a h o t , dry s u m m e r . T h e soils are primarily alluvium o r r e w o r k e d t r a n s p o r t e d alluvium which originates from t h e weathering of either basalt or l i m e s t o n e . In t h e clay soil area, m o n t m o r i l l o n i t e m a y be p r e s e n t in q u a n t i t i e s ranging from 4 0 t o 8 0 p e r c e n t of the soil. Mexico — Mexico City has a w o r l d - r e n o w n e d r e p u t a t i o n for s e t t l e m e n t p r o b l e m s . T h e p r o b l e m of expansive clays in Mexico is n o t considered t o be very serious t o d a t e . So

6


far, t h e y have been e n c o u n t e r e d in o n l y a b o u t five t o w n s of r a t h e r m e d i u m size, b u t t h e p r o b l e m is p o t e n t i a l l y m o r e serious because n e w t o w n s are being c o n s t r u c t e d and small t o w n s are being e x p a n d e d . S o u t h Africa — In S o u t h Africa, t h e p r o b l e m of expansive soils was b r o u g h t t o t h e a t t e n t i o n of the engineers as early as 1 9 5 0 . T h e S o u t h African I n s t i t u t i o n of Civil Engineers published

the

first

symposium

on

expansive

clays in

1 9 5 7 . Severe

foundation

m o v e m e n t p r o b l e m s were recorded at Leeuhof, Vereeniging, and Pretoria in Transvaal, where t h e fluvio-lacustrine deposits are t h e source of swelling soils. T h e Ecca shale, covering a large p a r t of S o u t h Africa, is responsible for the f o u n d a t i o n

movement

p r o b l e m s at Odendaalsrus in t h e Orange F r e e States Goldfields. Spain — In Spain, m a n y clay f o r m a t i o n s of s e d i m e n t a r y origin w i t h high plasticity can be found. In m o s t p a r t s of t h e c o u n t r y , t h e climate is arid and t h e é v a p o t r a n s p i r a t i o n is several times greater t h a n t h e precipitation resulting in swelling p h e n o m e n a . A m o n g the various regions w h e r e such p h e n o m e n a have been observed, t h e r e are t w o provinces which m a y be regarded as t y p i c a l ; Andalucia and Madrid. In t h e province of Madrid, t h e soils for the m o s t p a r t consist of m o n t m o r i H o n i t i c clays. These soils r e a c h a liquid limit of 2 5 0 , t h o u g h generally t h e y d o n o t go over 8 0 . In a great p a r t of the m e t r o p o l i t a n areas, t h e highly plasticity clays are covered w i t h a sufficient d e p t h of sandy clay s e d i m e n t s , therefore, present n o swelling p r o b l e m . Venezuela — T h e first r e p o r t of swelling clays in Venezuela c a m e from t h e vicinity of t h e City of C o r o where m a n y buildings are badly cracked. In one instance near t h e city, shales with expansive properties are found. Some of these soils have swelling pressures of 13 t o n s per sq. ft. and occasionally u p t o 2 8 t o n s per sq. ft.

Distribution

of expansive

soils in the United

States

In t h e United States, from t h e Gulf of Mexico t o t h e Canadian Border and from Nebraska t o the Pacific Coast, the a b u n d a n c e of M o n t m o r i l l o n i t e is c o m m o n in b o t h clays and claystone shales. T h e r e p o r t e d p r o b l e m areas are m o s t l y located in t h e regionally a b u n d a n t m o n t m o r i l l o n i t e areas indicated in figure 2. Research has been carried o u t o n expansive soils in m a n y states t h r o u g h o u t t h e United States. Figure 4 indicates t h e states where t h e State Highway D e p a r t m e n t s have sponsored research c o n c e r n i n g expansive soils [ 4 ] . It is interesting t o n o t e t h e similarity b e t w e e n figures 2 and 4 . T h e states t h a t experience various degrees of expansive soil p r o b l e m s are listed as follows: Severe:

Colorado Texas Wyoming

Moderate:

California Utah Nebraska South Dakota

NATURE OF EXPANSIVE SOILS 7

Figure 4 . State Highway Departments that are sponsoring or have recently sponsored research concerning expansive clay soils. (After Sallbert and Smith)

8


Mild:

Oregon Montana Arizona Oklahoma Kansas Alabama Mississippi

D A M A G E C A U S E D BY E X P A N S I V E S O I L S J o n e s and Holtz r e p o r t e d in A S C E in 1 9 7 3 [ 5 ]

the estimated damage a t t r i b u t e d

to

expansive soil m o v e m e n t as follows: E s t i m a t e d average annual loss, millions of dollars

C o n s t r u c t i o n category Single-family h o m e s

$

C o m m e r c i a l buildings

300 360

Multi-story buildings

80

Walks, drives, parking areas

110

Highway and streets

1,140

U n d e r g r o u n d utilities and service

100

Airports

40

U r b a n landslides

25

Others

100 Total

$

2,255

According t o t h e above e s t i m a t e , expansive soil damages n o w exceed t h e c o m b i n e d average annual damages from floods, hurricanes, e a r t h q u a k e s , and t o r n a d o s . A great deal of s t r u c t u r a l m o v e m e n t has been u n d u l y b l a m e d on expansive soils. M a n y floor slabs c o n s t r u c t e d in an expansive soil area crack and s o m e t i m e s heave d u e t o i m p r o p e r l y designed concrete. It is a well k n o w n fact t h a t i m p r o p e r curing of c o n c r e t e , in a d d i t i o n to t h e lack of expansion j o i n t s , will cause cracking. Curling of c o n c r e t e slabs has a strong resemblance t o heaving floors caused b y swelling soils. This is especially true for large warehouse floors w h e r e p r o p e r curing and design is essential. In expansive soil areas, the soils are generally stiff, and t h e chance of lightly loaded structures cracking due t o s e t t l e m e n t is r a t h e r r e m o t e . At t h e same t i m e , there are a large n u m b e r of instances where heavy cracks have appeared in the b a s e m e n t walls t h a t were n o t caused b y f o u n d a t i o n heaving b u t by e a r t h pressure exerted o n the wall, generally c o m p o u n d e d b y seepage pressure. In m o s t cases where vertical or h o r i z o n t a l cracks developed in the b a s e m e n t wall, e a r t h pressure p r o b l e m s are suspect. Diagonal cracks t h a t develop b e l o w w i n d o w s and above d o o r s are a strong indication of swelling m o v e m e n t .

9


S o m e t i m e s , b a s e m e n t wall cracks are caused b y careless c o n s t r u c t i o n crews. B a c k h o e or o t h e r e a r t h moving e q u i p m e n t b u m p i n g against the wall can cause vertical or h o r i z o n t a l cracks. Expansive soils are o f t e n t i m e s

blamed

for arching of a wall w h e n actually

improper

r e i n f o r c e m e n t and restraint is t h e real p r o b l e m . Backfill should n o t be placed against t h e wall until t h e wall has been p r o p e r l y restrained at t o p and b o t t o m . Failure t o d o so m a y result in an arched c o n d i t i o n . Such p h e n o m e n o n s o m e t i m e s is e r r o n e o u s l y i n t e r p r e t e d as h o r i z o n t a l swelling pressure being e x e r t e d against t h e wall. While it is possible t h a t a large a m o u n t of swelling pressure can be e x e r t e d h o r i z o n t a l l y against a wall, generally backfill is so loosely c o m p a c t e d t h a t distress caused b y lateral e x p a n s i o n of backfill is very u n c o m m o n . Structural defects are s o m e t i m e s m i s t a k e n for distress caused b y swelling soils. Split level houses are generally c o n s t r u c t e d w i t h grade b e a m s placed at different levels. Such grade b e a m s , if n o t p r o p e r l y tied t o g e t h e r w i t h r e i n f o r c e m e n t , can result in cracks and m o v e m e n t . While it is t r u e t h a t swelling soils are p r o b a b l y responsible for m o s t of the cracking and m o v e m e n t of lightly loaded s t r u c t u r e s , o t h e r aspects of f o u n d a t i o n m o v e m e n t c a n n o t a n d should n o t be ignored.

CLAY MINERALS Most soil classification

systems arbitrarily

define

clay particles as having an

effective

d i a m e t e r of t w o m i c r o n s ( 0 . 0 0 2 m m ) or less. Particle size alone does n o t d e t e r m i n e clay mineral. Probably t h e m o s t i m p o r t a n t grain p r o p e r t y of fine-grained soils is t h e minealogical c o m p o s i t i o n [ 6 1 . F o r small size particles, t h e electrical forces acting on t h e surface of t h e particle are m u c h greater t h a n the gravitational force. These particles are said t o be in t h e colloidal state. T h e colloidal particle consists primarily of clay minerals t h a t were derived from p a r e n t r o c k b y weathering. T h e three m o s t i m p o r t a n t groups of clay minerals are m o n t m o r i l l o n i t e , illite, and kaolinite, which are crystalline h y d r o u s aluminosilicates. M o n t m o r i l l o n i t e is t h e clay mineral t h a t presents m o s t of t h e expansive soil p r o b l e m s . T h e n a m e ' ' m o n t m o r i l l o n i t e " is used c u r r e n t l y b o t h as a g r o u p n a m e for all clay minerals w i t h an e x p a n d i n g lattice, e x c e p t vermiculite, and also as a specific mineral n a m e [ 7 ] . A b s o r p t i o n of w a t e r b y clays leads t o e x p a n s i o n . F r o m t h e mineralogical s t a n d p o i n t , t h e m a g n i t u d e of e x p a n s i o n d e p e n d s u p o n t h e kind and a m o u n t of clay minerals p r e s e n t , their exchangeable ions, electrolyte c o n t e n t of a q u e o u s phase, and t h e internal s t r u c t u r e . Formation

of clay

minerals

T h e clay minerals are formed t h r o u g h a c o m p l i c a t e d process from an a s s o r t m e n t of p a r e n t materials. T h e p a r e n t materials include feldspars, micas, and l i m e s t o n e . T h e a l t e r a t i o n process t h a t takes place on land is referred t o as w e a t h e r i n g and t h a t on t h e sea floor or lake b o t t o m as halmyrolysis. T h e alteration process includes disintegration, o x i d a t i o n , h y d r a t i o n , and leaching.


10

T o u r t e l o t [31 p o i n t e d o u t t h a t t h e setting for t h e f o r m a t i o n of m o n t m o r i l l o n i t e is e x t r e m e disintegration, strong h y d r a t i o n , and restricted leaching. T h e situations in w h i c h m o n t m o r i l l o n i t e can form require t h a t leaching be restricted, so t h a t m a g n e s i u m , calcium, s o d i u m , and iron cations m a y a c c u m u l a t e in t h e s y s t e m . T h u s , t h e f o r m a t i o n of m o n t m o r i l l o n i t i c minerals is aided b y an alkaline e n v i r o n m e n t , presence of magnesium ions, and a lack of leaching. Such c o n d i t i o n s are favorable in semi-arid regions w i t h relatively low rainfall or highly seasonal m o d e r a t e rainfall, particularly w h e r e e v a p o r a t i o n exceeds p r e c i p i t a t i o n . U n d e r these c o n d i t i o n s , e n o u g h w a t e r is available for the alteration process, b u t t h e a c c u m u l a t e d cations will n o t be r e m o v e d b y flush rain. T h e p a r e n t minerals for t h e f o r m a t i o n of m o n t m o r i l l o n i t e often consist of ferromagnesium minerals, calcic feldspars, volcanic glass, and m a n y volcanic r o c k s . B e n t o n i t e is a clay c o m p o s e d primarily of m o n t m o r i l l o n i t e which has b e e n formed b y the chemical weathering of volcanic ash. Swelling clays are c o m m o n l y

referred

t o as b e n t o n i t i c soils b y l a y m e n . Since c o m m e r c i a l

b e n t o n i t e is w h i t e , the w h i t e calcium streaks present in stiff clays are often m i s t a k e n

for

b e n t o n i t e . Actually, clays w i t h an a b u n d a n c e of calcium seldom exhibit swelling characteristics. Cation

exchange

Clay minerals have t h e p r o p e r t y of sorbing certain anions and cations and retaining t h e m in an exchangeable s t a t e . T h e exchangeable ions are held a r o u n d the outside of the silica-alumina clay-mineral structural u n i t , and t h e exchange reaction does n o t affect t h e s t r u c t u r e of t h e silica-alumina p o c k e t . In clay minerals, t h e m o s t c o m m o n exchangeable cations are Ca^, Mg**", FT, K +, N H 4 +, Na +, frequently in a b o u t t h a t order of general relative a b u n d a n c e . T h e existence of such charges is indicated b y t h e ability of clay t o absorb ions from t h e solution. Cations (positive ions) are m o r e readily absorbed t h a n anions (negative ions); h e n c e , negative charges m u s t be p e r d o m i n a n t on t h e clay surface. A cation, such as Na +, is readily a t t r a c t e d from a salt solution and a t t a c h e d t o a clay surface. However, t h e absorbed Na + ion is n o t p e r m a n e n t l y a t t a c h e d ; it can be replaced by K + ions if the clay is placed in a s o l u t i o n of potassium chloride KCL. T h e process of r e p l a c e m e n t by excess cations is called cation exchange [8]. T h e cation exchange capacity is t h e charge or electrical a t t r a c t i o n for cation per unit mass as measured in millequivalent p e r 100 grams of soil. T h e cation exchange capacity of different t y p e s of clay minerals m a y be m e a s u r e d b y washing a sample of each w i t h a solution of a salt such as a m m o n i u m chloride N H 4 C L and the a m o u n t

of adsorbed N H ^ b y measuring t h e difference b e t w e e n t h e original and the final

c o n c e n t r a t i o n of t h e washing solution. Typical ranges of cation exchange capacities of various clay minerals are s h o w n in table 1. F r o m table 1, it is seen t h a t m o n t m o r i l l o n i t e s are 10 times as active in absorbing cations as kaolinites. This is caused b y t h e large net negative charge carried by t h e m o n t m o r i l l o n i t e particle and its greater specific surface as c o m p a r e d w i t h kaolinite and illite. Certain relationships exist b e t w e e n soil p r o p e r t i e s such as A t t e r b e r g limits, t h e t y p e of clay mineral, and t h e n a t u r e of t h e adsorbed i o n . Table 2 indicates t h e liquid limit and t h e plasticity index of each g r o u p of clay minerals. F r o m tables 1 and 2, it is seen t h a t the cation exchange


11

Table 1 - . Ranges of cation exchange capacities of various clay minerals Kaolinite

Illite

Montmorillonite

Particle thickness

0.5-2 microns

0.003 - 0 . 1 microns

Less than 9.5 A

Particle diameter

0.5-4 microns

0.5 - 10 microns

0.05 - 10 microns

Specific surface (sq. meter/gram)

10-20

65 - 180

50 - 840

3-15

10-40

70-80

Cation exchange capacity (milliequivalents per 100g)

(After Woodward-Clyde & Associates, 1967)

capacity of a clay has definite relation with t h e A t t e r b e r g limits. T h e greater t h e cation exchange capacity of clay, t h e greater the effect of changing t h e adsorbed c a t i o n . Cation exchange p h e n o m e n o n takes place in everyday life. A simple and well k n o w n e x a m p l e of t h e i o n exchange reaction is t h e softening of water b y t h e use of p e r m u t i t e s or c a r b o n exchangers. T h e basic principle involved in the chemical stabilization of expansive soil is the increase in t h e ionic c o n c e n t r a t i o n in t h e free w a t e r and base e x c h a n g e p h e n o m e n o n .

Clay

structure Philip L o w [9] p o i n t e d o u t t h e t w o f u n d a m e n t a l m o l e c u l a r s t r u c t u r e s as the basic u n i t s of

the lattice s t r u c t u r e . These are t h e silica t e t r a h e d r o n and t h e alumina o c t a h e d r o n . T h e silica t e t r a h e d r o n consists of a silicon a t o m s u r r o u n d e d tetrahedrally b y four o x y g e n ions as s h o w n on figure 5a. T h e alumina o c t a h e d r o n consists of an a l u m i n u m a t o m s u r r o u n d e d octahedrally b y six o x y g e n ions as s h o w n on figure 5 b . When each o x y g e n a t o m is shared b y t w o t e t r a h e d r a , a plate-shaped layer is f o r m e d . Similarly, when each a l u m i n u m a t o m is shared b y t w o o c t a h e d r o n , a sheet is f o r m e d . T h e silica sheets and t h e alumina sheets c o m b i n e t o form the basic s t r u c t u r a l u n i t s of t h e clay particle. Various clay minerals differ in t h e stacking configuration. T h e results of studies using t h e e l e c t r o n m i c r o s c o p e and X-ray diffraction t e c h n i q u e s s h o w t h a t t h e clay minerals have a lattice s t r u c t u r e in w h i c h t h e a t o m s are arranged in several sheets, similar t o t h e pages of a b o o k . T h e a r r a n g e m e n t and t h e chemical c o m p o s i t i o n of these sheets d e t e r m i n e t h e t y p e of clay mineral. T h e basic building blocks of t h e clay minerals are t h e silica t e t r a h e d r o n and t h e alumina o c t a h e d r o n . T h e b l o c k s c o m b i n e i n t o t e t r a h e d r a l and o c t a h e d r a l sheets to p r o d u c e t h e various types of clays. Kaolinite is a typical two-layer mineral having a single t e t r a h e d r a l sheet j o i n e d b y a single octahedral sheet t o form w h a t is called a 2 t o 1 lattice s t r u c t u r e .


12

Table 2 - . Atterberg-limit values of clay minerals with various adsorbed cations Na+

Cation

Ca"

Mg"

Liquid

Plasticity

Liquid

Plasticity

Liquid

Plasticity

Liquid

limit,

index,

limit,

index,

limit,

index,

limit,

index,

percent

percent

percent

percent

percent

percent

percent

percent

Plasticity

Clay mineral Kaolinte

29

1

35

7

34

8

39

11

Illite

61

27

81

38

90

50

83

44

344

251

161

104

166

101

158

99

Montmorillonite

(After W.A. White, 1958)

Figure 5. Polyhedra composing the structure of montmorillonite: (b) the alumina octahedron. (After Philip Low, 1973)

(a) the silica tetrahedron,


M o n t m o r i l l o n i t e is a three-layer mineral having a single o c t a h e d r a l sheet

13

sandwiched

b e t w e e n t w o t e t r a h e d r a l sheets t o give a 2 t o 1 lattice s t r u c t u r e as s h o w n on figure 6. Illite has similar s t r u c t u r e w i t h t h a t of m o n t m o r i l l o n i t e , b u t s o m e of t h e silican a t o m s are replaced b y a l u m i n u m , a n d , in a d d i t i o n , p o t a s s i u m ions are present b e t w e e n t h e t e t r a h e d r a l sheet and adjacent crystals. In t h e clay-water-air s y s t e m , the w a t e r within t h e clay is called adsorbed water, t h e w a t e r and ions w i t h t h e clay lattice c o n s t i t u t e t h e diffuse d o u b l e layer. T w o forces exist in t h e system, the attractive and the repulsive forces. T h e closer t h e dipolar w a t e r molecules and cations are t o t h e flat plate surface, t h e m o r e strongly t h e y are a t t r a c t e d . A t small interlayer distances, t w o attractive forces p r e d o m i n a t e .

1.

Electrostatic force - d e p e n d s o n t h e c o m p o s i t i o n of the mineral.

2.

Van der Waals' force — d e p e n d s on t h e distance b e t w e e n t h e layers.

T h e high c o n c e n t r a t i o n of cations near the surface of t h e clay particle creates a repulsive force b e t w e e n t h e diffuse double-layer system. T h e interlayer solution has a higher c o n c e n t r a t i o n of dissolved e l e c t r o l y t e t h a n the e x t e r n a l solution and the s u b s e q u e n t e n t r y of water b y osmosis. T h e resulting repulsive pressure is, therefore, t h e o s m o t i c pressure. T h e double-layer t h e o r y assumes t h a t t h e clay particle is a flat, charged c o n d e n s e r plate and t h e ions are assumed t o be non-interacting p o i n t charges. H e n c e , it is possible t o use Poisson's e q u a t i o n from t h e t h e o r y of electrostatics. By c o m b i n i n g Poisson's e q u a t i o n w i t h B o l t z m a n n ' s e q u a t i o n of o s m o t i c pressure, the resulting e q u a t i o n is the Poisson-Boltzmann e q u a t i o n and is t h e

Figure 6. Model of a layer of montmorillonite. (After Philip Low, 1973)


14

basic differential e q u a t i o n of t h e double-layer t h e o r y . T h e typical result of t h e i n t e g r a t i o n of t h e Poisson-Boltzmann

equation

is given in figure

7, in w h i c h t h e surface

charge density is

d e t e r m i n e d b y dividing the cation ion exchange capacity b y the surface area. It is seen from figure 7 t h a t t h e calculated repulsive pressures increase rapidly as the half-distance b e t w e e n the particles decreases. Warkentin and Bolt [101 observed t h a t e x p e r i m e n al curves of swelling pressure versus interlayer half-distance for N a - m o n t m o r i l l o n i t e has t h e same shape as figure 7. T h e basic relation b e t w e e n d r y density and swelling pressure developed by the a u t h o r in figure 2 8 also assumes t h e same p a t t e r n . Osmotic

pressure

Osmosis is t h e passage of solvent t h r o u g h a semi-permeable m e m b r a n e from a s o l u t i o n of lesser c o n c e n t r a t i o n t o one of higher c o n c e n t r a t i o n , and o s m o t i c pressure is t h e pressure w h i c h m u s t be applied t o t h e solution t o prevent t h e flow of solvent which tries t o dilute t h e solution. O s m o t i c pressure can be evaluated b y V a n ' t Hoff e q u a t i o n as follows:

In w h i c h :

P0

=

PQ

=

O s m o t i c pressure

R T ( C i - C 2)

Gas c o n s t a n t ( B o l t z m a n n c o n s t a n t )

R

=

Τ

=

Absolute temperature

Cj

=

C o n c e n t r a t i o n of a n y ionic species (in ions p e r c m 3 )

C2

=

Ionic c o n c e n t r a t i o n of the ionic species in the e x t e r n a l s o l u t i o n (in ions p e r c m 3 )

It is well recognized t h a t o s m o t i c pressure can be e x p e c t e d t o take place in t h e soil-water s y s t e m . Assuming t h a t the d o u b l e layer system exists in t h e soil lattice, the c o n c e n t r a t i o n of ions being held b y t h e attractive force prevents t h e ions from moving away from the d o u b l e layer. However,

water

is able

to

move

in

and

dilute t h e c o n c e n t r a t i o n , a n d , c o n s e q u e n t l y ,

a

semi-permeable m e m b r a n e effect is achieved. Research m a d e in t h e last decade strongly suggests t h a t o s m o t i c pressure indeed develops in t h e soil-water system and is responsible for the swelling m e c h a n i s m . G. H. Bolt, as early as 1 9 5 6 , [ 1 1 ] c o n c l u d e d t h a t t h e swelling of b o t h illitic clays and m o n t m o r i l l o n i t e clays is caused b y the excess o s m o t i c pressure in t h e adsorbed layer of ions. Bolt claimed t h a t t h e o s m o t i c pressure of the s y s t e m might r e a c h a value of 50 t o 100 t o n s per square foot. It is therefore, n o t surprising t h a t t h e swelling pressure of expansive clays s o m e t i m e s reaches m o r e t h a n 25 t o n s per square foot. Based on t h e t h e o r y t h a t o s m o t i c pressure is t h e only internal pressure acting b e t w e e n particles, if t h e soil is subjected t o e x t e r n a l pressure, the distance b e t w e e n particles will decrease and w a t e r will be squeezed o u t . As a result, t h e ion c o n c e n t r a t i o n b e t w e e n the particles will increase and t h e o s m o t i c pressure in t u r n increases. A n equilibrium is finally reached w h e n t h e o s m o t i c pressure equals the e x t e r n a l pressure. T h e reverse process involves the decrease of


15

Figure 7. Calculated repulsive pressures at different half-distances between adjacent montmorillonite particles (layers) for two values of the surface charge density (cr). (After Philip Low)


16

e x t e r n a l pressure and t h e s u c t i o n of liquid b y o s m o t i c pressure b e t w e e n t h e particles t o dilute t h e c o n c e n t r a t i o n of ions. T h e distance b e t w e e n the particles w o u l d increase, resulting in volume increase and a r e d u c t i o n of o s m o t i c pressure. This process c o n t i n u e s until a n e w equilibrium is established. T h e i m b i b a t i o n of w a t e r is the m o s t i m p o r t a n t cause of swelling.

RECOGNITION O F EXPANSIVE SOILS There are three different m e t h o d s of classifying p o t e n t i a l l y expansive soils. T h e first, mineralogical identification, can be useful in t h e evaluation of t h e material b u t is n o t sufficient in itself w h e n dealing w i t h n a t u r a l soils. T h e various m e t h o d s of mineralogical identification are i m p o r t a n t in a research l a b o r a t o r y in exploring t h e basic p r o p e r t i e s of clays, b u t are impractical and u n e c o n o m i c a l for practicing engineers. A n o t h e r g r o u p includes t h e indirect m e t h o d s , such as t h e i n d e x p r o p e r t y , PVC m e t h o d , and activity m e t h o d w h i c h are valuable tools in evaluating t h e swelling p r o p e r t y . Soil s u c t i o n m a y prove to be very useful w i t h m o r e general application and improved testing t e c h n i q u e s . N o n e of the indirect m e t h o d s should be used i n d e p e n d e n t l y . E r r o n e o u s conclusions can be drawn w i t h o u t the benefit of direct tests. T h e third m e t h o d , direct m e a s u r e m e n t , offers the m o s t useful d a t a for a practicing engineer. T h e tests are simple t o perform and d o n o t require a n y costly and e x o t i c l a b o r a t o r y e q u i p m e n t . A w o r d of c a u t i o n should be i n t r o d u c e d h e r e . Testing should be p e r f o r m e d on a n u m b e r of samples r a t h e r t h a n of a few t o avoid e r r o n e o u s conclusions. Mineralogical

identificatioη

T h e mineralogical c o m p o s i t i o n of expansive soils has an i m p o r t a n t bearing on t h e swelling p o t e n t i a l as explained u n d e r "Clay S t r u c t u r e . " T h e negative electric charges o n the surface of t h e clay minerals, t h e strength of t h e interlayer b o n d i n g , and t h e cation exchange capacity all c o n t r i b u t e to the swelling p o t e n t i a l of t h e clay. H e n c e , it is claimed b y t h e clay mineralogist t h a t t h e swelling p o t e n t i a l of a n y clay can b e evaluated b y identification of t h e c o n s t i t u e n t mineral of this clay. T h e five t e c h n i q u e s w h i c h m a y be used are as follows: X-ray diffraction, Differential t h e r m a l analysis, Dye adsorption, Chemical analysis, and E l e c t r o n m i c r o s c o p e resolution.

The

various

methods

listed

above

should

generally

be

used

in

combination.

Using

c o m b i n a t i o n s of t h e m e t h o d s , t h e different t y p e s of clay minerals present in a given soil can be evaluated q u a n t i t a t i v e l y . U n f o r t u n a t e l y , t h o u g h a great deal of research has been d o n e in t h e various fields of mineralogical s t u d y , t h e test results require e x p e r t i n t e r p r e t a t i o n and the


17

specialized a p p a r a t u s r e q u i r e d are costly a n d n o t e c o n o m i c a l l y available in m o s t soil testing laboratories. A brief description of the various t e c h n i q u e s is as follows:

X-Ray Diffraction M e t h o d . T h e X-ray diffraction m e t h o d used in d e t e r m i n i n g t h e p r o p o r t i o n of the various minerals present in a colloidal clay consists essentially of c o m p a r i n g the ratios of t h e intensities of diffraction lines from t h e different minerals w i t h t h e intensities of lines from t h e standard

substance. G. W. Brindley

[12]

claimed t h a t the use of self-recording

counter

s p e c t r o m e t e r s in lieu of p h o t o g r a p h i c t e c h n i q u e s increases considerably b o t h the a c c u r a c y and t h e convenience of t h e X-ray m e t h o d . Brindley also believes t h a t t h e X-ray m e t h o d

for

q u a n t i t a t i v e d e t e r m i n a t i o n s should be applied w i t h considerable c i r c u m s p e c t i o n , a n d t h a t in favorable cases t h e possibility of 1 identifying species b y X-ray analysis can be regarded w i t h restrained o p t i m i s m .

Differential T h e r m a l Analysis. Differential t h e r m a l analysis w h e n used in c o n j u n c t i o n w i t h X-ray diffraction a n d chemical analysis enables t h e identification of o t h e r w i s e difficult materials. It is well established as a t e c h n i q u e for t h e c o n t r o l of materials w h i c h u n d e r g o characteristic changes on heating. T h e use of differential t h e r m a l analysis t e c h n i q u e in identifying expansive soil is n o t always accurate [ 1 3 ] .

Dye A d s o r p t i o n . Dyestuffs and o t h e r reagents which e x h i b i t characteristic colors w h e n adsorbed b y clay have been used t o identify clay. When a clay sample has been p r e t r e a t e d w i t h acid, t h e color assumed b y t h e adsorbed d y e d e p e n d s o n t h e base exchange capacity of t h e various clay minerals present. T h e presence of m o n t m o r i l l o n i t e can be d e t e c t e d if its a m o u n t is greater t h a n a b o u t 5 t o 10 p e r c e n t . T h e relatively simple testing p r o c e d u r e and speed of d y e staining tests c o m p a r e d w i t h X-ray diffraction and differential t h e r m a l analysis justify wider a p p l i c a t i o n of t h e color m e t h o d .

Chemical Analysis. Chemical analysis can be a valuable s u p p l e m e n t t o o t h e r m e t h o d s such as X-ray analysis in identifying clays. In t h e m o n t m o r i l l o n i t e g r o u p of clay minerals, chemical analysis can be used t o d e t e r m i n e t h e n a t u r e of i s o m o r p h i s m and t o show t h e origin a n d l o c a t i o n of the charge o n t h e lattice. A c c o r d i n g t o Kelley [ 1 4 ] , t h e i s o m o r p h o u s c h a r a c t e r of t h e m o n t m o r i l l o n i t e g r o u p can p r o b a b l y be s h o w n in n o o t h e r way. T h e i s o m o r p h i s m involves t h r e e basic variations in t h e s u b s t i t u t i o n : t h e s u b s t i t u t i o n for Al for Si in t e t r a h e d r a l p o s i t i o n s in t h e lattice; t h e s u b s t i t u t i o n of F e for Al in t h e o c t a h e d r a l c o o r d i n a t i o n ; and t h e s u b s t i t u t i o n of Mg for Al in t h e o c t r a h e d r a l positions.

Electron Microscope

R e s o l u t i o n . Microscopic e x a m i n a t i o n

of clay minerals offers a direct

observation of t h e material. T w o clays m a y give t h e same X-ray p a t t e r n and t h e same differential t h e r m a l curve b u t will s h o w u p distinct m o r p h o l o g i c a l characteristics u n d e r e l e c t r o n m i c r o s c o p e resolution. T h e main p u r p o s e of t h e microscopic e x a m i n a t i o n is t o d e t e r m i n e c o m p o s i t i o n , t e x t u r e , and i n t e r n a l s t r u c t u r e .

minéralogie

18

FOUNDATIONS ON EXPANSIVE SOILS Ravina [ 1 5 ] m a d e extensive s t u d y of t h e mineralogical c o m p o s i t i o n of expansive clays b y

the use of the scanning electron m i c r o s c o p e . It s h o w e d t h a t the nonswelling clays a p p e a r as flat, relatively thick plates while m o n t m o r i l l o n i t e s have a crinkly, ridged, h o n e y c o m b - l i k e t e x t u r e . It might be possible t o evaluate some p r o p e r t i e s of t h e expansive soil b y observing t h e degree of crinkling and interparticle b o n d i n g from scanning an electron m i c r o s c o p e . Single index

method

Simple soil p r o p e r t y tests can be used for the evaluation of the swelling p o t e n t i a l of expansive soils. Such tests are easy t o p e r f o r m and should be included as r o u t i n e tests in t h e investigation of building sites in t h o s e areas having expansive soil. Such tests m a y i n c l u d e : A t t e r b e r g limits tests, Linear shrinkage tests, Free swell tests, and Colloid c o n t e n t tests. A t t e r b e r g Limits. Holtz and Gibbs [ 1 6 ] d e m o n s t r a t e d in 1956 t h a t plasticity index and liquid limit

are

useful

indices

for d e t e r m i n i n g the swelling characteristics of m o s t clays.

Seed,

W o o d w a r d , and Lundgren [ 1 7 ] have d e m o n s t r a t e d t h a t t h e plasticity index alone can be used as a preliminary indication of swelling characteristics of m o s t clays. The swell p o t e n t i a l is defined as t h e percentage swell of a laterally confined sample w h i c h has soaked u n d e r a surcharge of 1 p o u n d per square inch after being c o m p a c t e d t o m a x i m u m density at o p t i m u m m o i s t u r e c o n t e n t according to t h e A A S H O c o m p a c t i o n test. F r o m this, Seed, W o o d w a r d , and L u n d g r e n established t h e following simplified relationship: S = 60K(PI)

M2

in w h i c h : and

S = Swell p o t e n t i a l Κ = 3.6 X 1 0 " 5 and is a c o n s t a n t .

T h e above e q u a t i o n applies o n l y t o soils w i t h clay c o n t e n t s b e t w e e n 8 and 6 5 p e r c e n t a n d the

computed

value is p r o b a b l y accurate t o within a b o u t

3 3 p e r c e n t of t h e

laboratory

d e t e r m i n e d swell p o t e n t i a l . Since liquid limit and swelling of clays b o t h d e p e n d o n the a m o u n t of water a clay tries t o i m b i b e , it is n o t surprising t h a t t h e y are related. Relation b e t w e e n swelling p o t e n t i a l of clays and plasticity i n d e x can be established as follows: Swelling p o t e n t i a l Low

Plasticity i n d e x 0 - 15

Medium

10 -

High

20-55

Very high

35

35 and Above

19


While it m a y be t r u e t h a t high swelling soil will manifest high index p r o p e r t y , t h e converse is n o t true. Linear Shrinkage. T h e swell p o t e n t i a l is p r e s u m e d t o be related t o t h e o p p o s i t e p r o p e r t y of linear shrinkage measured in a very simple test. In t h e o r y it appears t h a t t h e shrinkage characteristics of the clay should be a consistent and reliable i n d e x t o t h e swelling p o t e n t i a l .

It was suggested b y A l t m e y e r in 1955 [ 1 8 ] as a guide t o t h e d e t e r m i n a t i o n of p o t e n t i a l expansiveness for various values of shrinkage limits and linear shrinkage as follows:

Shrinkage limit as a percentage

Linear shrinkage as a percentage

Degree of expansion

Less t h a n 10

Greater than 8

Critical

10-12 G r e a t e r t h a n 12

5 - 8

Marginal

0 - 5

Non-critical

R e c e n t research, however, failed t o show conclusive evidence of t h e correlation b e t w e e n swelling p o t e n t i a l and shrinkage limit. F r e e Swell. F r e e swell tests consist of placing a k n o w n v o l u m e of d r y soil in w a t e r and n o t i n g t h e swelled v o l u m e after t h e material settles, w i t h o u t a n y surcharge, t o t h e b o t t o m of a g r a d u a t e d cylinder. T h e difference b e t w e e n t h e final and initial v o l u m e , expressed as a p e r c e n t a g e of initial v o l u m e , is t h e free swell value. T h e swell test is very crude and was used in t h e early d a y s w h e n refined testing m e t h o d s were n o t available. Experiments

conducted

b y Holtz

[16]

indicated

t h a t a good grade of high swelling

commercial b e n t o n i t e will have a free swell value of from 1 2 0 0 t o 2 0 0 0 p e r c e n t . Holtz suggested t h a t soils having free swell value as low as 100 p e r c e n t can cause considerable damage t o lightly loaded s t r u c t u r e s , and soils having free swell value below 5 0 p e r c e n t seldom exhibit appreciable v o l u m e change even u n d e r very light loadings. Colloid C o n t e n t . T h e grain size characteristics of a clay appear t o have a bearing on its swelling p o t e n t i a l , particularly t h e colloid c o n t e n t . Seed, W o o d w a r d , and L u n d g r e n [ 1 7 ] believed t h a t there is n o correlation* b e t w e e n swelling p o t e n t i a l and percentage of clay sizes. However, for a given clay t y p e , t h e a m o u n t of swell will increase w i t h t h e a m o u n t of clay present in t h e soil as s h o w n on figure 8. F o r a n y given clay t y p e , t h e relationship b e t w e e n t h e swelling p o t e n t i a l and p e r c e n t a g e of clay size can be expressed by t h e e q u a t i o n :

where:

S

=

S

=

KCX Swelling p o t e n t i a l , expressed as percentage of swell u n d e r

1-psi

surcharge for a sample c o m p a c t e d at o p t i m u m m o i s t u r e c o n t e n t t o m a x i m u m density in s t a n d a r d A A S H O c o m p a c t i o n test,


20 70

ι l 1 NOTE : Percent swell measured under 1 psi surcharge for sample compacted at optimum water content to maximum density IP «tnnHnrri Δ Δ ^ΠΗ t * * t

Clay co mponent: C >mmercial Εîentonite

J

\y I

50

% 40

/

h i Comme -cial Illite/ Bentonite

, ,6\

Com rcercial Kao unite/Senti >nite

^3-1 Com nercial III) e/Bentonit

•mmercial JL-C<

^<_|:|

A

20

1

I 1 lite

Commercial Illite/Kaol nite 1 Commercial Kaolinite 100

40 50 60 70 30 PERCENT CLAY SIZES (finer than 0 . 0 0 2 mm)

Figure 8. Relationship between percentage of swell and percentage of clay sizes for experimental soils. (After Seed, Woodward & Lundgren)

C

=

Percentage of clay sizes finer t h a n 0 . 0 0 2 m m ,

X

=

A n e x p o n e n t d e p e n d i n g on t h e t y p e of clay, a n d

Κ

=

Coefficient d e p e n d i n g o n t h e t y p e of clay.

Where t h e q u a n t i t y o f t h e clay size particles is d e t e r m i n e d b y a h y d r o m e t e r test, t h e quality o r kind of colloid, w h i c h is reflected b y X and Κ in t h e above e q u a t i o n , c o n t r o l s t h e a m o u n t of swell. Colloid c o n t e n t as well as A t t e r b e r g limits should be i n c l u d e d in t h e r o u t i n e l a b o r a t o r y investigation o n expansive soils. Classification

method

By utilizing r o u t i n e l a b o r a t o r y tests such as A t t e r b e r g limits, colloid c o n t e n t s , shrinkage limits,

and

others,

t h e swelling

potential

can b e evaluated

without

resorting

to

direct

m e a s u r e m e n t . Some of these m e t h o d s are as follows: U S B R M e t h o d — Developed b y H o l t z and G i b b s [ 1 6 ] is based o n t h e s i m u l t a n e o u s consideration of several soil p r o p e r t i e s . T h e typical relationships of these p r o p e r t i e s w i t h swelling p o t e n t i a l are s h o w n o n figure 9. Based o n t h e curves presented in figure 9 , H o l t z [ 1 9 ] p r o p o s e d t h e identification criteria of expansive clay as follows:


21

Table 3 - D a t a for making estimates of probable volume changes for expansive soils Data from index tests* Colloid content, percent minus 0.001 mm

Plasticity index

>28 20-13 13-23 >15

>35 25-41 15-28 <18

Shrinkage limit

Probable expansion, percent total vol. change

Degree of expansion

<11 7-12 10-16 >15

>30 20-30 10-30 <10

Very high High Medium Low

•Based on vertical loading of 1.0 psi. (After Holtz & Gibb)

It should be p o i n t e d o u t t h a t figure 9 is based o n actual e x p a n s i o n tests for only 4 5 u n d i s t u r b e d and r e m o l d e d samples and, t h e r e f o r e , t h e d a t a a c c u m u l a t e d is n o t sufficient t o form accurate empirical relationship b e t w e e n m e a s u r e d e x p a n s i o n and t h r e e i n d i c a t o r tests. Especially, consideration should be given in t h e differentiation of soil behavior b e t w e e n u n d i s t u r b e d and r e m o l d e d samples. T h e a u t h o r has over t h e past 15 years p e r f o r m e d m a n y t h o u s a n d s of tests o n p o t e n t i a l swell and index p r o p e r t i e s . F r o m t h e test results of 3 2 1 u n d i s t u r b e d samples, a regression curve can b e fitted

as s h o w n o n figure 10. T h e relationship b e t w e e n swell p o t e n t i a l and plasticity i n d e x can b e

expressed as follows: S = in w h i c h

B e A( p )i

A = 0 . 0 8 3 8 , and Β = 0.2558

F r o m figure 10 it is seen t h a t with increase of plasticity i n d e x , t h e increase of swelling p o t e n t i a l is m u c h less t h a n predicted b y H o l t z and Gibbs or from Seed, W o o d w a r d and L u n d g r e n . All tests refer to a surcharge pressure of 1 psi w i t h m o i s t u r e c o n t e n t b e t w e e n 15 and 2 0 p e r c e n t and dry density b e t w e e n 100 and 110 pcf. Activity M e t h o d . - T h e activity m e t h o d p r o p o s e d b y Seed, W o o d w a r d , and L u n d g r e n ( 1 6 ) was based on r e m o l d e d , artificially prepared soils c o m p o s e d of 2 3 m i x t u r e s of b e n t o n i t e , illite, kaolinite, and fine sand. T h e expansion was measured as p e r c e n t swell on soaking from

100

p e r c e n t m a x i m u m d e n s i t y and o p t i m u m m o i s t u r e c o n t e n t in standard A A S H O c o m p a c t i o n test u n d e r a surcharge of 1 psi. T h e activity for t h e artificially prepared sample was defined as:

Activity

=

J2_

In t h e above, C d e n o t e s t h e percentage clay size finer t h a n 0 . 0 0 2 m m . T h e p r o p o s e d classification chart is s h o w n on figure 1 1 . T h e activity m e t h o d a p p e a r s t o be an i m p r o v e m e n t over t h e U S B R m e t h o d in t h a t t h e shrinkage limit did n o t e n t e r in t h e evaluation of swell p o t e n t i a l . Also, an a t t e m p t has b e e n m a d e t o differentiate b e t w e e n u n d i s t u r b e d and r e m o l d e d samples.

22


/ ·

1

/.

!•/ ';•/

—/

•

1 τ 7

f - - 0

•

r

//

./

/

-/i

/

1 • 1.

\V . 1 · \ \

hAr

V \

//'

COLLOID CONTENT (% less than 0.001 mm)

λ* A

%

-/

•l

• 40

1

• •

• 20

40

24

PLASTICITY INDEX

SHRINKAGE LIMIT (%)

Figure 9. Relation of volume change to colloid content, plasticity index, and shrinkage limit (air-dry to saturated condition under a load of 1 lb. per sq. in.) (After Holtz and Gibbs)

Indirect

measurement

Indirect m e a s u r e m e n t of swelling p o t e n t i a l of expansive soils has b e e n a p p r o a c h e d b y m a n y investigations. T h e Ladd and L a m b e m e t h o d aided b y a PVC m e t e r is p r o b a b l y t h e simplest and quickest m e t h o d , while t h e soil suction m e t h o d is considered t o b e a n e w a p p r o a c h t o w a r d t h e m e a s u r e m e n t of swelling p o t e n t i a l and swelling pressure.

PVC Meter. - T h e d e t e r m i n a t i o n of t h e p o t e n t i a l v o l u m e change (PVC) of soil was developed b y T. W. L a m b e u n d e r t h e auspices of t h e Federal Housing A d m i n i s t r a t i o n [ 2 0 ] . R e m o l d e d samples were specified. T h e sample was first c o m p a c t e d in a fixed ring c o n s o l i d o m e t e r w i t h c o m p a c t i o n effort of 5 5 , 0 0 0 ft.-lbs. per cu. ft. T h e n an initial pressure of 2 0 0 psi was applied, and w a t e r added t o t h e sample w h i c h is partially restrained from vertical expansion b y a proving ring. T h e proving ring reading is t a k e n at t h e end of 2 h o u r s . T h e reading is converted t o pressure and is designated as Swell I n d e x . F r o m figure 12, t h e swell index can b e converted t o p o t e n t i a l v o l u m e change. L a m b e established t h e following categories of PVC rating:

PVC Rating Less t h a n 2 2 —4 4 - 6 Greater than 6

Category Non-critical Marginal Critical V e r y critical


Figure 10. Relationship of volume change to plasticity index as predicted by Holtz, Seed, and Chen.

23


24

*>

Swelling Potential = 2 5 % Swelling Potential = 5% Swelling Potential = 1.5% I l _ I Ο

10

20

30 Percent

40

50

60

70

80

90

100

Clay Sizes (finer than 0.002mm)

Figure 11. Classification chart for swelling potential (After Seed, Woodward & Lundgren)

T h e PVC m e t e r m e t h o d has b e e n widely utilized b y t h e Federal Housing A d m i n i s t r a t i o n as well as t h e C o l o r a d o State Highway D e p a r t m e n t . It should b e p o i n t e d o u t t h a t t h e PVC m e t e r test in itself d o e s n o t m e a s u r e t h e swell p o t e n t i a l . T h e t r u e swell p o t e n t i a l of clay measured can be m u c h greater t h a n t h e indicated value. T h e PVC m e t e r test should be used only as a comparison b e t w e e n various swelling soils. Ladd and L a m b e [ 2 1 ] p r o p o s e d a classification system in 1961 w h e r e b y soils are classified w i t h respect t o p o t e n t i a l v o l u m e change d u e t o b o t h swelling and shrinkage. T h e m e t h o d has n o t received wide a t t e n t i o n . Soil Suction. — In theoretical analysis, t h e t o t a l suction can be considered t o consist of t h e o s m o t i c (or solute) p o t e n t i a l , gravitational p o t e n t i a l , and m a t r i x or capillary p o t e n t i a l . In engineering practice, however, it is considered satisfactory t o c o n d u c t l a b o r a t o r y analysis b y simulating t h e actual capillary p o t e n t i a l in t h e soil. T h e capillary p o t e n t i a l can b e considered as being equivalent t o t h e negative p o r e pressure at low level of m a t r i x s u c t i o n . T h e capillary potential of an u n s a t u r a t e d soil is often identified in t e r m s of its soil s u c t i o n .


2 Non Critical

25

3

4

Marginal POTENTIAL

5

6

Critical VOLUME

7

8

Very

Critical

CHANGE

9

ΙΟ

II

12 _

IPVC)

Figure 12. Swell index versus potential volume change. (From "FHA Soil PVC Meter Publication," Federal Housing Administration Publication No. 701)


26

Soil suction is expressed in a term designated as p F which is t h e log of t h e equivalent capillary rise in c e n t i m e t e r s of water. T h u s , a p F of 2 represents 100 c e n t i m e t e r s of h y d r o s t a t i c heads ( 2 0 5 psf), p F of 4 represents 10,000 c e n t i m e t e r s ( 2 0 , 5 0 0 psf), and so forth [ 2 2 ] . T h e a m o u n t of soil suction of a sample at equilibrium w i t h free w a t e r is z e r o . U p o n drying, the a m o u n t of soil suction rises rapidly. At oven d r y c o n d i t i o n , t h e value m a y b e several thousand atmospheres. O b e r m e i e r [ 2 3 ] claimed t h a t for a saturated clay mass, the stress release during excavation can result in significantly m o r e negative p o r e pressure in t h e u n d e r l y i n g soils. T h u s , w a t e r can flow i n t o t h e soil b e n e a t h t h e excavated area and cause swelling. O b e r m e i e r further believed that b o t h shear and tensile stresses m a y have been an i m p o r t a n t c o n t r i b u t i o n t o t h e heaving of clay shales. T h e long-term heave p o t e n t i a l t h a t results from stress release during t h e excavation of clay-shales m a y s o m e d a y be predicted b y the use of a suction test. T h e u l t i m a t e goal of t h e m e a s u r e m e n t

of soil suction is t h e p r e d i c t i o n of m o i s t u r e

m o v e m e n t and m o i s t u r e equilibria r a t h e r t h a n t h e direct m e a s u r e m e n t of t h e swell p o t e n t i a l . O s m o t i c cell-consolidometer a p p a r a t u s has recently been developed t o m e a s u r e t h e swelling properties

of

soils

under

variable

suction

conditions.

Obermeier

developed

an

osmotic

c o n s o l i d o m e t e r which is similar t o t h a t used b y Kassiff and Ben-Shalom [ 2 4 ] b u t is inexpensive, simple in c o n s t r u c t i o n , and easily a d a p t e d t o existing l a b o r a t o r y e q u i p m e n t . It consists of t w o units separated b y a semi-permeable m e m b r a n e . A solution of p o l y e t h y l e n e glycol is placed in unit I and t h e soil sample in u n i t II. Since t h e m e m b r a n e is pervious t o ions of dissolved salts in t h e soil-water, t h e system controls m a t r i x suction. T h e disadvantage of such tests is t h a t there is a long t i m e required t o reach equilibrium. Better l a b o r a t o r y t e c h n i q u e s for measuring heaving potential of swelling soils subjected t o stress release are n e e d e d .

Direct Measurem en t T h e m o s t satisfactory and convenient m e t h o d of d e t e r m i n i n g t h e swelling p o t e n t i a l and swelling pressure of an expansive clay is b y direct m e a s u r e m e n t . Direct m e a s u r e m e n t of expansive soils can be achieved b y t h e use of t h e conventional one-dimensional c o n s o l i d o m e t e r . T h e c o n s o l i d o m e t e r can b e platform t y p e , scale t y p e , or o t h e r a r r a n g e m e n t . T h e load can b e applied with air as in t h e case of C o n b e l c o n s o l i d o m e t e r or b y direct weight as in t h e case of cantilever consolidometer. T h e soil sample is enclosed b e t w e e n t w o p o r o u s plates and confined in a m e t a l ring. T h e d i a m e t e r of t h e ring ranges from 2 t o 4 inches d e p e n d i n g u p o n t h e t y p e of sampling device. T h e thickness of t h e s a m p l e ranges from one-half t o 1 inch. T n e soil sample can b e flooded b o t h from t h e b o t t o m and from t h e t o p . Vertical expansion m e a s u r e m e n t is r e p o r t e d as percentage of t h e initial height of the sample and is frequently referred t o as t h e p e r c e n t of swell. Such a device enables an easy and accurate m e a s u r e m e n t of t h e swelling p o t e n t i a l of a clay u n d e r various c o n d i t i o n s . After t h e soil has reached its m a x i m u m v o l u m e increase, t h e sample can b e reloaded

and swelling pressure d e t e r m i n e d

( c h a p t e r 2 ) . T h u s , swelling pressure can be

evaluated easily w i t h o u t resorting t o devices t o hold t h e soil v o l u m e c o n s t a n t . A great

deal

of d a t a

has been a c c u m u l a t e d

in files of soil engineers, academic

or

governmental organizations o n expansive tests using a c o n s o l i d o m e t e r . U n f o r t u n a t e l y , dissimilar test p r o c e d u r e s have been used. T h u s , it is difficult t o evaluate and c o m p a r e t h e test d a t a [ 2 5 ] . A

27


s t a n d a r d i z a t i o n of test p r o c e d u r e of a one-dimensional swell test d o e s n o t a p p e a r difficult and will salvage m u c h of t h e valuable d a t a a c c u m u l a t e d in t h e h a n d s of t h e private c o n s u l t a n t s . In the p e r f o r m a n c e of a typical swell test, t h e m o r e i m p o r t a n t variables involved are as follows: 1. State of sample. F o r an u n d i s t u r b e d sample, this w o u l d include t h e c o n d i t i o n of t h e sample, sampling m e t h o d , and stress h i s t o r y of t h e s a m p l e . F o r r e m o l d e d samples, this would include t h e m e t h o d of c o m p a c t i o n , curing t i m e before and after c o m p a c t i o n , and compaction density. 2. Moisture c o n t e n t . T h e lower t h e initial m o i s t u r e c o n t e n t t h e higher t h e swell. T h e initial m o i s t u r e c o n t e n t is affected b y : (a) T h e t i m e allowed for t h e sample to remain in t h e ring before w e t t i n g , (a) T h e e x t e n t of evaporation allowed while t h e sample is in t h e ring, and (c) T h e t e m p e r a t u r e and h u m i d i t y of t h e l a b o r a t o r y . 3 . Surcharge load. Increasing t h e applied load will r e d u c e t h e m a g n i t u d e of swell. Surcharge load for m o s t l a b o r a t o r y practice ranges from 1 t o 10 psi. S o m e t i m e s , a t t e m p t s were m a d e to duplicate t h e surcharge load w i t h t h e actual footing dead load. 4. T i m e

allowed.

The

time

required

to

fully

complete

t h e swell process m a y

vary

considerably and d e p e n d s on t h e p e r m e a b i l i t y of t h e clay, t h e m o l d i n g w a t e r c o n t e n t , t h e dry d e n s i t y , and thickness of t h e sample. F o r an u n d i s t u r b e d sample having a thickness of 1 inch, it m a y require as m u c h as several d a y s t o c o m p l e t e t h e total available swell. U n d o u b t e d l y , t h e direct m e a s u r e m e n t m e t h o d is t h e m o s t i m p o r t a n t and reliable test on expansive soils. By standardizing the above variables, a reliable and r e p r o d u c i b l e test can b e o b t a i n e d . Also, if t h e c o n c e p t of swelling pressure as discussed in c h a p t e r 2 is fully u n d e r s t o o d , m a n y of the variables m e n t i o n e d above can be simplified.

PHYSICAL PROPERTIES O F EXPANSIVE SOILS It is well k n o w n t o soil engineers t h a t m o n t m o r i l l o n i t e clays swell w h e n t h e m o i s t u r e c o n t e n t is increased, while swelling is absent o r limited in illite and kaolinite. T h e t y p e s of soils, and t h e c o n d i t i o n s u n d e r w h i c h t h e m o s t critical situation exists, can b e outlined as follows: Moisture

content

Irrespective

of

high

swelling p o t e n t i a l , if t h e m o i s t u r e c o n t e n t

of t h e clay

remains

u n c h a n g e d , t h e r e will be n o v o l u m e change; and s t r u c t u r e s founded on clays w i t h c o n s t a n t m o i s t u r e c o n t e n t will n o t be subject to m o v e m e n t caused b y heaving. When t h e m o i s t u r e c o n t e n t of the clay is changed, v o l u m e e x p a n s i o n , b o t h in the vertical and h o r i z o n t a l d i r e c t i o n , will take place. C o m p l e t e s a t u r a t i o n is n o t necessary t o accomplish swelling. Slight changes of m o i s t u r e c o n t e n t , in the m a g n i t u d e of o n l y 1 t o 2 p e r c e n t , are sufficient t o cause d e t r i m e n t a l swelling. In the l a b o r a t o r y , clay samples swell in the c o n s o l i d o m e t e r w i t h slight increase of h u m i d i t y . It is k n o w n t h a t floor slabs f o u n d e d on expansive soils cracked m o s t severely w h e n t h e m o i s t u r e c o n t e n t increased slightly d u e to local w e t t i n g . If t h e floor slab is flooded, as in t h e case of a rising w a t e r table, the floor will heave b u t the e x t e n t of cracking will n o t be severe.

28


T h e initial m o i s t u r e c o n t e n t of t h e expansive soils controls the a m o u n t of swelling. This is true

both

for

soils in

undisturbed

and in r e m o l d e d

states. As previously discussed,

the

relationship b e t w e e n t h e initial m o i s t u r e c o n t e n t and t h e capability of swelling has b e e n studied b y Holtz [ 1 6 ] , Seed [ 1 7 1 , and m a n y o t h e r s . V e r y d r y clays w i t h n a t u r a l m o i s t u r e c o n t e n t below 15 p e r c e n t usually indicate danger. Such clays will easily absorb m o i s t u r e t o as high as 35 p e r c e n t w i t h resultant damaging e x p a n s i o n t o s t r u c t u r e s . Conversely, clays w i t h m o i s t u r e c o n t e n t s above 3 0 p e r c e n t indicate t h a t m o s t of t h e expansion has already t a k e n place and further e x p a n s i o n will be small. However, m o i s t clays m a y desiccate d u e to lowering of w a t e r table or o t h e r changes in physical c o n d i t i o n s and u p o n s u b s e q u e n t w e t t i n g will again exhibit swelling p o t e n t i a l . Dry density

'

Directly related t o initial m o i s t u r e c o n t e n t , the dry density of the clay is a n o t h e r i n d e x of expansion. Soils w i t h d r y densities in excess of 110 pcf generally exhibit high swelling p o t e n t i a l . R e m a r k s m a d e b y excavators complaining t h a t the soils are as hard as a r o c k is an indication t h a t soils inevitably will p r e s e n t e x p a n s i o n p r o b l e m s . T h e d r y density of t h e clays is also reflected b y the standard p e n e t r a t i o n resistance test results. Clays w i t h p e n e t r a t i o n resistance in excess of 15 usually possess some swelling p o t e n t i a l . In the highly expansive clay areas of Denver, p e n e t r a t i o n resistances as high as 3 0 are n o t uncommon. Index

properties T h e a u t h o r has a c c u m u l a t e d years of test d a t a o n expansive soils in t h e R o c k y M o u n t a i n

area and found

t h a t it is m o r e c o n v e n i e n t t o correlate t h e expansive properties w i t h t h e

percentage of silt and clay ( - 2 0 0 ) , liquid limit, and field p e n e t r a t i o n resistance. Since m o s t lightly loaded s t r u c t u r e s will exert a m a x i m u m dead load pressure of a b o u t 1,000 psf on t h e footings, it is realistic to use a vertical load of 1,000 psf t o gauge the swelling p o t e n t i a l . Table 4 is a guide for estimating t h e p r o b a b l e v o l u m e changes of expansive soils. T h e simplified

classification

of the expansive p r o p e r t i e s can be conveniently used b y

engineers as a guide for t h e choice of t y p e of f o u n d a t i o n on expansive soils. F o r e x a m p l e , for

Table 4 - . Data for making estimates of probable volume changes for expansive soils Laboratory and field data Percentage passing No. 200 sieve

Liquid limit, percent

Standard penetration resistance, blows/ft

>35 60-95 30-60 <30

>60 40-60 3040 <30

>30 20-30 10-20 <30

Probable expansion, percent total volume change >10 3-10 1-5 < 1

Swelling pressure, ksf

Degree of expansion

>20 5-20 3-5 1

Very high High Medium low


29

soils w i t h a low degree of e x p a n s i o n , spread footing t y p e f o u n d a t i o n s can usually be used, if sufficient r e i n f o r c e m e n t is provided in t h e f o u n d a t i o n walls t o c o m p e n s a t e for slight m o v e m e n t s . F o r soils of m e d i u m degree of e x p a n s i o n , individual footings or p a d s can b e used w h e r e t h e dead load of t h e s t r u c t u r e can be c o n c e n t r a t e d to an i n t e n s i t y of 3 , 0 0 0 t o 5 , 0 0 0 psf. F o r soils of high-to-very-high degree of e x p a n s i o n , special c o n s i d e r a t i o n should be given as t o t h e f o u n d a t i o n t y p e . Piers w i t h sufficient d e a d load pressure and e n o u g h anchorage as described in c h a p t e r 4 should be used. Fatigue of

swelling

A clay sample is subjected t o full swelling in t h e c o n s o l i d o m e t e r , allowed t o desiccate t o its initial m o i s t u r e c o n t e n t , t h e n is saturated again. This is repeated for a n u m b e r of cycles. It was observed t h a t t h e soil showed signs of fatigue after each cycle of d r y i n g and w e t t i n g [ 2 6 ] . This p h e n o m e n o n has n o t b e e n u n d e r full investigation. It has b e e n n o t e d t h a t p a v e m e n t s f o u n d e d on expansive clays w h i c h have u n d e r g o n e seasonal m o v e m e n t d u e t o w e t t i n g and drying have a t e n d e n c y t o reach a p o i n t of stabilization after a n u m b e r of years. T h e fatigue of swelling p r o b a b l y can furnish t h e answer. Figure 13 shows a typical l a b o r a t o r y fatigue curve of swelling. Fatigue of swelling was also observed b y C h u [ 2 7 ] in his research o n controlled suction test. Chu believed t h a t if d r y i n g and w e t t i n g cycles are r e p e a t e d , t h e swelling d u r i n g t h e first cycle w o u l d be appreciably higher t h a n t h a t in s u b s e q u e n t cycles. 5

ι

1

1

ζ ο
1

1

1

1

1

I

< £ UJ

Claystone Soil Sample. Sample saturated to allow full expansion, then dessicated to initial moisture content ( 1 1 . 5 % ) , then allow full expansion a g a i n .

Ο

1

2

4

3

NUMBER

OF

CYCLES

OF

WETTING

5

8

DRYING

Figure 13. Fatigue of swelling (After Chen, 1965)

6

7

30


REFERENCES

[I] [2] [3] [4]

[5] [6] [71 [81 [9]

Holtz, W. G. and Gibbs, H. J., "Engineering Properties of Expansive Clays," Proceedings, ASCE, Vol. 80, 1954. Donaldson, G. W., "The Occurrence of Problems of Heave and the Factors Affecting its Nature." Second International Research and Engineering Conference on Expansive Clay Soils, Texas A & M Press, 1969. Tourtelot, Η. Α., "Geologic Origin and Distribution of Swelling Clays," Proceedings of Workshop on Expansive Clay and Shale in Highway Design and Construction, Vol. I, 1973. Salbert, J. R. and Smith, P.C., "Pavement Design over Expansive Clay: Current Practices and Research in the United States," Engineering Effects of Moisture Change in Soils, International Research and Engineering Conference on Expansive Clay Soils, Texas A & M Press, 1965. Jones, D. E. Jr., and Holtz, W.G., "Expansive Soils - The Hidden Disaster," Civil Engineering, Aug. 1973, Vol. 43, Nov. 8. Peck, R., Hanson, W. and Thornburn, T., "Foundation Engineering," John Wiley & Sons, 1974. Mielenz, R. C. and King, M. E., "Physical-Chemical Properties and Engineering Performance of Clays," Clay and Clay Technology, Bulletin 169, 1955, State of California, Dept. of Natural Resources. Grim, R. E., "Clay Mineralogy," McGraw-Hill Book Co., 1968. Low, P.F., "Fundamental Mechanisms Involved in Expansion of Clays as Particularly Related to Clay Mineralogy," Proceedings of Workshop on Expansive Clays and Shales in Highway Design and Construction, Vol. I, 1973.

[101 Warkentine, B.P., Bolt, G. H. and Miller, R. D., "Swelling Pressure of Montmorillonite," Soil Science Society of America, Proceeding 2 1 , 1957. [ I I ] Bolt, G. H., "Physical-Chemical Analysis of the Compressibility of Pure Clays," Geotechnique, Vol. 6, No. 2, 1956. [12] Brindley, G. W., "Identification of Clay Minerals by X-Ray Diffraction Analysis," Clays and Clay Technology, Div. of Mines Bulletin 169, 1955. [13] Dodd, C. G., "Dye Adsorption as Method of Identification of Clays," Clays and Clay Technology, Div. of Mines, Bulletin 169, 1955. [14] Kelley, W. P., "Interpretation of Chemical Analysis of Clays," Clays and Clay Technology, Div. of Mines, Bulletin 169, 1955. [15] Ravina, I., "Swelling of Clays, Mineralogical Composition and Microstructure," Proceedings of the Third International Conference on Expansive Soils, Haifa, Israel, 1973. [16] Holtz, W. G. and Gibbs, J. J., "Engineering Properties of Expansive Clays," ASCE Transactions Paper No. 2814, Vol. 121, 1956. [17] Seed, H. B., Woodward, R. J. and Lundgren, R., "Prediction of Swelling Potential for Compacted Clays," Journal ASCE, Soil Mechanics and Foundations Div., Vol. 88, 1962. [18] Altmeyer, W. T., "Discussion of Engineering Properties of Expansive Clays," Proceedings ASCE, Vol. 81, Separate No. 658, March, 1955. [19] Holtz, W. G., "Expansive Clay - Properties and Problems," Colorado School of Mines Quarterly, Vol. 54, No. 4, 1959. [20] "The Character and Identification of Expansive Soils." A Report Completed for the Technical Studies Program of the Federal Housing Administration, May, 1960. [21] Ladd, C. C. and Lambe, T. W., "The Identification and Behavior of Expansive Clays," Proceedings, 5th International Conference on Soil Mechanics and Foundation Engineering, Paris, Vol. I, 1961. [22] "A Review Paper on Expansive Clay Soils," by Woodward-Clyde & Assoc., Vol. I, 1967. [23] Obermeier, S. I., "Evaluation of Laboratory Techniques for Measurement of Swell Potential of Clays," Proceedings of Workshop on Expansive Clay and Shale in Highway Design and Construction, Vol. I, 1973. [24] Kassiff, G. and Ben-Shalom, Α., "Apparatus for Measuring Swell Potential Under Controlled Moisture Intake," ASTM Journal of Materials, March, 1971.


31

[25] Kraynski, L. M., "The Need for Uniformity in Testing of Expansive Soils," Proceedings of Workshop on Expansive Clays and Shales in Highway Design and Construction, Vol. I, 1973. [26] Chen, F. H., "The Use of Piers to Prevent the Uplifting of Lightly Loaded Structures Founded on Expansive Soils," Engineering Effects of Moisture Changes in Soils, Concluding Proceedings International Research and Engineering Conference on Expansive Clay Soils, Texas A & M Press, 1965. [27] Chu, T. and Mou, C. H., "Volume Change Characteristics of Expansive Soils Determined by Controlled Suction Tests," Proceedings of the Third International Conference on Expansive Soils, Haifa, Israel, 1973.

Chapter

2

MECHANICS OF SWELLING

INTRODUCTION In c h a p t e r 1, t h e origin, mineralogical c o m p o s i t i o n , and t h e basic s t r u c t u r e of expansive soil were o u t l i n e d . Obviously, if t h e e n v i r o n m e n t of t h e expansive soil has n o t b e e n changed, swelling will n o t t a k e place. E n v i r o n m e n t a l change can consist of pressure release d u e t o excavation, desiccation caused b y t e m p e r a t u r e increase, and v o l u m e increase because of t h e i n t r o d u c t i o n of m o i s t u r e . By far t h e m o s t i m p o r t a n t e l e m e n t and of m o s t c o n c e r n t o t h e practicing engineer is t h e effect of w a t e r on expansive soils. Kraynski [ 2 2 ] stated in his review p a p e r o n expansive soils, " T h e r e m u s t be a p o t e n t i a l gradient which can cause w a t e r migration and a c o n t i n u o u s passage t h r o u g h w h i c h w a t e r transfer can t a k e p l a c e . " With t h e i n t r o d u c t i o n of water, v o l u m e t r i c e x p a n s i o n takes place. If pressure is applied t o p r e v e n t e x p a n s i o n , t h e pressure required t o m a i n t a i n t h e initial v o l u m e is t h e swelling pressure. This c h a p t e r p r e s e n t s a discussion o n t h e m i g r a t i o n of w a t e r , swelling p o t e n t i a l , and swelling pressure.

MOISTURE MIGRATION The

pattern

of

moisture

migration

depends

on

the

geological

formations,

climatic

c o n d i t i o n s , t o p o g r a p h i c features, soil t y p e s , and g r o u n d - w a t e r level. I m p o r t a n t differences in t h e m o i s t u r e migration p a t t e r n b e t w e e n covered and n a t u r a l areas have b e e n studied extensively b y t h e C o m m o n w e a l t h Scientific and Industrial Research Organization in Australia. Much research has b e e n c o n d u c t e d in r e c e n t years b y highway organizations in Australia, S o u t h Africa, and t h e United States in an a t t e m p t t o stabilize p a v e m e n t s c o n s t r u c t e d in expansive soil areas. Moisture

transfer

T h e m o s t c o m m o n m e t h o d of m o i s t u r e transfer is b y gravity. T h e seepage of surface w a t e r , p r e c i p i t a t i o n , and snow m e l t i n g i n t o t h e soil are c o m m o n e x a m p l e s . T h e m o i s t u r e migration can o c c u r in all directions. U n d e r artesian c o n d i t i o n s , t h e flow can be u p w a r d . In stiff clays and in shale b e d r o c k , t h e flow generally occurs in t h e b e d d i n g planes or follows c o n t i n u o u s fractures and fissures. Shrinkage cracks w h i c h develop due t o surface desiccation provide easy access of w a t e r i n t o t h e d e e p soils. In fine grained soils, capillary force is a significant m e a n s of w a t e r transfer. T h e height of w a t e r rise i n t o t h e capillary fringe varies inversely w i t h t h e radius of t h e capillary t u b e . In clean, coarse gravel, t h e capillary rise is insignificant. In clean sands, t h e rise is a few inches; in fine


34

sands, t h e rise is 1 or 2 feet; in silt, u p t o 10 t o 12 feet; and in clay, a rise of m o r e t h a n 1000 feet is theoretically possible. It is well recognized b y observant soil engineers t h a t t h e heaving of expansive soils m a y t a k e place w i t h o u t t h e presence of free w a t e r . V a p o r transfer plays an i m p o r t a n t role in providing the m e a n s for t h e v o l u m n e increase of expansive soils. Water v a p o r at a t e m p e r a t u r e higher t h a n its surroundings will migrate t o w a r d t h e cooler area t o equalize the t h e r m a l engery of t h e t w o areas. When w a t e r reaches t h e cooler area, generally the covered area b e n e a t h a s t r u c t u r e , c o n d e n s a t i o n can t a k e place and provide sufficient m o i s t u r e t o initiate swelling. T h e r m a l gradients can also cause m o i s t u r e migration t h r o u g h t h e liquid p h a s e of t h e soils. E x p e r i m e n t s c o n d u c t e d at P r i n c e t o n University show t h a t a t e m p e r a t u r e differential of Γ C was at least equivalent t o a h y d r o s t a t i c head of 3 feet in its ability t o cause m o i s t u r e m i g r a t i o n . T h e r m a l gradient reaches m a x i m u m efficiency w h e n t h e m o i s t u r e c o n t e n t in t h e soil is near plastic limit. Covering t h e ground surface a r o u n d t h e building w i t h plastic m e m b r a n e s creates a thermal

gradient

which

may

encourage

moisture

from

lawn watering t o transfer t o

the

f o u n d a t i o n soils. V a p o r and liquid m o i s t u r e transfer u n d e r t h e r m a l gradient can be an i m p o r t a n t cause of t h e swelling of moisture-deficient soils. Moisture

equilibria

In n a t u r a l g r o u n d , t h e m o i s t u r e c o n t e n t

of t h e partially saturated

soil is in general

equilibrium w i t h t h e applied stress, t h e forces d u e t o evaporation and transpiration at ground surface, and t h e capillary forces. When t h e area is covered b y a building o r p a v e m e n t , t h e evaporation and t r a n s p i r a t i o n

forces are eliminated and a new set of equilibrium m u s t be

established. T h e new equilibrium

requires t h e flow

of m o i s t u r e c o m p a t i b l e w i t h t h e new

c o n d i t i o n . T h e force causing t h e m o i s t u r e change or flow is t e r m e d soil suction. T h e review panel of engineering c o n c e p t s of Moisture Equilibria and Moisture Changes in Soil B e n e a t h Covered Areas [ 2 8 ] states, " T h e major change induced in soil b y a surface cover is caused b y an alteration in t h e rates and q u a n t i t i e s of w a t e r able t o e n t e r and leave t h e soil at t h e surface." Soil suction as briefly described in c h a p t e r 1 is considered a basic p a r a m e t e r t o soil-water equilibria and m o v e m e n t of w a t e r in soil. T h e p o r e pressure of an u n d i s t u r b e d sample o b t a i n e d at t h e g r o u n d surface (above t h e w a t e r table w i t h n o externally applied stress) is less t h a n a t m o s p h e r i c . T h e difference b e t w e e n t h e p o r e pressure and a t m o s p h e r i c pressure is t h e value of soil suction [ 2 9 ] . Below t h e g r o u n d surface or u n d e r structural load, the value of soil suction is reduced b y the o v e r b u r d e n pressure a n d / o r t h e e x t e r n a l stress. T h e m a g n i t u d e of p o r e pressure in t h e soil can be obtained from t h e soil suction as follows:

where:

u

=

aP-s

a

=

fraction of t h e n o r m a l pressure effective in changing t h e

Ρ

=

s

=

t h e m e a s u r e d soil s u c t i o n , and

u

=

t h e p o r e w a t e r pressure.

value of soil s u c t i o n , t h e t o t a l n o r m a l stress o n t h e soil e l e m e n t ,


35

In s a t u r a t e d clay a is u n i t y - i n n o n c o n s o l i d a t a b l e soil a is z e r o . In u n s a t u r a t e d clay, t h e value of a varies b e t w e e n zero and u n i t y . T h e soil suction c o n c e p t provides a m e a n s of d e t e r m i n i n g , w i t h o u t direct m e a s u r e m e n t , t h e soil m o i s t u r e c o n t e n t . T h e calculated m o i s t u r e will c o r r e s p o n d t o m e a s u r e d field

moisture

c o n t e n t if field p o r e pressure can b e accurately d e t e r m i n e d [ 3 0 ] . Aichison and R i c h a r d s [ 3 1 ] claim t h a t in u n s a t u r a t e d soils, t h e p o r e pressures are negative and t h a t t h e p e r m e a b i l i t y Κ is n o longer assumed t o b e a c o n s t a n t . C o n s e q u e n t l y , t h e classic D a r c y ' s Law should be modified t o c o n f o r m with flow of w a t e r in u n s a t u r a t e d soils. Aichison presented empirical m e t h o d s predicting equilibrium soil s u c t i o n u n d e r s t r u c t u r e s b y utilizing t h e results of a broad-scale field investigation b e n e a t h sealed p a v e m e n t t h r o u g h o u t Australia. S t u d y showed t h a t t h e soils u n d e r a covered area are isolated from rapid changes and tend t o w a r d a stable equilibrium m o i s t u r e d i s t r i b u t i o n . T h e t o t a l suction b e n e a t h t h e covered area t e n d s t o change w i t h climatic c o n d i t i o n s .

Depth

of moisture

fluctuation

K r a y n s k i [ 2 2 ] explained t h e m o i s t u r e c o n t e n t variation w i t h d e p t h in a h o m o g e n e o u s soil b y figure 14. In a covered area, t h e m o i s t u r e profile is s h o w n b y curve 1. T h e r e is n o gain or loss of m o i s t u r e t o t h e a t m o s p h e r e . T h e m o i s t u r e c o n t e n t of t h e soil decreases w i t h d e p t h . Curve 2 indicates t h e m o i s t u r e c o n t e n t variation w i t h d e p t h in t h e same area in uncovered

natural

c o n d i t i o n s . E v a p o r a t i o n causes loss of m o i s t u r e in t h e soil n e a r t h e ground surface. However, t h e influence of e v a p o r a t i o n decreases w i t h d e p t h and at s o m e d e p t h , H ^ , t h e m o i s t u r e c o n t e n t equilibrium remains t h e same as t h e covered c o n d i t i o n . K r a y n s k i referred t o this d e p t h as D e p t h of Desiccation. T h e value of H ^ d e p e n d s c n t h e climate c o n d i t i o n , t h e t y p e of soil, and t h e location of t h e w a t e r table. This d e p t h represents t h e t o t a l thickness of material w h i c h has a p o t e n t i a l t o e x p a n d because of w a t e r deficiency. It is impossible t o d e t e r m i n e t h e value of H(j. T h e h o t t e r and drier t h e climate, t h e greater t h e d e p t h of desiccation. T h e m a x i m u m d e p t h of H(j is equal t o t h e d e p t h t o t h e w a t e r table, and t h e m i n i m u m d e p t h is equal t o t h e d e p t h of t h e seasonal m o i s t u r e c o n t e n t fluctuation described b e l o w . During w e t m o n t h s w i t h heavier p r e c i p i t a t i o n and higher h u m i d i t y , t h e m o i s t u r e c o n t e n t of near-surface soil increases and t h e m o i s t u r e profile represented b y curve 2 alters its shape t o curve 3. T h e u p p e r p o r t i o n of curve 3 can e x t e n d b e y o n d curve 1 in very w e t seasons and b e h i n d curve 1 in dry seasons. The* d e p t h of seasonal m o i s t u r e c o n t e n t f l u c t u a t i o n , H s, indicated in figure 14 d e p e n d s on t h e variation of surface m o i s t u r e , permeability of t h e soils, and climatic c o n d i t i o n s . In areas w h e r e p r e c i p i t a t i o n and evaporation are fairly c o n s t a n t , t h e H s d e p t h m a y be only a few feet. When a long d r o u g h t is followed b y an intense rainfall, t h e H s d e p t h can reach 10 feet or more. It should b e n o t i c e d t h a t in t h e above evaluation of t h e d e p t h H s, n o c o n s i d e r a t i o n has been given t o t h e m a n - m a d e e n v i r o n m e n t . T h e watering of lawns, planting of trees and s h r u b s , discharge of r o o f drains, f o r m a t i o n of drainage channels and swales, and t h e possibility of utility line leakage will all increase t h e value of H s. It is n o t u n c o m m o n t h a t H s d e p t h can reach as m u c h as 25 feet (see c h a p t e r 7).

36


INCREASING MOISTURE CONTENT OF SOIL

2 - Dessicated Moisture Content 3 - Wet Season Moisture Content 2 - Dessicated Moisture Content H $- Depth of Seasonal Moisture Content Fluctuation H d-Depth of Dessication Figure 14. Moisture content variation with depth below ground surface. (After Kraynski, 1967)


37

When areas are covered b y structures such as buildings, p a v e m e n t s , sidewalks o r a p r o n s , evaporation is blocked or partially r e t a r d e d . T h e m o i s t u r e c o n t e n t b e n e a t h t h e covered area increases d u e t o gravitational migration, capillary action, v a p o r and liquid t h e r m a l transfer a n d , in the course of several years, t h e d e p t h of seasonal m o i s t u r e c o n t e n t fluctuation H s can a p p r o a c h t o d e p t h of desiccation H ^ . The

shifting

of

t h e m o i s t u r e profile of a swelling soil from

t h e natural

conditions

represented b y curves 1 and 2 t o curve 3 in figure 14 for covered c o n d i t i o n s is t h e cause of significant d a m a g e . Since m o i s t u r e transfer is a slow process, it is n o t surprising t h a t distress of a building often takes place several years after o c c u p a n c y . In t h e course of investigating a cracked building, it is n o t u n u s u a l t o find t h a t t h e m o i s t u r e c o n t e n t of t h e soils b e n e a t h t h e covered area or in the vicinity of t h e covered area had substantially increased. Shrinkage In reviewing literature o n expansive soil, m u c h a t t e n t i o n has b e e n focused b y various investigators o n t h e m e c h a n i c s of shrinkage. It has b e e n claimed t h a t shrinkage is t h e m i r r o r reflection of e x p a n s i o n . It is e x p e c t e d b y measuring soil s u c t i o n w h i c h evaluates t h e t o t a l negative pressures in t h e soil-water s y s t e m , t h e p h e n o m e n o n of swelling and shrinkage can be explained at t h e same t i m e . It is believed t h a t seasonal m o i s t u r e c o n t e n t fluctuation can result in heaving and settling of t h e s t r u c t u r e . T h e cyclic u p and d o w n m o v e m e n t is believed to o c c u r in phases, b u t lags b e h i n d rainfall [ 3 2 ] . Theoretically, shrinkage can result in s e t t l e m e n t , b u t t h e r e is very little evidence that

t h e r e is appreciable d o w n w a r d m o v e m e n t u n d e r covered areas in a building. Seasonal

fluctuation of m o i s t u r e c o n t e n t along t h e edges of highway p a v e m e n t o r parking areas can be e x p e c t e d , b u t at t h e central p o r t i o n of a covered area, shrinkage seldom takes place even u n d e r a prolonged arid climate. M o v e m e n t m e a s u r e m e n t s of buildings in good c o n d i t i o n have seldom been

performed.

Monitoring cracked building m o v e m e n t c a n n o t reflect t h e effect of seasonal m o i s t u r e variation. An u n u s u a l o p p o r t u n i t y afforded t h e m a k i n g of a precise m e a s u r e m e n t o f a school building founded o n piers. T h e m o v e m e n t of t h e piers was m e a s u r e d each m o n t h for a period of 11 m o n t h s . T h e m o v e m e n t of b o t h t h e interior and e x t e r i o r piers was r e c o r d e d and p l o t t e d along w i t h t h e average m o n t h l y p r e c i p i t a t i o n , as s h o w n in figure 15. T h e d r y and w e t p e r i o d s coincide fairly well w i t h t h e u p w a r d and d o w n w a r d m o v e m e n t of the

e x t e r i o r piers. However,

for t h e interior piers, t h e m o v e m e n t

graph lags b e h i n d

the

p r e c i p i t a t i o n graph. This is e x p e c t e d since it takes a d d i t i o n a l t i m e for m o i s t u r e or v a p o r t o migrate from t h e e x t e r i o r of t h e building t o t h e interior piers. In t h e school building previously discussed, it should b e n o t e d t h a t over a period of 11 m o n t h s , t h e difference b e t w e e n t h e m a x i m u m and m i n i m u m pier m o v e m e n t seldom exceeded one-half inch. Such m o v e m e n t was insufficient t o manifest noticeable d a m a g e t o t h e building. In fact, such m o v e m e n t w o u l d n o t have been n o t e d had n o t a precise leveling b e e n c o n d u c t e d . T h e end result of shrinkage a r o u n d or b e n e a t h a covered area seldom causes any s t r u c t u r a l d a m a g e , h e n c e , is n o t an i m p o r t a n t item t o be considered b y soil engineers. It is the c o n t i n u o u s increase in m o i s t u r e c o n t e n t b e n e a t h t h e covered area t h a t i n t r o d u c e s the d a m a g e in an expansive soil area.


38

„

MOVEMENT OF PIER

MONTHS (SHIFTED)

ι N

1 O

1 J

1

1

1

1

F

M

A

M

1 J MONTHS

1

1 J

1

1 A

1 S

1 O

1 N

J

D

Figure 15. Movement of exterior and interior piers in a school building with respect to precipitation. SWELLING POTENTIAL A l t h o u g h t h e swelling p h e n o m e n o n has b e e n fully recognized for m a n y years, a definite m e t h o d of measuring t h e swelling p o t e n t i a l of clay has n o t been established. In 1 9 6 2 , Seed [ 1 7 ] defined swelling p o t e n t i a l as t h e p e r c e n t a g e of swell of a laterally confined sample on soaking u n d e r a 1-psi surcharge, after being c o m p a c t e d t o m a x i m u m density at o p t i m u m w a t e r c o n t e n t in t h e standard A A S H O c o m p a c t i o n test. L a m b e [ 3 3 ] used t h e swell index t o m e a s u r e t h e expansion characteristics of clay. T h e swell index is defined as t h e slope of t h e e-log ρ curve. T h e pressure i n c r e m e n t from 1.0 t o 0.1 kg per c m 2 w a s used. It is clear t h a t b o t h definitions have their limitations. Seed's definition is confined t o r e m o l d e d soil while t h e slope of t h e e-log ρ curve d e p e n d s o n initial-moisture c o n t e n t as well as surcharge pressure. T h e difficulty

in providing a suitable yardstick for measuring swelling

characteristics is t h e presence of t h e n u m e r o u s variables involved. C o n s e q u e n t l y , t o this d a t e , w h e n high swelling soil in San A n t o n i o , T e x a s is discussed, it is n o t t h e same standard as t h e swelling of t h e clay shale in t h e R o c k y M o u n t a i n area. T h e r e is an u r g e n t need for u n i f o r m i t y in testing expansive soils. A s t a n d a r d m e t h o d for general use was p r o p o s e d b y W. G. H o l t z [ 3 4 ] . T h e p r o p o s e d m e t h o d utilizes t h e c o n v e n t i o n a l fixed-ring

consolidometer

for c o n d u c t i n g t h e tests. Procedures are given for testing

both

u n d i s t u r b e d and r e m o l d e d specimens. T h e standard p r o c e d u r e involves t h e testing of t w o similar samples, o n e for l o a d e d - a n d - e x p a n d e d test and t h e o t h e r for e x p a n d e d - a n d - l o a d e d test. T h e m e t h o d also involves shrinkage tests and permeability tests. T h e m e t h o d has b e e n s u b m i t t e d for consideration as a standard t o ASTM C o m m i t t e e D-18 and is given in A p p e n d i x A.

MECHANICS OF SWELLING Factors affecting

volume

39

change

In t h e preceding c h a p t e r u n d e r t h e "Physical P r o p e r t i e s of Expansive S o i l , " t h e i m p o r t a n c e of

initial

moisture

content

and

initial d r y d e n s i t y has b e e n discussed. These and

other

e n v i r o n m e n t a l c o n d i t i o n s are e x t r e m e l y i m p o r t a n t in d e t e r m i n i n g t h e a m o u n t of swell. T h e following factors influence t h e results o b t a i n e d in loaded swell tests o n soils of any mineralogical composition: 1. Initial m o i s t u r e c o n t e n t . In testing u n d i s t u r b e d samples, care should b e t a k e n in selecting the sample w i t h t h e m o s t critical m o i s t u r e c o n t e n t . Usually, tests should b e p e r f o r m e d o n t h e driest sample. F r e q u e n c y of testing is i m p o r t a n t so as t o cover all possible c o n d i t i o n s . F o r r e m o l d e d samples, it is o b v i o u s t h a t t h e initial m o i s t u r e c o n t e n t will c o n t r o l t h e v o l u m e change. Field c o n d i t i o n and c o n s t r u c t i o n specifications d i c t a t e t h e m o i s t u r e requirement.

In a d d i t i o n , a great deal of a t t e n t i o n should b e directed t o t h e time

e l e m e n t - t h e t i m e elapsed b e t w e e n sampling and testing and t h e t i m e elapsed w h e n t h e sample is placed in t h e c o n s o l i d o m e t e r , and w h e n t h e w e t t i n g takes place or w h e n t h e load is applied. 2. Initial d r y d e n s i t y . T h e single m o s t i m p o r t a n t factor affecting swelling characteristics of swelling soils is d e n s i t y . Detailed discussions are given later u n d e r "Swelling P r e s s u r e . " In r e m o l d e d tests, t h e initial c o m p a c t i o n c o n d i t i o n is critical. Swell tests m a y d e c i d e t h e degree of c o m p a c t i o n required in t h e p l a c e m e n t of fill. Since m o i s t u r e c o n t e n t and d r y d e n s i t y are closely related, t h e y should b e e x a m i n e d c o n c u r r e n t l y . 3 . Surcharge pressure. A small surcharge load in t h e range of 0.35 t o 1 psi has b e e n suggested for a seating load in t h e swell test. Since swell is very sensitive t o changes in pressure in t h e lower ranges of pressure (less t h a n a b o u t 1 psi), t h e use of low surcharge pressure m a y lead t o erratic and e r r o n e o u s test results. Since m o s t footing f o u n d a t i o n s can exert a pressure of a b o u t 1,000 psf o n the soil, it is r e c o m m e n d e d t h a t this value b e used for a surcharge load. 4 . T i m e allowed for swell. T h e t i m e required for t h e soil t o reach its m a x i m u m swell p o t e n t i a l m a y vary considerably d e p e n d i n g essentially o n t h e initial d e n s i t y , p e r m e a b i l i t y , and t h e thickness of t h e sample. F o r r e m o l d e d samples, generally 2 4 h o u r s is sufficient t o o b t a i n 9 5 p e r c e n t of t h e total available swell. A t t h e s a m e t i m e , for u n d i s t u r b e d high d e n s i t y clay shale, it m a y require several d a y s or even a w e e k before c o m p l e t e s a t u r a t i o n can

be

achieved.

For

remolded

samples,

t h e initial a d d e d w a t e r m u s t b e

evenly

d i s t r i b u t e d . This requires a m i n i m u m curing time of 6 h o u r s for r e p r o d u c i b l e results. 5. Size and thickness. Sample thickness affects t h e t i m e required for t o t a l s a t u r a t i o n . T o e x p e d i t e testing t i m e , sample thickness of less t h a n

1 inch should be used. G r e a t e r

thickness m a y i n t r o d u c e excessive side friction. A t t h e same t i m e , in a c u t t i n g from an u n d i s t u r b e d s a m p l e , small thickness m a y i n t r o d u c e surface d i s t u r b a n c e and e x c l u d e t h e possible effect of granular particles, fissures, and seams in t h e soil. T h e d i a m e t e r of t h e sample is also significant, a l t h o u g h o n l y vertical rise is m e a s u r e d in t h e c o n s o l i d o m e t e r . T h e smaller t h e d i a m e t e r , t h e larger t h e effect of side friction. Sample d i a m e t e r is c o n t r o l l e d b y a sampling device. In general, t h e d i a m e t e r should n o t b e less t h a n 2 inches. S t a n d a r d i z a t i o n o n b o t h size and thickness o f sample is a p p a r e n t l y necessary for c o m p a r a b l e results.

40


F r o m t h e preceding analysis, it is obvious t h a t for a certain clay w i t h k n o w n p r o p e r t i e s , t h e swelling characteristics will vary greatly w i t h t h e variation of one or m o r e of t h e above environmental

or

placement

conditions.

It w o u l d

be e r r o n e o u s to c o m p a r e

the

swelling

characteristics of different soils w i t h o u t first clearly defining t h e p l a c e m e n t c o n d i t i o n . A reliable and r e p r o d u c i b l e test w h i c h is t o be considered as a basis for t h e classification of p o t e n t i a l expansive soil m u s t b e s t a n d a r d i z e d at least for t h e following e n v i r o n m e n t a l c o n d i t i o n s : F o r u n d i s t u r b e d sample: Surcharge pressure Size and thickness T i m e required for t h e test Initial m o i s t u r e c o n t e n t and density m u s t b e qualified F o r r e m o l d e d sample: Initial m o i s t u r e c o n t e n t Initial d r y d e n s i t y M e t h o d of c o m p a c t i o n Surcharge pressure Size and thickness T i m e required for t h e test Curing t i m e allowed Clearly, o n e set of s t a n d a r d s can hardly cover t h e very c o m p l i c a t e d variables involved. With the i n t r o d u c t i o n of t h e c o n c e p t of swelling pressure, t h e above e n v i r o n m e n t a l c o n d i t i o n can be simplified and test results can be c o m p a r e d as later explained u n d e r "Swelling P r e s s u r e . "

Total

heave T h e d e t e r m i n a t i o n of swelling p o t e n t i a l in t h e l a b o r a t o r y , as outlined above, leads t o t h e

prediction of t o t a l heave o r t h e m a x i m u m p o t e n t i a l m a g n i t u d e of heaving of a s t r u c t u r e . T h e in situ variables involved are always m o r e c o m p l i c a t e d t h a n those used in l a b o r a t o r y tests, especially w h e r e only a single sample is used t o represent an entire f o u n d a t i o n . S o m e of t h e variables are described as follows: 1. Climate. Climate c o n d i t i o n s involving p r e c i p i t a t i o n , e v a p o r a t i o n and transpiration affect t h e m o i s t u r e in t h e soil as previously discussed u n d e r " M o i s t u r e T r a n s f e r . " T h e d e p t h and degree of desiccation

affects

the amount

of swell in a given soil h o r i z o n . Climate

c o n d i t i o n partially affects t h e desiccation. 2. Soil profile. T h e thickness of expansive soil u n d o u b t e d l y affects t h e m a g n i t u d e of t o t a l heave. If t h e thickness of high swell p o t e n t i a l soils is thin, the p o t e n t i a l total heave will be small. At t h e same t i m e , for a thick s t r a t u m of low swelling soil, t h e t o t a l heave can be considerable over a long period of t i m e . As long as t h e thickness of t h e swelling soil is less t h a n 2 4 inches, little d a m a g e t o t h e f o u n d a t i o n can t a k e place. 3 . G r o u n d w a t e r . Evidently, swelling soil located b e l o w g r o u n d w a t e r will n o t pose a p r o b l e m t o t h e s t r u c t u r e . T h e thickness of t h e swelling soil s t r a t u m is, therefore, limited b y the d e p t h t o g r o u n d water. However, g r o u n d w a t e r fluctuates, and receding g r o u n d w a t e r s o m e t i m e s c o n t r i b u t e s to a d d i t i o n a l swelling. Damaging swelling soil p r o b l e m s are


41

seldom e n c o u n t e r e d for a soil profile w i t h g r o u n d w a t e r located s h o r t distances b e n e a t h t h e footings. Capillary m o v e m e n t and v a p o r transfer will b e of such m a g n i t u d e t h a t it takes only a s h o r t period of t i m e before t h e thin layer of swelling soil below t h e s t r u c t u r e becomes completely saturated. 4 . Drainage. As e x p e c t e d , good surface drainage a r o u n d t h e s t r u c t u r e r e d u c e s t h e swelling p r o b l e m as discussed in detail in c h a p t e r 8. V a r i o u s m e t h o d s have b e e n p r o p o s e d t o predict t h e a m o u n t of total heave u n d e r a given structural load. These are t h e d o u b l e o e d o m e t e r m e t h o d [ 3 5 1 , t h e McDowell m e t h o d [ 3 6 ] , t h e L a m b e and W h i t m a n m e t h o d [ 3 3 ] , and the F H A m e t h o d [ 3 7 ] . All suggested m e t h o d s have limitations. T h e d o u b l e o e d o m e t e r m e t h o d developed b y Jennings and Knight is based on t h e c o n c e p t of effective stress and has received wide a t t e n t i o n . T h e general test p r o c e d u r e is given as follows: T w o c o n s o l i d o m e t e r rings are filled w i t h u n d i s t u r b e d samples from adjacent l o c a t i o n s . T h e first sample is k e p t at its n a t u r a l m o i s t u r e c o n t e n t and a confined c o m p r e s s i o n test is p e r f o r m e d . T h e second sample is flooded w i t h w a t e r at low pressure of 2 0 psf. After t h e sample is fully w e t t e d , a c o n s o l i d a t i o n test is perfor«med in t h e c o n v e n t i o n a l m a n n e r . T h e t w o c o m p r e s s i o n curves are p l o t t e d o n t h e same diagram and o n e of t h e curves is selected for vertical a d j u s t m e n t so as t o coincide w i t h t h e virgin sections of t h e curves as s h o w n o n figure 16. A soil sample t a k e n at d e p t h ζ has an o v e r b u r d e n pressure P

0

= Yz w h e r e Y is t h e d e n s i t y of

t h e soil. T h e void-ratio at t h e o v e r b u r d e n pressure is e G. S e t t l e m e n t d u e t o load i n c r e m e n t Δ Ρ can be calculated from t h e change of void-ratio e 0 - t l . If n o load is applied, t h e soil u n d e r a covered area will gain m o i s t u r e a n d swelling will t a k e place. T h e c o n d i t i o n will alter P 0 , resulting in e 0 having a n e w effective pressure P 0 + U L represented in t h e u p p e r saturated curve b y P 0 + U L and e 2. If D is t h e d e p t h t o t h e w a t e r table and Y w is t h e u n i t weight of water, U L = Y w (D - z). T h e effect of t h e load i n c r e m e n t is t h e n again t a k e n i n t o c o n s i d e r a t i o n and t h e final values are ( P 0 + U L + Δ Ρ and e 3 ). T h e final c o n d i t i o n s of m o v e m e n t m a y t h e n be p r e d i c t e d b y adding t h e voidration changes over t h e w h o l e profile. T h e d o u b l e o e d o m e t e r m e t h o d is based o n t h e a s s u m p t i o n t h a t t h e r e is a p o i n t d u r i n g c o m p r e s s i o n at w h i c h t h e initially u n s a t u r a t e d soils pass from an applied pressure t o an effective p h e n o m e n o n and t h e c o m p r e s s i o n curve joins w i t h t h e virgin consolidation curve. Typical heave calculations are s h o w n o n table 5. T h e search b y various investigators for a reliable m e t h o d for predicting t h e t o t a l heave is p r o b a b l y affected b y t h e c o n c e p t of u l t i m a t e s e t t l e m e n t in t h e t h e o r y of c o n s o l i d a t i o n . F o r m a n y years engineers have b e e n familiar w i t h t h e calculation of u l t i m a t e s e t t l e m e n t

and

differential s e t t l e m e n t of a s t r u c t u r e f o u n d e d on clay, and it is assumed t h a t t h e total heave can also be p r e d i c t e d . T h e r e are s o m e f u n d a m e n t a l differences b e t w e e n t h e behavior of settling and heaving soil. S o m e of t h e m are as follows: 1. S e t t l e m e n t of clay u n d e r load will t a k e place w i t h o u t the aid of w e t t i n g , while e x p a n s i o n of clay will n o t be realized w i t h o u t m o i s t u r e increase. 2. T h e t o t a l a m o u n t of heave d e p e n d s o n t h e e n v i r o n m e n t a l c o n d i t i o n , such as t h e e x t e n t of w e t t i n g , t h e d u r a t i o n of w e t t i n g , and t h e p a t t e r n of m o i s t u r e m i g r a t i o n . Such variables c a n n o t b e ascertained, and c o n s e q u e n t l y , a n y t o t a l heave p r e d i c t i o n c a n b e entirely erroneous.

42


Figure 16. Log P curves showing adjustment to bring straight line portions coincident. (After Jennings & Knight, 1958)

3. Differential settlement is usually described as a percent of the ultimate settlement. However, in the case of expansive soils, one corner of the building may be subjected to maximum heave due to excessive wetting while another corner may have no movement. Therefore, in the case of swelling soils, differential heaving can equal the total heave. No correlation between differential and total heave can be established. Effective stress Terzaghi developed the principle of effective stress in the early 1920's. It is believed that the principle can be applied to all soil behaviors. In gernal terms, the principle of the effective stress states that the behavior of an element of soil mass depends not on the total stress applied to the element but rather on the difference between this total stress and the stress present in the pore fluid. For saturated soil, the effective stress is defined as: = G-yL

where:

- effective stress - total normal stress M = stress in pore fluid or pore water pressure


43 Table 5-. Results of heave calculations

1

2

3

4

5

6

8

9

10 <5

UL at mean depth, ton/sq. ft.

ΔΡ at mean depth, ton/sq. ft.

P0 at mean depth, ton/sq. ft.

•o

1 ζ •j

«5

A f 5 6

Χ)

ei

e2

ο

.s

ο

o

C

^

Ο Ο)

ι

g ^

c Ό

e3 >

Β Ό

ra

CO

DU

4.006.56 6.568.87 8.8710.80 10.8013.35 13.3518.81 18.81-

Β S) Β

ο

e

13

ο
Depth of double Oedom. test, feet

Depths at top and bottom of layer, feet

Layer No.

7

12

11

VOID RATIO

ο

I-H

Β & ë ι ο s κ *

00

— Iπ

=• 1

r »°

ζ

1+

5.25

0.30

0.10

0.97

0.419

0.418

0.492

0.481

1.585

0.021

1.342

7.87

0.45

0.07

0.89

0.428

0.427

0.460

0.455

0.623

0.019

0.525

9.87

0.59

0.06

0.84

0.418

0.417

0.449

0.447

0.506

0.016

0.474

11.74

0.73

0.04

0.78

0.544

0.543

0.563

0.562

0.362

0.020

0.343

14.85

0.97

0.04

0.70

0.626

0.625

0.640

0.639

0.565

0.040

0.524

18.87

1.21

0.04

0.59

0.523

0.522

0.524

0.523

0.017

0.017

0

3.658

0.133

3.208

(After Jennings & Knight, 1958)

TOTALS:

T h e preceding e q u a t i o n is based on saturated soils. F o r partly saturated soils, c o n s i d e r a t i o n should b e given t o t h e fact t h a t p o r e pressure acts o n l y o n p a r t of t h e area of a n y plane t h r o u g h t h e soil. In a d d i t i o n , an electro-chemical force of a t t r a c t i o n and repulsion force m a y act t h r o u g h t h e spaces n o t filled w i t h w a t e r . C o n s e q u e n t l y , t h e effective e q u a t i o n should b e modified b y adding t h e v e c t o r sum of attractive forces and s u b t r a c t i n g t h e v e c t o r sum of repulsive forces. Aichison and Richards [ 3 1 ] s t a t e d , " I t is n o t o n l y possible b u t essential for t h e effective stress principle t o be used in t h e quantification of expansive soil b e h a v i o r . " L a m b e [ 3 3 ] stated in 1959 after m a k i n g in-depth research on t h e application of t h e c o n c e p t of effective stress t o explain t h e swelling soil behavior, " O n e w o n d e r s w h e t h e r , for e x a m p l e , we are hindering o u r u n d e r s t a n d i n g of the n a t u r e of t h e e x t r e m e l y plastic swelling soils b y forcing t h e m t o fit t h e effective stress c o n c e p t . " T h e a u t h o r shares t h e t h i n k i n g of L a m b e and believes t h a t t h e m e c h a n i c s of expansive soil are totally different from t h e t h e o r y of c o n s o l i d a t i o n and t h e shearing s t r e n g t h of soils. It should be considered as a n e w phase of soil m e c h a n i c s .

SWELLING PRESSURE In the course of t h e last 15 years, t h o u s a n d s of swell tests were c o n d u c t e d b y t h e a u t h o r o n various k i n d s of expansive soils found in t h e R o c k y M o u n t a i n area. T h e s t a n d a r d p r o c e d u r e used t o c o n d u c t these tests is t o place t h e u n d i s t u r b e d sample in a c o n s o l i d o m e t e r u n d e r a surcharge


44

load of 1,000 psf ( a b o u t 7 psi) for 2 4 h o u r s , s a t u r a t e t h e sample, m e a s u r e and record t h e a m o u n t of v o l u m e change. T h e a m o u n t of v o l u m e change exhibited b y various soils u n d e r various p l a c e m e n t c o n d i t i o n s varies greatly. F u r t h e r m o r e , it was found t h a t soil o b t a i n e d from b e n e a t h a s t r u c t u r e t h a t has u n d e r g o n e severe f o u n d a t i o n m o v e m e n t m a y n o t possess high swell p o t e n t i a l . It was suspected t h a t t h e r e m u s t b e a single soil p r o p e r t y t h a t governs t h e swelling characteristics. After t h e sample had swelled t o its m a x i m u m e x t e n t , the specimen was loaded until it r e t u r n e d to its initial v o l u m e and t h e pressure r e q u i r e d t o d o this was designated as swelling pressure. It is suspected t h a t t h e swelling pressure is t h e built-in p r o p e r t y of expansive soil and will n o t be affected b y p l a c e m e n t c o n d i t i o n or e n v i r o n m e n t a l c o n d i t i o n . Test

procedure T h e clay selected for m a k i n g this s t u d y is a c l a y s t o n e shale typical of t h o s e found in

s o u t h e a s t Denver. Such soil has caused a great deal of d a m a g e t o lightly loaded s t r u c t u r e s such as residential houses. Most of t h e h o u s e s in this area are f o u n d e d w i t h piers b o t t o m e d in a z o n e where a change of m o i s t u r e c o n t e n t is unlikely t o t a k e p l a c e ; however, slabs placed on such soil have experienced generally severe m o v e m e n t . Uplift m o v e m e n t in excess of 6 inches is n o t uncommon. T h e physical p r o p e r t i e s of such clay are as follows: Liquid limit

44.4%

Plasticity i n d e x

24.4%

Shrinkage limit

14.5%

Sand

0%

Silt

63.0%

Clay ( p e r c e n t smaller t h a n 0 . 0 0 5 m m )

37.0%

Optimum moisture content

19.5%

M a x i m u m d r y d e n s i t y (standard P r o c t o r test)

108.4 pcf

F r e e swell ( U S B R m e t h o d )

75.0%

Specific gravity

2.67

In a d d i t i o n t o t h e above, t h e mineral c o n t e n t , in p e r c e n t , of such clay is as follows: Montmorillonite

25.0

Calcite

5.0

Quartz

25.0

Feldspar Kaolinite

10 t o 2 5 . 0 5.0

Because of t h e erratic f o r m a t i o n of n a t u r a l soil strata, for research p u r p o s e s , it will be necessary t o use only r e m o l d e d samples so t h a t the p l a c e m e n t c o n d i t i o n can b e d u p l i c a t e d . T h e air d r y sample is prepared b y passing it t h r o u g h a N o . 4 0 sieve. Moisture c o n t e n t is added t o t h e air d r y clay and t h e n allowed t o age in a sealed c o n t a i n e r for a period of 4 8 h o u r s .


45

The sample is compacted in a 2-inch-diameter, 1-inch-thick consolidometer ring in three layers with predetermined compaction effort. The required density is obtained as near as possible by an experienced technician. It should be noted that in the test results presented, some deviation of density has taken place which results in some erratic test results. Surcharge pressure It is a well recognized fact that if sufficient load is applied on an expansive clay, the detrimental volume increase can be controlled. The surcharge pressure applied to the soil sample in the consolidometer simulates the dead load pressure exerted on the footings or pier foundation. Figure 17 and table 6 indicate that with a surcharge pressure of 1,000 psf, upon wetting the clay swelled 5.9 percent with a swelling pressure of 12,000 psf. By increasing the surcharge pressure to 5,000 psf, the amount of volume increase was limited to 1.6 percent, but the swelling pressure remains unchanged. Figure 18 indicates the relationship between volume change with surcharge pressure. These curves have a hyperbolic shape and the intersection of the curves with the abscissa indicates the pressure required for zero volume change. This pressure by definition is the swelling pressure.

Figure 17. Relationship between surcharge pressure and volume increase for constant density and moisture content samples.


46

Table 6—. Effect of varying pressure on volume change and swelling pressure for constant density and moisture content samples. Applied pressure (psf)

Moisture content, percent Initial

Initial density, pcf

Final

Volume increase, percent

Swelling pressure, psf

1,000

11.90

24.58

105.58

5.90

12,000

2,000

11.90

25.08

106.08

3.90

13,000

3,000

11.90

24.94

105.96

2.80

12,000

5,000

11.90

25.02

105.90

1.60

12,500

1.00

7,000

11.90

25.49

105.96

Average

11.90

25.02

105.95

SURCHARGE

12,500 12,400

PRESSURE ( p s f )

Figure 18. Effect of varying pressure on volume change for constant density and moisture content sample.

Surcharge

load

is essential

to

control foundation

m o v e m e n t . With swelling

pressure

d e t e r m i n e d , a reasonable f o u n d a t i o n design can be a p p r o a c h e d . If t h e swelling pressure is n o t excessive, say, on t h e o r d e r of 5 , 0 0 0 psf, a spread footing f o u n d a t i o n can be used. T h e r e q u i r e m e n t will b e t o assign a m i n i m u m dead load pressure of 5,000 psf so t h a t the v o l u m e change of t h e soil will n o t be allowed even in excessive w e t t i n g c o n d i t i o n . In t h e design of t h e


47

footing f o u n d a t i o n , it m a y be possible t o allow certain a m o u n t s of uplift m o v e m e n t so as t o minimize t h e required dead load pressure. Uplift m o v e m e n t can be tolerated in certain s t r u c t u r e s in t h e same m a n n e r as s o m e s e t t l e m e n t can be t o l e r a t e d in m o s t s t r u c t u r e s . A differential uplift of three-fourths of an inch generally is considered t o b e tolerable. With 3/4-inch allowable differential uplift, t h e required dead load pressure can b e drastically r e d u c e d . F o r a rational design, it is advisable t o actually d e t e r m i n e t h e swelling pressure. F r o m t h e pressure versus v o l u m e change curve and t h e tolerable uplift, a w o r k i n g swelling pressure can be established. F o r highly swelling soil w i t h swelling pressure in excess of 5,000 psf, a pier f o u n d a t i o n will be required t o c o n c e n t r a t e t h e dead load pressure o n a small area. It is n o t difficult t o assign a dead load pressure in excess of 2 0 , 0 0 0 psf in a small d i a m e t e r pier. However, a t t e n t i o n m u s t be given t o t h e additional swelling effect o n t h e shaft of t h e pier e m b e d d e d in swelling soil. T h e swelling pressure e x e r t e d o n t h e shaft of t h e pier can b e m a n y times greater t h a n t h e pressure e x e r t e d at t h e b o t t o m of the pier. Dead load pressure alone generally is n o t sufficient t o prevent t h e uplifting of t h e pier. A n c h o r a g e of t h e pier in a z o n e n o t affected b y m o i s t u r e change should b e used t o assist the dead load pressure r e q u i r e m e n t . Since t h e v o l u m e change for surcharge pressure of 1 psi and t h e v o l u m e change u n d e r a surcharge pressure of 10 psi will vary considerably, t h e m e r i t of using swelling pressure as a direct m e a s u r e m e n t of t h e swelling characteristics can be seen at o n c e . Degree of

saturation

This series of tests was p e r f o r m e d t o s t u d y t h e effect of d u r a t i o n of w e t t i n g o n swelling characteristics. It is a well established p h e n o m e n o n t h a t p r o l o n g e d w e t t i n g will result in m o r e damage t o a s t r u c t u r e t h a n s h o r t d u r a t i o n w e t t i n g . Since it is difficult in a short sample height t o c o n t r o l t h e d u r a t i o n of w e t t i n g , t o achieve t h e same effect in t h e l a b o r a t o r y t h e degree of s a t u r a t i o n on the sample was varied. T h e samples were c o m p a c t e d

in t h e c o n s o l i d o m e t e r w i t h u n i f o r m d e n s i t y and m o i s t u r e c o n t e n t , and a

m e a s u r e d a m o u n t of w a t e r was t h e n i n t r o d u c e d i n t o the sample. Sufficient t i m e was allowed for all a d d e d w a t e r t o soak i n t o t h e sample. T h e a m o u n t of v o l u m e change and t h e swelling pressure recorded are s h o w n o n table 7. F r o m figure 19 it is seen t h a t t h e a m o u n t of v o l u m e change Table 7 - . Effect of varying degree of saturation on volume change and swelling pressure for constant density and moisture content samples. Moisture content, percent

rage

Degree of saturation, percent

Initial

Final

Initial density, pcf

9.66

13.07

106.6

1.83

16,000

61.0

9.66

14.53

106.0

3.35

15,500

67.0 82.0



9.66

17.58

105.6

4.35

12,000

9.66

18.50

106.7

5.53

17,000

86.3

9.66

19.93

105.9

6.25

15,000

93.0

9.66

106.2

15,100

48


Figure 19. Effect of varying degree of saturation on volume change for constant density and moisture content sample.

49


increases in direct p r o p o r t i o n t o t h e degree of s a t u r a t i o n at t h e end of test. Figure 20 indicates t h a t t h e swelling pressure is c o n s t a n t , or the pressure required t o m a i n t a i n c o n s t a n t v o l u m e is i n d e p e n d e n t of t h e d u r a t i o n of w e t t i n g or the degree of s a t u r a t i o n . C o m p l e t e s a t u r a t i o n is n o t required t o result in a large v o l u m e change. This reflects directly in t h e m i s c o n c e p t i o n t h a t b y removing free w a t e r t h e swell can be controlled. It has been a c o m m o n practice t o install drain tiles a r o u n d the building in an a t t e m p t t o remove free w a t e r and to

stop

foundation

movement.

A

subdrain

will n o t

arrest

the

migration

of

moisture.

C o n s e q u e n t l y , swelling can be substantial. W i t h o u t s u p e r i m p o s e d load, t h e swelling of t h e soil c a n n o t be controlled even w i t h a m i n i m u m a m o u n t of m o i s t u r e change. Since the swelling pressure remains c o n s t a n t , a short d u r a t i o n w e t t i n g can cause equally heavy damage t o lightly loaded s t r u c t u r e s as long d u r a t i o n wetting. This is t h e reason w h y it is so difficult t o c o n t r o l slab m o v e m e n t for slab-on-ground construction. Initial moisture

content

Expansive soils will n o t be subject t o v o l u m e change unless t h e r e is an increase in m o i s t u r e c o n t e n t . A drier soil will swell m o r e t h a n a wet soil. In this series of tests, an a t t e m p t is m a d e t o

Figure 20. Relationship between degree of saturation and volume increase for constant density and moisture content samples.


50

d e t e r m i n e t h e effect of increasing t h e initial m o i s t u r e c o n t e n t o n t h e v o l u m e change as well as swelling pressure. Figure 21 s h o w s the results of a series of tests indicating the a m o u n t of v o l u m e change of. soil samples c o m p a c t e d

at c o n s t a n t density b u t varying m o i s t u r e c o n t e n t s . As

e x p e c t e d , t h e soils w i t h low initial m o i s t u r e c o n t e n t swell m o s t . T h u s , the slope of t h e e-log ρ curve decreases as t h e initial m o i s t u r e c o n t e n t increases. However, t h e swelling pressure required for zero v o l u m e change r e m a i n e d practically c o n s t a n t . Kassiff & Baker in 1971 [ 3 8 ] stated t h a t if clay is given e n o u g h time for aging, that for the same dry d e n s i t y , t h e swell pressure is n o t affected b y m o i s t u r e c o n t e n t . T h e result s h o w n in table 8 confirms this s t a t e m e n t . Table 8 indicates t h a t the swelling pressure for t h e various m o i s t u r e c o n t e n t s ranges from 7 , 0 0 0 t o 12,500 psf. D u e to variation in l a b o r a t o r y controlled c o n d i t i o n s , t h e initial d e n s i t y is n o t entirely c o n s t a n t , and s o m e variation in swelling pressure occurs. F o r all practical p u r p o s e s , however, t h e swelling pressure is a c o n s t a n t value. Figure 22 indicates t h e variation of m o i s t u r e c o n t e n t versus v o l u m e change. These tests indicate t h a t w i t h m o i s t u r e c o n t e n t slightly higher t h a n o p t i m u m m o i s t u r e c o n t e n t , t h e v o l u m e change should be negligible. T h e results of t h e above l a b o r a t o r y tests indicate t h a t the increase of m o i s t u r e c o n t e n t of an expansive soil is n o t a positive m e t h o d of controlling t h e e x p a n s i o n of the soil. Even w i t h high m o i s t u r e c o n t e n t , footings f o u n d e d o n swelling soil will experience the same swelling pressure

Figure 21. Relationship between initial moisture content and volume increase for constant density samples.


Ο» 0

51

1

1

5

10 Moisture

I 15

I 20

\ 25

Content (%)

Figure 22. Effect of varying moisture content on volume changes from constant density samples.

52


Table 8 - . Effect of varying moisture content on volume change and swelling pressure for constant density samples. Initial density, pcf


Final


Swelling pressure, psf 9,500

106.97

5.84

20.34

7.71

105.93

9.95

20.77

5.55

9,500

106.27

10.77

18.75

5.03

12,500 9,500

105.60

12.48

22.09

4.30

106.47

12.92

20.54

3.48

9,000

106.37

14.84

19.59

3.30

10,500

105.46

17.97

18.50

2.15

7,000

105.73

18.59

19.41

1.38

7,500

106.35

19.37

20.18

0.75

9,000

îrage 106.13

20.02

9,333

and the same a m o u n t of dead load pressure will be required to insure zero v o l u m e change. High moisture c o n t e n t soils will experience less uplift, b u t t h e pressure required t o maintain c o n s t a n t volume will n o t be altered. This also indicates t h a t t h e c o m m o n l y accepted p r o c e d u r e of prewetting t h e f o u n d a t i o n excavation t o eliminate t h e swelling characteristics is n o t a reliable p r o c e d u r e . Wetting of t h e f o u n d a t i o n soil, if it can be accomplished, can only serve t o decrease the a m o u n t of swelling. A f o u n d a t i o n placed on such soil will still require t h e same a m o u n t of dead load pressure. Soil engineers are often deceived b y the low volume change of a high m o i s t u r e c o n t e n t soil. If such soils possess a high swelling pressure, t h e y will cause severe damage t o structures if allowed t o dry and are s u b s e q u e n t l y w e t t e d . Stratum

thickness

L a b o r a t o r y research has been further e x t e n d e d t o explore t h e effect of s t r a t u m thickness on the a m o u n t of v o l u m e change and swelling pressure. In this series of tests, t h e sample thickness ranged from

1/2 t o 1-1/2 inches. Again, t h e samples were c o m p a c t e d t o uniform m o i s t u r e

c o n t e n t and density and sufficient t i m e was allowed for c o m p l e t e s a t u r a t i o n of t h e thickest sample. As could be p r e d i c t e d , from t h e results s h o w n in figure 2 4 , t h e m a g n i t u d e of t h e v o l u m e change is p r o p o r t i o n a l t o t h e sample thickness and the percentage of v o l u m e increase remains c o n s t a n t . T h e shape of t h e e-log ρ curve remains almost identical for various sample thicknesses (fig. 2 3 ) and t h e swelling pressure is c o n s t a n t (table 9 ) . This series of tests indicates t h a t if t h e weight of a s t r u c t u r e is capable of exerting pressure t o various d e p t h s b e n e a t h t h e footing w i t h equal i n t e n s i t y , t h e n t h e v o l u m e increase can be arrested. U n f o r t u n a t e l y , d e a d load pressure e x e r t e d on t h e footing can only c o n t r o l v o l u m e change of t h e near surface soils. A t lower d e p t h s , pressure exerted on t h e footing is distributed


53

over a larger area and is n o t effective in preventing v o l u m e change. D e e p seated swelling is controlled o n l y b y t h e weight of t h e o v e r b u r d e n soil and n o t b y dead load pressure e x e r t e d o n the foundation system.

Table 9—. Effect of varying sample thickness on volume change and swelling pressure for constant density and moisture content sample. Initial density, pcf


Final

Sample thickness, in.


Volume increase, in.

Swelling pressure, psf 11,000

105.20

10.10

22.30

0.504

5.66

0.0285

106.33

10.10

20.92

0.748

5.75

0.0430

11,500

105.31

10.10

21.14

1.007

5.15

0.0520

11,000

106.05

10.10

20.49

1.250

5.60

0.0700

15,000

106.05

10.10

20.58

1.500

5.60

0.0840

12,500

Avg. 105.78

10.10

21.08

5.54

12,200

Figure 23. Relationship between sample thickness and volume increase for constant density and moisture content samples.

54


Ο

0.50

0.75 Sample

1.00 Thickness

1.25

1.50

(Inches)

Figure 24. Effect of varying sample thickness on volume change for constant density and moisture content samples.


Initial

55

density Initial d e n s i t y , w h e t h e r u n d i s t u r b e d or r e m o l d e d , is t h e o n l y e l e m e n t t h a t affects t h e

swelling pressure. As seen in figure 25 and table 10 for c o n s t a n t m o i s t u r e samples, t h e v o l u m e change increases w i t h dry d e n s i t y , as does t h e swelling pressure. T h e family of curves have t h e same shape and are a p p r o x i m a t e l y parallel t o each o t h e r . A similar relationship was found b y Kassiff and Shalom [ 3 9 ] . Figure 2 6 establishes a straight line relationship b e t w e e n dry d e n s i t y and v o l u m e change. T h e relationship b e t w e e n t h e d r y density and swelling pressure can b e p l o t t e d either in semi-log scale o r in rectangular scale. F o r t h e semi-log scale, t h e curve is a straight line as s h o w n in figure 2 7 . T h e curve can be expressed as: log y = a x - b where:

y=

swelling pressure,

χ = d r y d e n s i t y , and a and b = c o n s t a n t s d e p e n d i n g o n soil p r o p e r t y and " a " is t h e slope of t h e curve. D r y density and swelling pressure relationship w h e n p l o t t e d in rectangular scale are s h o w n o n figure 2 8 . T h e curve can be expressed b y t h e following e x p o n e n t a l f o r m : y = kcx where:

b

k = 10" c=

10a

It is seen t h a t w h e n d r y d e n s i t y decreases, swelling pressure rapidly a p p r o a c h e s zero. W h e n d r y d e n s i t y increases, swelling pressure rapidly increases and a p p r o a c h e s infinity. T h e soil engineer is i n t e r e s t e d , only w i t h i n a n a r r o w range of d r y d e n s i t y , ranging from 100 t o 130 pcf. Table 1 0 - . Effect of varying density on volume change and swelling pressure for constant moisture content samples. Moisture content, percent Initial density, pcf

Initial

Final

Initial degree of saturation, percent



94.3

12.93

21.27

45.0

2.7

2,600

99.4

12.20

24.92

48.1

3.8

4,600 5,000

100.2

12.93

19.93

52.1

4.2

103.3

12.93

20.51

56.3

5.1

7,000

109.1

12.93

20.56

65.4

6.7

13,000

110.8

12.20

19.03

64.7

7.3

14,000

114.5

12.20

19.17

71.6

8.2

21,000

12.20

17.08

81.2

8.6

35,000

12.55

21.08

118.9 Average

56


Figure 25. Relationship between density and volume increase for constant initial moisture content samples.

Since the foregoing established that the swelling pressure of a given soil is a constant and varies only with the dry density, swelling pressure can be conveniently used as a yardstick to measure the swelling characteristics of the soils. For undisturbed soil, dry density is the in situ characteristic. Therefore, the swelling pressure at the in situ dry density can be used directly to describe the swelling characteristics. For remolded soil, the swelling pressure varies with degree of compaction. It is useful to introduce maximum Proctor density as a guide. In other words, the swelling pressure of remolded clay can be defined as the pressure required to keep the volume of a soil at its Proctor density constant. In this particular case, the standard Proctor density is 108.4 pcf. At this density the swelling pressure is 12,000 psf. It is intresting to note that at the site where the sample is taken in its undisturbed state, the dry density of the soil is 110 pcf corresponding to a swelling pressure of 14,000 psf. Conclusions 1. The swelling pressure of a clay is independent of the surcharge pressure, initial moisture content, degree of saturation, and the thickness of the stratum. 2. The swelling pressure increases with the increase of initial dry density.


90

57

95

100

105

110

115

Dry Density (pcf) Figure 26. Effect of varying density on volume change for constant moisture content samples.

120

58


40000

2000

100

105

Dry Density (pcf) Figure 27. Effect of varying density on swelling pressure for constant moisture content samples.


59

70,000

60,000

0

20

40

60 80 DRY DENSITY (pcf)

100

120

Figure 28. Effect of varying density on swelling pressure for constant moisture content samples.

140

60


3. F o r u n d i s t u r b e d soil, t h e swelling pressure can be defined as the pressure required t o k e e p t h e v o l u m e of a soil at its n a t u r a l dry density c o n s t a n t . 4. F o r r e m o l d e d soil the swelling pressure can be defined as t h e pressure required t o k e e p t h e volume of a soil at its m a x i m u m P r o c t o r density c o n s t a n t . 5. Swelling pressure can be used as a yardstick for measuring swelling soil. Swelling pressure reflects only t h e swelling characteristics of the soil and will n o t b e changed b y p l a c e m e n t c o n d i t i o n s or e n v i r o n m e n t a l c o n d i t i o n s .

REFERENCES [28] "Moisture Equilibria and Moisture Changes in Soils Beneath Covered Areas," Butterworth, Australia, 1965. [29] Croney, D. and Coleman, J. D., "Soil Moisture Suction Properties and their Bearing on the Moisture Distribution in Soils," Proceedings, Third International Conference on Soil Mechanics and Foundation Engineering, Zurich, Vol. I, 1953. [30] "A Review of Literature on Swelling Soils," Department of Highways, State of Colorado, University of Colorado and Bureau of Public Roads. [31] Aichison, G. D. and Richards, B. G., "The Fundamental Mechanics Involved in Heave and Soil Moisture Movement and the Engineering Properties of Soils which are Important in Such Movement," Second International Research-and Engineering Conference on Expansive Clay Soils, Texas A & M Press, 1969. [32] Jennings, J. E., "The Theory and Practice of Construction on Partially Saturated Soils as Applied to South Africa Conditions." "Engineering Effects of Moisture Changes in Soils," International Research and Engineering Conference on Expansive Clay Soils, Texas A & M Press, 1965. [33] Lambe, T. W. and Whitman, R. V., "The Role of Effective Stress in the Behavior of Expansive Soils," Quarterly of the Colorado School of Mines, Vol. 54, No. 4, October, 1959. [34] "Special Procedures for Testing Soil and Rock for Engineering Purposes," 5th Edition, 1970, ASTM, STP 479,1970. [35] Jennings, J. E. and Knight, K., "The Prediction of Total Heave from the Double Oedometer Test," 1957-58 Symposium on Expansive Clays, South African Institution of Civil Engineers, Johannesburg. [36] McDowell, C , "Interrelationship of Load, Volume Change and Layer Thickness of Soils to the Behavior of Engineering Structures," Proceeding, Highway Research Board, Vol. 35, 1956. [37] "Criteria for Selection and Design of Residential Slab-on-Ground," Building Research Advisory Board, Report No. 33 to the Federal Housing Administration, 1968. [38] Kassiff, G. and Baker, R., "Aging Effects on Swell Potential of Compacted Clay," Journal of the Soil Mechanics and Foundation Division, ASCE, Vol. 97, SM 3. Proc. March, 1971. [39] Kassiff, G. and Shalom, A. B., "Experimental Relationship Between Swell Pressure and Suction," Geotechnique, Vol. XXI, No. 3, September, 1971.

Chapter

3

FIELD AND LABORATORY INVESTIGATIONS

INTRODUCTION T h e stability of a s t r u c t u r e f o u n d e d o n expansive soil d e p e n d s u p o n t h e subsoil c o n d i t i o n s , g r o u n d surface features, t y p e of c o n s t r u c t i o n , and possibly even t h e meteorological variations. Subsoil c o n d i t i o n s can be e x p l o r e d b y drilling and sampling, seismic surveying, excavating test pits, and b y s t u d y i n g existing d a t a . G r o u n d surface features are controlled b y surface geology and physiography. A s t u d y of existing structures in t h e i m m e d i a t e vicinity of t h e site can be of prime i m p o r t a n c e in d e t e r m i n i n g t h e t y p e of c o n s t r u c t i o n in an expansive soil area. This is necessary in the investigation of soil for a building a d d i t i o n . Elaborate

site

investigation o f t e n t i m e s

c a n n o t be c o n d u c t e d

d u e t o limited

assigned

c o n s t r u c t i o n costs. F o r very favorable sites, e l a b o r a t e site investigation m a y n o t be w a r r a n t e d . However, if t h e area is suspected of having swelling soil p r o b l e m s , extensive soil investigation will b e necessary even for very m i n o r s t r u c t u r e s . Generally, it is t h e small building w i t h i n a d e q u a t e funding,

insufficient

planning,

and

low-bidding

contractor

who

unwisely

economizes

in

constructing the building t h a t presents t h e m o s t p r o b l e m s . T h e soil engineer should n o t accept j o b s in an expensive soil area w h i c h will n o t allow a t h o r o u g h subsoil investigation.

SITE I N V E S T I G A T I O N Before initiating t h e site investigation, t h e soil engineer should o b t a i n i n f o r m a t i o n regarding site t o p o g r a p h y , surficial geology, and existing s t r u c t u r e s . This can be accomplished b y reviewing available d a t a , s t u d y i n g t o p o g r a p h i c and geologic m a p s , and m a k i n g a reconnaissance survey. Topography T h e t o p o g r a p h i c c o n d i t i o n is an essential part of t h e site investigation. Generally, for larger projects a t o p o g r a p h i c survey is available. However, care m u s t be t a k e n t o insure t h a t t h e survey is correct. Many times, site grading has c o m p l e t e l y altered t h e survey. T h e elevation of each test hole should be r e c o r d e d . O n e i m p o r t a n t aspect is t h e selection of a b e n c h m a r k . Every effort should be m a d e t o tie in t h e elevations w i t h t h e a r c h i t e c t ' s reference p o i n t . Since t h e floor level governs t h e selection of f o u n d a t i o n t y p e , this aspect c a n n o t be overemphasized. Whenever possible, t h e b e n c h m a r k should be referenced t o establish d a t u m such as an existing building, m a n h o l e invert, cross on a sidewalk, t o p of fire h y d r a n t and so forth.

62


T h e l o c a t i o n of n a t u r a l and m a n m a d e drainage features is also of considerable i m p o r t a n c e . Erecting a s t r u c t u r e across a n a t u r a l gully always poses a future drainage p r o b l e m . T h e w a t e r level in a n y n e a r b y streams and rivers should be m e a s u r e d and r e c o r d e d . Irrigation d i t c h e s carry large a m o u n t s of w a t e r d u r i n g irrigation season. Water leaking from t h e d i t c h can s u p p l y m o i s t u r e t o t h e f o u n d a t i o n soil and cause swelling of footings and slabs. Pier uplift d u e t o t h e infiltration of w a t e r from an irrigation d i t c h is n o t u n c o m m o n . Water leaking from t h e d i t c h e s can also cause o t h e r p r o b l e m s such as b a s e m e n t d a m a g e . T h e l o c a t i o n and elevation of t h e d i t c h should be included as a p a r t of t h e field records. Streams and n e a r b y rivers naturally are of i m p o r t a n c e in t h e site investigation. O f particular interest, in avoiding flooding, is t h e e x t e n t of flood plains. Such preliminary i n f o r m a t i o n can usually be o b t a i n e d from t h e U. S. Geological Survey and t h e U. S. D e p a r t m e n t of Agriculture soil survey r e p o r t s . T h e steepness of valley slopes is of special c o n c e r n for sites t o be located in m o u n t a i n o u s areas. S o m e e n v i r o n m e n t a l agents classify valley slopes in excess of 30° as p o t e n t i a l hazard areas. Slope stability d e p e n d s u p o n t h e slope angle of t h e r o c k and soil f o r m a t i o n , evidence of past slope m o v e m e n t , and drainage features. T h e field engineer should be aware of the possible slope p r o b l e m s associated w i t h landslides, local slope failure, m u d flow o r o t h e r p r o b l e m s .

The

vegetative cover on t h e slope, shape of t h e trees, and t h e behavior of any neighboring s t r u c t u r e s should also b e observed.

Surficial

geology

General surficial geology of t h e area includes t h e s t u d y of slopes, t r i b u t a r y valleys, landslides,

springs

and

seeps,

sinkholes,

exposed

rock

sections, and

the

nature

of

the

unconsolidated overburden. A n inspection of u p l a n d and valley slopes m a y provide clues t o t h e thickness and sequence of f o r m a t i o n s and rock s t r u c t u r e . T h e shape and c h a r a c t e r of channels and t h e n a t u r e of t h e soil (residual, colluvial, alluvial, etc.) provide evidence of t h e past geologic activity. A n engineering geologist should identify and describe all geologic f o r m a t i o n s visible at t h e surface and n o t e their t o p o g r a p h i c positions. T h e local dip and strike of t h e formations should be d e t e r m i n e d and n o t e m a d e of any stratigraphie relationships or s t r u c t u r a l features t h a t m a y cause p r o b l e m s of seepage, excessive w a t e r loss, o r sliding of t h e e m b a n k m e n t . T o insure a d e q u a t e p l a n n e d d e v e l o p m e n t of a subdivision, Senate Bill 35 of C o l o r a d o requires the subdivider t o s u b m i t i t e m s such as: 1. R e p o r t s concerning streams, lakes, t o p o g r a p h y , geology, soils, and vegetation. 2. R e p o r t s concerning geologic characteristics of the area t h a t w o u l d significantly affect t h e land use and t h e d e t e r m i n a t i o n of t h e i m p a c t of such characteristics on t h e p r o p o s e d subdivision. 3 . Maps and tables concerning suitability of t y p e s of soil in t h e p r o p o s e d subdivision. Similar bills exist in m a n y o t h e r states in response t o e n v i r o n m e n t a l zeal of t h e n a t i o n . In m o u n t a i n areas, t h e m a p p i n g of surficial geologic features is highly desirable. J o h n s o n suggested t h a t t h e features t o be s h o w n o n t h e m a p should i n c l u d e : 1. T e x t u r e of surficial deposits,

[40]


63

2. S t r u c t u r e of b e d r o c k , including d i p and strike, faults or features, stratification, p o r o s i t y and p e r m e a b i l i t y , schistosity, and w e a t h e r e d zones, 3 . G r o u n d - w a t e r features, including seeps, springs, observable w a t e r table, and drainage, 4 . Area of m o d e r n d e p o s i t (result of accelerated e r o s i o n ) , 5. Unstable slopes, slips, and landslides, and 6. F a u l t s and fault z o n e s . Of special i m p o r t a n c e is t h a t t h e geologist should b e able t o recognize a p o t e n t i a l swelling soil p r o b l e m . F o r i n s t a n c e , t h e red siltstone f o r m a t i o n in Laramie, W y o m i n g , will n o t pose a swelling p r o b l e m while a few miles t o t h e east w h e r e t h e claystone of t h e Pierre F o r m a t i o n is e n c o u n t e r e d , swelling will be critical. Existing

structures

T h e behavior of existing s t r u c t u r e s has an i m p o r t a n t bearing o n t h e selection of t h e p r o p o s e d s t r u c t u r e . All possible i n f o r m a t i o n should be o b t a i n e d concerning s t r u c t u r e s in t h e i m m e d i a t e p r o x i m i t y . I n q u i r y should be m a d e as to t h e c o n d i t i o n of t h e s t r u c t u r e , age, and t y p e of

the

foundation.

If

adjacent

existing

s t r u c t u r e s have experienced

water problems,

the

possibility of a high-water table c o n d i t i o n in t h e area exists. If t h e existing s t r u c t u r e e x h i b i t s cracks, this usually indicates t h a t the existing f o u n d a t i o n system is n o t a d e q u a t e . A n experienced field engineer can readily identify t h e t y p e of distress w h i c h has occurred in t h e existing buildings and t h e n d e t e r m i n e if it is associated w i t h swelling soils. T h e age of t h e building should also be considered. If t h e cracks are old and n e w cracks have n o t a p p e a r e d in t h e p a t c h e d areas, t h e f o u n d a t i o n m o v e m e n t m a y have been stabilized. However, even if t h e existing s t r u c t u r e is in excellent c o n d i t i o n , this d o e s n o t m e a n t h a t t h e existing f o u n d a t i o n system should be d u p l i c a t e d in new c o n s t r u c t i o n . T h e existing s t r u c t u r e could have b e e n originally overdesigned. This is especially t r u e for old s t r u c t u r e s w h e r e massive f o u n d a t i o n s y s t e m s were usually used.

DRILLING AND SAMPLING Geophysical t e c h n i q u e s are generally used for preliminary subsoil investigation. B o t h t h e seismic

and

the

resistivity geophysical m e t h o d s have b e e n successfully

used.

Geophysical

e q u i p m e n t has b e e n c o m m o n l y used in d e t e r m i n i n g t h e surface of b e d r o c k , major changes in subsoil

conditions,

and

ground-water

depth.

However,

such

a survey

should

always

be

s u p p l e m e n t e d b y drilling. P r o b a b l y , t h e m o s t a c c u r a t e subsoil investigation m e t h o d is b y t h e opening of test pits. In a test pit, the field engineer can e x a m i n e in detail t h e subsoil strata, stratification, layers and lenses, as well as taking samples at the desired location. However, t h e d e p t h of t h e test pit is limited t o the reach of a b a c k h o e , generally 12 feet. Also w h e n t h e w a t e r table is high, test pit investigation b e c o m e s useless. In l o c a t i o n s w h e r e t h e subsoils consist essentially of large b o u l d e r s and cobbles, t h e use of test pit investigation is m o s t favorable. Auger drilling t h r o u g h b o u l d e r s and cobbles is difficult, if n o t impossible, and in a d d i t i o n t h e cost of r o t a r y drilling m a y n o t be w a r r a n t e d for small projects.

64


A n o t h e r possible investigation m e t h o d

is t o drill a large-diameter caisson hole to t h e

required d e p t h . By physically entering t h e caisson h o l e , t h e subsoil strata can be clearly e x a m i n e d , and u n d i s t u r b e d samples can b e o b t a i n e d at t h e desired d e p t h . T h e d r a w b a c k of b o t h t h e test pit m e t h o d and t h e caisson h o l e m e t h o d is t h a t standard penetration

tests c a n n o t be p e r f o r m e d .

Probably 95 percent

of all site investigations are

c o n d u c t e d b y drilling test holes, either b y auger drilling, r o t a r y drilling, percussion drilling, or other methods.

Test

holes T h e n u m b e r of test holes required for a project d e p e n d s u p o n t h e t y p e of f o u n d a t i o n

system, u n i f o r m i t y of subsoil c o n d i t i o n s , and t o s o m e e x t e n t t h e i m p o r t a n c e of t h e s t r u c t u r e . If preliminary investigation indicates t h a t shallow spread footings will be t h e m o s t likely t y p e of f o u n d a t i o n , t h e n it will be desirable t o drill shallow test holes close t o g e t h e r t o b e t t e r evaluate t h e subsoil c o n d i t i o n s within t h e loaded d e p t h of the footings. If, however, t h e shallow f o u n d a t i o n system is n o t feasible and a d e e p f o u n d a t i o n system is likely t o be required, t h e n t h e n u m b e r of test holes can b e decreased and t h e spacing increased. As a rule of t h u m b , test holes should be spaced at a distance of 5 0 t o 100 feet. In n o case should the test holes b e spaced m o r e t h a n 100 feet apart w h e n in an expansive soil area. T h e client s o m e t i m e s has the m i s c o n c e p t i o n t h a t drilling of test holes is t h e major cost of the subsoil investigation, and c o n s e q u e n t l y , t h e r e is a t e n d e n c y to drill as few holes as possible, preferably only o n e . T h e risk involved in such an u n d e r t a k i n g is e n o r m o u s . Erratic subsoil c o n d i t i o n s can exist b e t w e e n widely spaced test h o l e s ; t h e r e f o r e , logs of these test holes c a n n o t b e considered as representative of t h e overall site subsoil c o n d i t i o n . Closely spaced test holes are especially i m p o r t a n t w h e r e t h e presence of expansive soils is suspected. F o r instance, if s a n d s t o n e and siltstone b e d r o c k are e n c o u n t e r e d at a shallow d e p t h , a logical r e c o m m e n d a t i o n is t o found t h e s t r u c t u r e directly on t h e b e d r o c k w i t h spread footings designed for high pressure. However, if m o r e test holes had b e e n drilled, it is q u i t e possible t h a t claystone b e d r o c k containing high swelling p o t e n t i a l might have been revealed. T h e presence of swelling soil, even in only o n e test hole, can change t h e f o u n d a t i o n r e c o m m e n d a t i o n s completely. T h e d e p t h of test holes required is generally governed b y t h e t y p e of f o u n d a t i o n . Before drilling, t h e t y p e of f o u n d a t i o n c a n n o t be c o m p l e t e l y envisioned. Therefore, t h e first hole t h e field engineer drills should b e a d e e p h o l e , d e e p e n o u g h t o provide i n f o r m a t i o n p e r t i n e n t to b o t h shallow and d e e p f o u n d a t i o n systems. Samples in t h e h o l e should be t a k e n at frequent vertical intervals, preferably n o t m o r e t h a n 5 feet apart. After t h e c o m p l e t i o n of t h e d e e p test h o l e , t h e field engineer should have a fairly good idea of t h e possible f o u n d a t i o n system. C o n s e q u e n t l y , for t h e s u b s e q u e n t holes, m o r e a t t e n t i o n should b e directed to t h e u p p e r soil if a shallow f o u n d a t i o n is likely. If, however, a d e e p f o u n d a t i o n system is c o n t e m p l a t e d , drilling t o b e d r o c k would be necessary for all holes. O f t e n t i m e s , t h e d e p t h t o b e d r o c k is a criterion as t o t h e d e p t h of test holes. Where b e d r o c k is w i t h i n e c o n o m i c a l reach, say, within 4 0 feet, it is advisable t o drill a few holes i n t o t h e b e d r o c k . In C o l o r a d o , t h e d e p t h t o t h e t o p of shale b e d r o c k is e x t r e m e l y erratic, and t h e d e p t h

65


t o b e d r o c k can increase as m u c h as 30 feet w i t h i n a short distance of 100 feet. This will have t o b e t a k e n i n t o consideration w h e n d e t e r m i n i n g t h e l o c a t i o n and d e p t h of test holes. In s o m e cases, d e e p holes are required, n o t from t h e subsoil r e q u i r e m e n t b u t for the d e t e r m i n a t i o n of t h e w a t e r table elevation. F o r d e e p b a s e m e n t c o n s t r u c t i o n , t h e d e p t h of test holes should be at least 2 0 feet t o preclude t h e possibility of ground w a t e r b e c o m i n g a p r o b l e m in t h e lower floor. Penetration

test

In performing t h e standard p e n e t r a t i o n test, a soil sampler k n o w n as a split s p o o n is used. It is an open-ended steel cylinder w h i c h is split longitudinally i n t o halves. These t w o halves are held t o g e t h e r b y a c u t t i n g shoe at t h e lower end and a coupling w h i c h c o n n e c t s t h e sampler t o the drill rod. T h e split s p o o n is driven 18 inches i n t o t h e g r o u n d b y m e a n s of a 140-pound h a m m e r falling a free height of 3 0 inches. T h e n u m b e r of h a m m e r blows required t o drive 12 inches is called t h e standard p e n e t r a t i o n resistance Ν w h i c h represents t h e n u m b e r of b l o w s p e r foot, or t h e blow c o u n t . T h e s t a n d a r d p e n e t r a t i o n test has been and will c o n t i n u e t o be an i m p o r t a n t and practical field test. T h e following should be considered in using t h e p e n e t r a t i o n test: 1. T h e value of Ν in cohesionless soils is influenced b y t h e d e p t h at w h i c h t h e test is m a d e . A t a great d e p t h , t h e same soil w i t h t h e same relative d e n s i t y w o u l d give a high p e n e t r a t i o n resistance. T h e influence of o v e r b u r d e n pressure can be a p p r o x i m a t e d b y t h e following e q u a t i o n : Ν = N' \P+10/ where:

Ν = Adjusted value of s t a n d a r d p e n e t r a t i o n resistance, N ' = S t a n d a r d p e n e t r a t i o n resistance as actually r e c o r d e d , and ρ = Effective o v e r b u r d e n pressure, psi

T h e above e q u a t i o n was derived b y t h e U. S. Bureau of R e c l a m a t i o n t h r o u g h actual e x p e r i m e n t and has b e e n extensively used all over t h e w o r l d . T h e a u t h o r found t h a t while t h e increase in p e n e t r a t i o n resistance at greater d e p t h c a n n o t be ignored, t h e calculated values are r a t h e r high. Using t h e Bureau of

Reclamation

e q u a t i o n and a value of N ' = 12, at 20 feet b e l o w t h e g r o u n d surface w i t h Ύ = 110 pcf, the adjusted

penetration

resistance is almost d o u b l e d . This results in t h e designing

engineer using a p e n e t r a t i o n resistance value larger t h a n required for a safe design. 2. P e n e t r a t i o n resistance is reliable o n l y if t h e driving c o n d i t i o n is n o t abused. T h e h a m m e r should b e entirely free falling w i t h o u t being subject t o u n d u e friction. T h e standard p e n e t r a t i o n barrel should n o t be p a c k e d b y overdriving since this forces t h e soil against t h e sides of t h e barrel and causes incorrect readings. A n increase in b l o w c o u n t b y as m u c h as 50 p e r c e n t can s o m e t i m e s be caused b y a p a c k e d barrel. Driving a s t a n d a r d barrel i n t o gravelly soils p r e s e n t s a p r o b l e m . T h e barrel will b o u n c e w h e n driving on cobbles,

66


and h e n c e , n o useful value can be o b t a i n e d . S o m e t i m e s , a small piece of gravel will j a m in t h e barrel t h e r e b y preventing t h e e n t r a n c e of soil i n t o t h e barrel, t h u s substantially increasing t h e b l o w c o u n t . 3 . Considerable

economy

can

be

achieved

by

c o m b i n i n g p e n e t r a t i o n resistance

with

sampling. A slight m o d i f i c a t i o n in t h e design of t h e barrel will allow t h e insertion of thin wall lining in t h e barrel and provide a b l o w c o u n t as well as an u n d i s t u r b e d sample. T h e modified barrel is c o m m o n l y referred t o as a California sampler. Field tests have been c o n d u c t e d c o m p a r i n g t h e results of t h e p e n e t r a t i o n resistance of t h e California sampler w i t h t h a t of t h e s t a n d a r d

p e n e t r a t i o n tests. T h e tests indicate t h a t t h e results are

c o m m e n s u r a b l e w i t h t h e e x c e p t i o n of very soft soil

(N < 4 ) and very stiff or dense soil

(N > 3 0 ) . By c o m b i n i n g p e n e t r a t i o n resistance test w i t h t h e sampling device, m o r e tests can be m a d e and u n d i s t u r b e d samples can be o b t a i n e d w i t h o u t resorting t o the use of Shelby tubes.

Sampling S o m e c o n t r a c t s call for a p e n e t r a t i o n test for every 5 feet and sampling for t h e same interval o r every change of soil s t r a t u m . This m a y prove unnecessary. T h e field engineer should use his j u d g m e n t t o guide t h e frequency of sampling and avoid unnecessary sampling so t h a t t h e cost of investigation can be held t o a m i n i m u m . Samples in t h e u p p e r 10 or 15 feet are i m p o r t a n t , as this is generally t h e site of shallow f o u n d a t i o n s ; also, soil characteristics at this level govern t h e slab-on-ground c o n s t r u c t i o n a n d earth-retaining s t r u c t u r e s . Sampling and p e n e t r a t i o n tests at l o w e r d e p t h , say, in b e d r o c k , b e c o m e critical w h e n a d e e p f o u n d a t i o n system is r e q u i r e d . Sampling above t h e p r o p o s e d floor level should be limited since this material will be e x c a v a t e d . S o m e t i m e s , for d e e p b a s e m e n t c o n s t r u c t i o n , n o sampling or p e n e t r a t i o n tests will be necessary for t h e first 15 feet b e l o w g r o u n d level. Sampling and testing at an i n t e r m e d i a t e d e p t h generally is n o t t o o critical, e x c e p t w h e n friction piers are u n d e r c o n s i d e r a t i o n . Instead of assigning t h e field engineer definite instructions as t o the frequency of sampling, it w o u l d be b e t t e r to leave the m a t t e r u p to his discretion. A reasonably good sample can be o b t a i n e d w h e n driving i n t o shale b e d r o c k . Auger drilling in m o s t cases can be successfully c o n d u c t e d in shale b e d r o c k . F o r o t h e r t y p e s of b e d r o c k , such as l i m e s t o n e and granite, r o t a r y drilling will be necessary and rock cores o b t a i n e d . Core samples are b r o u g h t u p b y t h e drill and can be visually e x a m i n e d . T h e general characteristics, particularly the p e r c e n t a g e of recovery, are of i m p o r t a n c e t o f o u n d a t i o n design and c o n s t r u c t i o n .

LABORATORY TESTING Soil testing is essential in establishing t h e design criteria. Distinction m u s t be m a d e b e t w e e n t h e needs of t h e practicing engineer and t h o s e of the research engineer. F o r a practicing engineer, t h e p u r p o s e of l a b o r a t o r y testing is m a i n l y to confirm his preconceived c o n c e p t derived from field drilling, p e n e t r a t i o n tests, visual e x a m i n a t i o n of t h e samples, and personal experience in the


67

area. E x o t i c l a b o r a t o r y e q u i p m e n t and refined analysis are in t h e realm of t h e research engineer. N e i t h e r t i m e n o r b u d g e t will allow t h e practicing engineer t o follow t h e researcher's p r o c e d u r e . Swell

test T h e m o s t i m p o r t a n t l a b o r a t o r y test on expansive soils is t h e swell test. T h e s t a n d a r d

one-dimensional c o n s o l i d a t i o n test a p p a r t u s is similar t o t h a t used in m o s t soil laboratories for consolidation studies. This a p p a r a t u s , k n o w n as a c o n s o l i d o m e t e r , is s h o w n on figure 29 and is o f t h e fixed-ring t y p e [ 4 1 ] . It can a c c o m m o d a t e a r e m o l d e d o r u n d i s t u r b e d sample from 2 t o 4 . 2 5 inches in d i a m e t e r and from 0.75 t o 1.25 inches in thickness. P o r o u s stones are provided at each end of t h e specimen for drainage or s a t u r a t i o n . T h e sample u n i t is placed on a platform scale table and t h e load is applied b y a y o k e a c t u a t e d b y a screw jack. T h e load imposed o n t h e specimen is m e a s u r e d b y t h e scale b e a m and a dial gage is provided t o m e a s u r e t h e vertical movement. T h e advantage of such a p p a r a t u s is t h a t it is possible t o hold t h e u p p e r loading b a r at a c o n s t a n t v o l u m e and allow t h e m e a s u r e m e n t of t h e m a x i m u m uplift pressure of t h e soil w i t h o u t

Figure 29. Platform scale type consolidometer.

68


volume change. This requires a c o n s t a n t load a d j u s t m e n t b y an o p e r a t o r . A m o r e advanced scheme is t o use t h e c o n s o l i d o m e t e r w i t h a triaxial frame along w i t h t h e a t t a c h m e n t s and electrical circuits [ 4 2 ] . Such a device will allow t h e a u t o m a t i c load i n c r e m e n t and m e a s u r e swelling pressure w i t h o u t allowing v o l u m e change to take place. T h e c o n s o l i d o m e t e r can also b e used t o m e a s u r e t h e a m o u n t of e x p a n s i o n u n d e r various loading c o n d i t i o n s . Since swelling pressure can b e evaluated b y loading t h e swelled sample t o its original

volume

as explained

in

chapter

2, it is simple

to

convert

the

platform

scale

c o n s o l i d o m e t e r i n t o a lever-type c o n s o l i d o m e t e r as s h o w n o n figure 3 0 . A simple c o n s o l i d o m e t e r can

be

m a d e locally w i t h low cost. T h e average soil l a b o r a t o r y should have a train

of

consolidometers t o speed u p testing p r o c e d u r e s .

Interpretation

of test

Laboratory

results

testing of disturbed

and u n d i s t u r b e d soils can b e p e r f o r m e d in m o s t soil

laboratories b y an efficient l a b o r a t o r y technician. L a b o r a t o r y test results are reliable only t o the extent of t h e c o n d i t i o n of t h e sample. Results of testing o n badly d i s t u r b e d samples or samples n o t representative of t h e strata n o t only are useless b u t add confusion t o t h e c o m p l e t e analysis. Equally i m p o r t a n t t o testing of representative samples is t h e frequency of testing. Testing of a few samples on a single project and basing t h e final analysis o n such testing is n o t only undesirable b u t s o m e t i m e s d a n g e r o u s , especially w h e n evaluating swelling soils. T h e p o t e n t i a l of swelling generally c a n n o t be d e t e r m i n e d visually, and l a b o r a t o r y swell tests m u s t be c o n d u c t e d . By testing only a few samples, t h e high swelling sample m a y have been missed and e r r o n e o u s conclusions d r a w n . T o o little testing is worse t h a n n o testing at all.

Figure 30. Simplified lever-type consolidometer.


69

I n t e r p r e t a t i o n of test results should be c o n d u c t e d b y an experienced engineer, having t h e ability t o screen t h e test results and e x c l u d e t h e d u b i o u s ones, t o d e t e r m i n e w h e n t h e m i n i m u m o r m a x i m u m values are t o be used, and w h e n average values should be used.

REFERENCES [40] Johnson, A. I. "Suggested Method for Geologic Reconnaissance of Construction Site" Special procedures for testing soil & rock for engineering purposes. ASTM stp 479, 1970. [41 ] Lambe, T. W., "Soil Testing for Engineers," John Wiley & Son, Inc. [42] Agarwal, K.P. and Sharma, S. C , "A Method for Measuring Swelling Pressure of an Expansive Soil," Proceedings of the Third International Conference on Expansive Soils, Haifa, Israel.

Chapter 4

DRILLED PIER FOUNDATIONS

INTRODUCTION Drilled pier, o r drilled caisson, or caisson f o u n d a t i o n is widely used in t h e R o c k y M o u n t a i n area. Drilled piers, w h e n m a d e w i t h an enlarged base, are c o m m o n l y referred t o as belled piers and w h e n m a d e w i t h o u t an enlarged base are referred t o as straight-shaft piers. T h e drilled pier f o u n d a t i o n is used t o transfer t h e s t r u c t u r a l load from t h e u p p e r u n s t a b l e soil t o t h e lower stable soil. T h e use of drilled pier f o u n d a t i o n covers a wide range of possibilities; some of w h i c h follow: 1. Piers drilled i n t o hard b e d r o c k for s u p p o r t i n g high c o l u m n load. 2. F r i c t i o n piers b o t t o m e d on stiff clays for s u p p o r t i n g light s t r u c t u r e s , 3 . Belled piers b o t t o m e d o n sand and gravel for s u p p o r t i n g m e d i u m c o l u m n load, and 4 . Long, small-diameter piers drilled i n t o a z o n e unaffected b y m o i s t u r e change in swelling soil areas. T h e drilled pier f o u n d a t i o n is a rational s o l u t i o n t o c o m b a t t h e p r o b l e m of expansive soils; however, t h e design and c o n s t r u c t i o n m u s t b e closely controlled. As stated b y Richard W o o d w a r d [ 4 3 1 , " I f investigation and design were always p e r f e c t ; supervision always c o m p e t e n t and a d e q u a t e , inspection always c o n t i n u o u s , e x p e r i e n c e d and alert; and t h e c o n t r a c t o r ' s p e r s o n n e l always e x p e r t and conscientious—if all t h e s e c o n d i t i o n s prevailed, all t h e time on a project, t h e n t h e r e w o u l d be n o defective p i e r s . "

PIER CAPACITY Piers bearing o n b e d r o c k or shale have b e e n designed using m o s t l y empirical c o n s i d e r a t i o n s derived from e x p e r i e n c e , limited load test d a t a , and t h e behavior of t h e existing s t r u c t u r e s . F o r instance, because of its erratic characteristics, t h e u l t i m a t e load carrying capacity of t h e D e n v e r Blue Shale has never been d e t e r m i n e d . N o n e t h e l e s s , piers s u p p o r t i n g a c o l u m n load in excess o f 1,000 kips are c o m m o n l y designed and c o n s t r u c t e d in t h e R o c k y M o u n t a i n area w i t h satisfactory performance. W o o d w a r d , G a r d e n e r and G r e e n [ 4 3 ] stated o n empirical design of piers, " M a n y piers, particularly w h e r e r o c k bearing is used, have b e e n designed using strictly empirical c o n s i d e r a t i o n s which are derived conditions

are

well

from

regional e x p e r i e n c e . " T h e y further stated t h a t , "Where

established

c o n s t r u c t i o n s well d o c u m e n t e d satisfactory."

and are relatively u n i f o r m ,

subsurface

and t h e p e r f o r m a n c e

of

past

t h e design b y e x p e r i e n c e a p p r o a c h is usually found t o b e

72


T h e bearing capacity of drilled piers b o t t o m e d

o n b e d r o c k is a c o m b i n a t i o n of the

end-bearing capacity and t h e skin friction developed b e t w e e n the pier wall and b e d r o c k . Bearing

capacity

T h e bearing capacity of b e d r o c k for piles or piers can be found in m o s t building codes. T h e values given are generally conservative and t h e t y p e of b e d r o c k usually n o t clearly defined. T h e m e t h o d s for d e t e r m i n i n g t h e bearing capacity are as follows: 1. P e n e t r a t i o n resistance of b e d r o c k , 2. U n c o n f i n e d compressive s t r e n g t h or triaxial shear s t r e n g t h of u n d i s t u r b e d b e d r o c k core sample, 3 . Consolidation tests o n b e d r o c k core sample u n d e r high load, 4 . A c t u a l record of s e t t l e m e n t of existing building f o u n d e d o n similar b e d r o c k , 5. Load test o n b e d r o c k , and 6. Menard p r e s s u r e m e t e r test on b e d r o c k in t h e test holes. U n f o r t u n a t e l y , all of t h e above a p p r o a c h e s have their limitations. P e n e t r a t i o n resistance o n hard b e d r o c k involves blow c o u n t s in excess of 100. When a soft seam or a very hard lens is e n c o u n t e r e d , the actual p e n e t r a t i o n resistance of t h e s t r a t u m c a n n o t b e accurately d e t e r m i n e d . Therefore, t o o b t a i n and accurate d e t e r m i n a t i o n of p e n e t r a t i o n resistance, m a n y tests, possibly m o r e t h a n 5 0 , should be c o n d u c t e d . After t h e representative p e n e t r a t i o n resistance value has been d e t e r m i n e d , t h e soil engineer should convert t h e blow c o u n t t o allowable bearing capacity. One widely used m e t h o d is: Ν %= where:

2

q a = allowable bearing capacity in ksf, and Ν = blow count

F o r instance, if t h e representative blow c o u n t is 4 0 , t h e allowable bearing capacity selected will be 2 0 , 0 0 0 psf. This a p p r o a c h is q u i t e conservative. Extensive l a b o r a t o r y testing indicates t h a t the m o r e realistic value for q a should be in t h e range of Ν t o 0 . 7 5 N . T h a t is, w i t h Ν = 4 0 , the allowable bearing capacity should be m o r e nearly 3 0 , 0 0 0 t o 4 0 , 0 0 0 psf. Allowable bearing capacity derived b y p e n e t r a t i o n resistance d a t a should be checked b y b o t h the u n c o n f i n e d compressive s t r e n g t h test and the consolidation test. Unconfined

compressive

strength test at best can only represent t h e lower limit of t h e actual bearing capacity of b e d r o c k . Unconfined compressive s t r e n g t h p e r f o r m e d o n drive samples can only be used for comparative purposes. Samples o b t a i n e d from large d i a m e t e r cores o b t a i n e d from core drilling are far m o r e reliable, b u t t h e presence of slickensides and o t h e r effects of d i s t u r b a n c e limit the validity of such tests. High capacity c o n s o l i d a t i o n test o n a reasonably good sample can s o m e t i m e s be used t o d e t e r m i n e t h e a m o u n t of pier s e t t l e m e n t . T h e actual s e t t l e m e n t value t a k e n as a percentage of l a b o r a t o r y consolidation value should b e left to t h e j u d g m e n t of t h e soil engineer. Probably,

the

most

reliable

method

of

estimating

bedrock

bearing

capacity

is

the

observation of t h e behavior of existing structures. U n f o r t u n a t e l y , such records are scarce. In the

73


Denver area, piers bearing of Denver Blue Shale F o r m a t i o n designed for a bearing capacity of 6 0 , 0 0 0 psf have p e r f o r m e d satisfactorily w i t h long-term s e t t l e m e n t of less t h a n 1 inch. Load tests o n piers are costly and t i m e c o n s u m i n g , especially for large-diameter piers. Limited d a t a on t h e testing of small-diameter piers indicates t h a t the c o n v e n t i o n a l a p p r o a c h e s are e x t r e m e l y conservative. T h e m o s t direct and r e c o m m e n d e d m e t h o d in d e t e r m i n i n g bearing capacity is b y t h e insertion of a Menard p r e s s u r e m e t e r i n t o t h e b o t t o m of t h e test hole and evaluating t h e actual bearing capacity. A n e x p e r i e n c e d o p e r a t o r will be required t o c o n d u c t t h e tests.

Skin friction

of shale

T h e load-carrying capacity of a pier d e p e n d s n o t only u p o n its end bearing value b u t t o a great e x t e n t o n t h e skin friction or side shear b e t w e e n c o n c r e t e and its s u r r o u n d i n g soils. T h e skin friction value is u n f o r t u n a t e l y difficult

to d e t e r m i n e as discussed later u n d e r " F r i c t i o n

Piers." Soil engineers in t h e R o c k y M o u n t a i n area chose t h e e s t a b l i s h m e n t of skin friction value b y t h e following t w o a s s u m p t i o n s : 1.

Assuming t h a t skin friction value in b e d r o c k is o n e - t e n t h of the end bearing value of b e d r o c k , t h e n for an end bearing value of 6 0 , 0 0 0 psf, t h e skin friction value is 6 , 0 0 0 psf.

This value obviously has exceeded

t h e cohesion of c l a y s t o n e and t h e t o t a l

h o r i z o n t a l pressure against the shaft surface. It is generally t h o u g h t t h a t t h e skin friction value m a y n o t d e v e l o p fully along t h e surface of the shaft b u t t h a t a lubricated surface m a y exist b e t w e e n t h e soil and shaft. C o n s e q u e n t l y , a t t e m p t s have b e e n m a d e t o provide a shear ring in t h e p o r t i o n of the pier in b e d r o c k . Shear rings are m a d e b y t h e use of grooving tools w h i c h cut slots a r o u n d t h e circumference of t h e h o l e . T h e circular slots are a b o u t 1-inch d e e p and 3/4-inch thick at intervals of 8 inches. It is believed t h a t b y grooving t h e hole, a d e q u a t e d e v e l o p m e n t of skin friction can be assured. E x p e r i e n c e shows t h a t such an installation is seldom justified. The walls of a pier hole are seldom s m o o t h , w i t h e x c e p t i o n of t h o s e drilled i n t o oily shale. If it is f o u n d t h a t a pier hole is t o o s m o o t h , a t o o t h p r o t r u d i n g a b o u t one-half inch can be inserted o n t o an auger and w i t h several t u r n s , t h e surface of the hole can t h u s be r o u g h e n e d artificially. Load tests o n small d i a m e t e r piers ( 1 2 inches) drilled i n t o b e d r o c k w i t h n o e n d bearing (pier p o u r e d on elastic material) indicate t h a t t h e skin friction value exceeds t h e design value b y several times. It is believed t h a t for small d i a m e t e r piers, skin friction has actually t a k e n all t h e load e x e r t e d o n t h e pier w i t h o u t transferring t h e load t o t h e pier b o t t o m . 2.

In designing t h e pier load, t h e skin friction developed in t h e p o r t i o n of pier e m b e d d e d in the o v e r b u r d e n soil is usually ignored. Such values are considered t o be a d d e d factors of safety in t h e pier system. Actually, for long piers, the skin friction value developed in the o v e r b u r d e n soils can be considerable. Assuming t h e drilling of a 42-inch-diameter pier t h r o u g h 2 0 feet of dense sand and gravel i n t o t h e b e d r o c k , t h e unit skin friction value b e t w e e n t h e c o n c r e t e pier and t h e granular soil is at least 4 0 0

74


psf. Total skin friction value can exceed 9 0 kips. This added factor of safety t o t h e drilled pier system is r e c o m m e n d e d . F o r major s t r u c t u r e s , it is i m p o r t a n t t h a t an a d e q u a t e factor of safety be i n c o r p o r a t e d

t o allow for unforeseen

construction

contingencies. Design

capacity

After end bearing and skin friction values have b e e n established, it is n o t difficult t o calculate t h e total load-bearing capacity of a pier. Experience w i t h shale b e d r o c k in the R o c k y M o u n t a i n area generally indicates t h a t t h e hardness of b e d r o c k increases w i t h d e p t h . This has been

verified

by

penetration

resistance

and

by

pressuremeter

tests. Therefore,

p e n e t r a t i n g d e e p i n t o b e d r o c k , a higher load-carrying capacity can be assigned.

for

piers

Experience

indicates t h a t b o t h t h e end-bearing capacity and t h e skin friction value increases w i t h d e p t h at the

rate

of

3 percent

per

foot.

Using this a s s u m p t i o n , t h e load-carrying capacity of a

straight-shaft pier can be expressed as follows:

Q = A ( E + 0.03 E L ) + 0.1 (E + 0.03 E L ) C L in w h i c h :

Q = t o t a l pier carrying capacity (kips) A = end area of pier (sq. ft.) Ε = end bearing capacity of pier (psf) L = d e p t h of p e n e t r a t i o n i n t o b e d r o c k (ft.) C = p e r i m e t e r of pier (ft.)

Figure 31 was p r e p a r e d using t h e above e q u a t i o n , and it was assumed t h a t t h e end bearing value of t o p of b e d r o c k is 5 0 , 0 0 0 psf. It should be p o i n t e d o u t t h a t t h e above e q u a t i o n has its limitations as n o t e d :

1.

T h e bearing capacity of the t o p of b e d r o c k should be carefully evaluated. Oftentimes, t h e t o p of shale is highly w e a t h e r e d and only a low value can be assigned. S o m e t i m e s , d e e p excavation will e x p o s e t h e t o p of shale and t h e material will be subject t o severe disintegration. T h e r e f o r e , it is usually safe t o assign a bearing capacity value for t h a t p o r t i o n of pier 2 t o 4 feet below t h e surface of b e d r o c k , and t h e n increases t h e bearing value for additional p e n e t r a t i o n .

2.

The

increase

of bearing capacity w i t h d e p t h of p e n e t r a t i o n has its limitations.

According t o t h e formula, t h e bearing capacity of the pier will be d o u b l e d w h e n the d e p t h of p e n e t r a t i o n exceeds 3 0 feet. Obviously, a load-carrying capacity of such m a g n i t u d e c a n n o t be assigned t o the pier. Extensive experience in t h e R o c k y M o u n t a i n area indicates t h a t t h e a b s o l u t e m a x i m u m of end bearing value for piers in shale is 1 0 0 , 0 0 0 psf or a b o u t 7 0 0 psi.

75


2,200

<

1,800

1,000

8

10 PENETRATION

12 INTO

14 BEDROCK · FEET

Figure 31. Design load versus depth into bedrock.

Using t h e above a p p r o a c h in designing drilled piers, t h e load-carrying capacity of a single 60-inch-diameter pier can easily reach 3 , 0 0 0 kips w i t h reasonable p e n e t r a t i o n . Such load is of sufficient m a g n i t u d e t o a c c o m m o d a t e m o s t c o l u m n loads of a high-rise s t r u c t u r e .

MECHANICS OF PIER UPLIFT As stated earlier in t h e c h a p t e r , t h e principle of t h e use of drilled piers is t o provide a relatively inexpensive w a y of transferring t h e s t r u c t u r a l loads d o w n t o stable material or t o a stable z o n e w h e r e m o i s t u r e changes are i m p r o b a b l e . There should be n o direct c o n t a c t b e t w e e n the soil and t h e s t r u c t u r e w i t h the e x c e p t i o n of t h e soils s u p p o r t i n g t h e piers. Figure 3 2 is a sketch of t h e grade b e a m and pier s y s t e m . It is seen from figure 3 2 t h a t t h e uplifting forces w h i c h t e n d t o pull t h e pier o u t of t h e g r o u n d are a direct function of t h e swelling pressure. T h e w i t h h o l d i n g force consists of t h e dead load pressure e x e r t e d o n t h e pier a n d t h e skin friction along t h e u n w e t t e d p o r t i o n of t h e pier. F o r safe pier design, t h e w i t h h o l d i n g force m u s t balance t h e uplifting force. These forces are analyzed as follows.

76


Dead load pressure Reinforcement for - tension Grade Beam

Air space beneath Grade Beam Uplifting Pressure U = uf Swelling Pressure

Assumed circular plane of failure

Skin friction

2r

STRAIGHT SHAFT-PIER FOUNDATION

BELL - PIER FOUNDATION

Figure 32. Grade beam and pier system. Uplifting

force

T h e t o t a l uplifting force of t h e soils s u r r o u n d i n g t h e pier can be w r i t t e n as follows: U = 27rrfu(D-d) where:

r = radius of t h e pier (ft.) d = d e p t h of t h e z o n e of soils unaffected b y w e t t i n g , (ft.) D = t o t a l length of t h e pier, (ft.) u = swelling pressure, (psf) f = coefficient of uplift b e t w e e n c o n c r e t e and soil, U = t o t a l uplifting force, (lbs.)

The

soils s u r r o u n d i n g

t h e pier e x p a n d b o t h vertically and h o r i z o n t a l l y u p o n

wetting.

P a r c h e r and Liu [441 have s h o w n t h a t c o m p a c t e d clay soils e x h i b i t e d greater u n i t swelling in t h e h o r i z o n t a l direction t h a n in t h e vertical d i r e c t i o n . Since t h e m a g n i t u d e of t h e difference

of

swelling is small, we can assume t h a t t h e vertical swelling pressure can be used in estimating t h e uplifting force o n t h e piers. The coefficient of uplift b e t w e e n t h e soil a n d surface of t h e c o n c r e t e pier is n o t k n o w n . With t h e m o d e l pier test described in t h e s u b s e q u e n t section, it was a t t e m p t e d t o establish a rational value for t h e coefficient of uplift for design p u r p o s e s . It should be p o i n t e d o u t here t h a t t h e uplifting pressure of t h e pier m a y n o t b e c o n t r o l l e d b y t h e shear s t r e n g t h of t h e clay. O u r

77


e x p e r i m e n t discussed later u n d e r " M o d e l Test for Pier U p l i f t " indicates t h a t t h e pier actually slips o u t of t h e soil in very m u c h t h e same m a n n e r as e x t r a c t i n g a pile. T h e c o n t a c t surface b e t w e e n t h e c o n c r e t e and t h e soil r e m a i n e d clean. Withholding

force

T h e w i t h h o l d i n g force t h a t k e e p s a pier from pulling o u t of t h e g r o u n d can be w r i t t e n as: W = πι2 ρ +

2πΓ

sd

w h e r e : ρ = unit dead-load pressure, (psf) s = skin friction s u r r o u n d i n g t h e pier, (psf) W = total w i t h h o l d i n g force, (lbs.) d = d e p t h of z o n e of soils unaffected b y w e t t i n g , (ft.) r = radius of t h e pier, (ft.) T h e skin friction of t h e soils s u r r o u n d i n g t h e pier is again a factor w h i c h c a n n o t be fully evaluated. According t o M o h a n and C h a n d r a [ 4 5 ] " T h e frictional resistance of b o r e d c o n c r e t e piles in m e d i u m t o h a r d clays is a b o u t half t h e average u n d i s t u r b e d shear s t r e n g t h of t h e clay along t h e pile shaft and has the same value in loading and p u l l i n g . " E x p e r i e n c e w i t h piers e m b e d d e d in shale indicates t h a t the skin friction along t h e shaft of t h e pier is equal t o at least 1/10 of t h e end bearing value of t h e pier. T h e value is conservative and d e p e n d s u p o n t h e roughness of t h e pier shaft and t h e compressive strength of t h e c o n c r e t e as previously discussed u n d e r " S k i n F r i c t i o n of S h a l e " . Zone of

wetting

In pier design it is i m p o r t a n t t o d e t e r m i n e t h e d e p t h of t h e z o n e of w e t t i n g . Field tests indicate t h a t expansive soils are so i m p e r m e a b l e t h a t surface w a t e r can p e n e t r a t e o n l y a b o u t 2 feet i n t o t h e soil. However, in actual cases, it was found t h a t soils s u r r o u n d i n g piers were w e t t o a d e p t h of as m u c h as 15 feet b e l o w t h e g r o u n d surface. This is possibly d u e t o t h e following reasons: 1. T h e source of w e t t i n g m a y n o t b e derived from surface w a t e r . A b r o k e n w a t e r m a i n o r sewer pipe can cause a m u c h m o r e severe w e t t i n g c o n d i t i o n t h a n surface w a t e r . 2. Surface w a t e r seeps t h r o u g h seams and fractures in t h e stiff fissured clays and clay shales. 3 . Water t e n d s t o seep along the walls of t h e pier shaft i n t o t h e s u r r o u n d i n g soils. F o r t h e above reasons, it is n o t possible t o precisely define t h e d e p t h of t h e z o n e of w e t t i n g . F o r design p u r p o s e s , 5 feet can be arbitrarily assigned as t h e p r o b a b l e d e p t h of w e t t i n g . Model test for pier uplift

forces

T h e soil sample used for c o n d u c t i n g t h e test was typical of Southeast Denver's highly expansive clay. T h e physical p r o p e r t i e s of t h e soil were as follows: Liquid limit, p e r c e n t

= 50.0

Plastic limit, p e r c e n t

= 24.3

78


Plasticity i n d e x , p e r c e n t

= 25.7

Passing n o . 2 0 0 sieve, p e r c e n t

= 82.4

Optimum moisture content, percent = 21.0 M a x i m u m d r y d e n s i t y , pcf

= 96.5

T h e soil was p a c k e d i n t o a steel c o n t a i n e r t o a d e p t h of 9 inches in thin layers and at a m o i s t u r e c o n t e n t of 14.0 p e r c e n t . T w o sets of holes were drilled in the soil. T h e first set of t w o piers was p r e p a r e d for s t u d y i n g t h e behavior of friction piers. T h e piers w e r e 2 inches in d i a m e t e r and were b o t t o m e d o n t h e steel c o n t a i n e r . T h e length of the pier was e m b e d d e d in t h e full d e p t h of soil w h i c h is 9 i n c h e s ; therefore, any uplift pressure exerted on the pier w o u l d be only along t h e pier shaft and n o n e o n the base. T h e second set of t w o piers was p r e p a r e d for s t u d y i n g t h e behavior of end-bearing piers. Holes 2-1/2 inches in d i a m e t e r and 6 inches d e e p were drilled in t h e soil sample. T h e piers, being o n l y 2 inches in d i a m e t e r , did n o t have any direct c o n t a c t w i t h the soil along their circumference, b u t were bearing directly on t h e soil at t h e b o t t o m of the h o l e . C o n s e q u e n t l y , t h e only pressure acting on these piers was at t h e base. T h e a r r a n g e m e n t of t h e tests is s h o w n on figures 3 3 and 3 4 . Piers A and Β are friction piers. T h e uplifting m o v e m e n t of Pier A was recorded by placing a dial gauge directly on t h e t o p of t h e pier. T h e uplifting pressure of Pier Β was m e a s u r e d w i t h a proving ring. T h e proving ring w a s , c e n t e r e d on t o p of t h e pier w i t h a ball bearing b e t w e e n t h e pier and proving ring. An adjustable screw was placed b e n e a t h t h e ball bearing. Dial A, w h i c h was placed on t o p of t h e pier, was carefully observed for m o v e m e n t ; as s o o n as uplifting m o v e m e n t was observed on Dial A, the adjustable screw was t u r n e d t o k e e p t h e pier in its original position t h r o u g h o u t t h e test. T h e stress registered b y t h e proving ring represents t h e t r u e uplifting pressure of t h e pier [ 4 6 ] . Piers C and D were end-bearing piers. T h e set u p for these t w o piers was identical t o Piers A and B.

Figure 33. Apparatus for the determination of the coefficient of uplift

79


Friction Pier A Strain Gage

End Bearing Pier C Strain Gage

ο

ο

ο

ο

°

φ ο

ο

° \

(φ)

° r>

ο

ο

ο

Drain Pipes

ο

ο

(| ι V \

Dial A - ^ 0 σ

ο

ι JJ ο / Μ— Proving Ring /

0

ί/

End Bearing Pier D Stress Gage

ο

r

ο

Dial Support

Friction Pier Β Stress Gage ο

° ο

Dial A

ο

ο — ^

(ι_4 \ > - y

ο LJ]

Ο

J

-^ZT^ο ο

PIER UPLIFTING TEST APPARATUS PLAN VIEW

Load Frame

SECTION

A A

Figure 34. Pier uplifting test apparatus—plan and section.

7/

80


Perforated plastic pipes were inserted a r o u n d t h e p e r i p h e r y of t h e piers as s h o w n o n figure 3 4 . Water was a d d e d t o t h e soil t h r o u g h t h e perforated pipes t o o b t a i n saturated c o n d i t i o n s . B o t h stress and strain readings were t a k e n at frequent intervals. T h e test was carried o u t for a period of 2 weeks. Results of t h e time-strain and time-stress relationships are s h o w n on figures 35 and 3 6 respectively. T h e results of t h e tests can be s u m m a r i z e d as follows:

Type

M a x i m u m uplifting

Maximum upward

Pier

of

pressure,

movement,

No.

pier

pounds

inches

A

Friction

_

Β

Friction

195.0

C

End Bearing

D

End Bearing

0.20 -

0.16

-

77.0

-

F o r c o m p l e t e w e t t i n g c o n d i t i o n s and w i t h o u t pressure being exerted o n t h e b o t t o m of t h e piers, t h e total uplifting force is: U = 27rrfuD T h e t o t a l uplifting

force exerted on an end-bearing pier w i t h t h e pier free from

the

s u r r o u n d i n g soils can be w r i t t e n as: U = 7rr2 u

Δ

0.20 FRICTK >N P I E R - ^

ο —

0

t-

•

-END BEAR! IG PIER

ί1. 4/17

4/18

4/19

4/20

4/21

4/22

4/23

4/25

DATE

Figure 35. Time-strain relationship of friction and end-bearing piers in clay.

4/26

4/27


81

300

250

FRI CTION

"

PIER-

ο ο

Ο

_

0

Ô

ο

ο

/

4/17

ΕND BEARING PIER "

•·

4/18

4/19

4/20

4/21

Λ

Λ

4/22

4/23

4/24

4/25

DATE

Figure 36. Time-stress relationship of friction and end-bearing piers in clay

S u b s t i t u t i n g t h e actual uplifting pressure m e a s u r e d in t h e m o d e l t e s t ; F o r end-bearing piers: 77 = π ( I ) 2 u u = 24.5 psi

F o r friction piers: 195 = 2 π

(l)fu(9)

fu = 3.45 psi.

Assuming t h a t t h e vertical swelling pressure is equal t o t h e h o r i z o n t a l swelling pressure, t h e n fu = 3.45 psi, from w h i c h t h e value f is d e t e r m i n e d t o equal 0 . 1 4 . F r o m t h e results of t h e e x p e r i m e n t , and from e x p e r i e n c e w i t h pier systems in this area, it appears t h a t t h e uplifting pressure along t h e surface of t h e c o n c r e t e e x e r t e d b y t h e soil in soil-concrete pier s y s t e m s is a b o u t 15 p e r c e n t of t h e vertical swelling pressure. This value is of significant m a g n i t u d e and can b e t h e governing factor in t h e design of grade b e a m and pier systems. T h e following e x a m p l e will illustrate t h e i m p o r t a n c e of uplifting pressure along t h e surface of t h e pier. A s s u m e a 12-inch-diameter pier e m b e d d e d

10 feet in expansive soils having a swelling

pressure of 1 0 , 0 0 0 psf. Assuming t h a t t h e u p p e r 5 feet of t h e soil b e c o m e s w e t t e d , t h e n t h e t o t a l uplifting force e x e r t e d o n t h e pier will b e : U = 27rrfu(D-d) = 2π ( 0 . 5 ) ( 0 . 1 5 ) ( 1 0 , 0 0 0 ) (10-5) = 2 3 , 6 0 0 lbs. F o r piers spaced o n a b o u t 10-to-l 5-foot c e n t e r s , t h e dead load pressure n o r m a l l y assigned t o t h e piers is a b o u t 2 0 , 0 0 0 psf. T h e t o t a l dead load pressure e x e r t e d o n a 12-inch-diameter pier is 15,700 lbs. T h e u n b a l a n c e d uplifting pressure is 2 3 , 6 0 0 - 1 5 , 7 0 0 = 7 , 9 0 0 lbs. C o m p e n s a t i o n m u s t


82

b e m a d e for this u n b a l a n c e d uplifting pressure. This is accomplished by t h e pier in t h e lower u n w e t t e d p o r t i o n of the soil w h e r e friction along t h e surface of t h e pier will provide a restraint t o uplift. In e x t r e m e cases, it can be considered t h a t t h e entire length of soil s u r r o u n d i n g t h e pier is w e t t e d . T h e total uplifting pressure t h e n will be 4 7 , 2 0 0 lbs., and t h e u n b a l a n c e d

uplifting

p r e s s u r e will be 3 1 , 5 0 0 lbs. U n d e r this c o n d i t i o n , uplifting of t h e pier is inevitable and structural c r a c k i n g results. It was c o n c l u d e d from t h e above described m o d e l pier tests t h a t the uplifting pressure along t h e surface of c o n c r e t e was a b o u t 15 p e r c e n t of t h e vertical swelling pressure as previously discussed. T h e o r e t i c a l analysis of t h e m a g n i t u d e of the uplifting pressure is n o t presently feasible since t h e m e c h a n i c s of swelling have n o t y e t b e e n translated i n t o a suitable m a t h e m a t i c a l m o d e l . It is interesting t o k n o w t h e shape of t h e failure plane w h e n t h e pier is lifted from the g r o u n d . O n e t h e o r y is t h a t w h e n the pier is being lifted, a cone of soil is carried b y t h e pier as s h o w n on figure 37a. By c o n v e n t i o n , the cone is assumed t o be 6 0 ° . A n o t h e r t h e o r y is t h a t the p i e r is pulled o u t from t h e g r o u n d relative t o t h e s u r r o u n d i n g soils as s h o w n o n figure 3 7 b . In o u r m o d e l test, w e found t h a t after t h e c o m p l e t i o n of t h e test, Pier A had actually pulled from the s u r r o u n d i n g soils leaving a gap b e t w e e n the b o t t o m of the pier and the surface of the container, a d i s t a n c e a p p r o x i m a t e l y equal t o the vertical m o v e m e n t of t h e pier, as indicated o n the strain gauge. This indicated t h a t the failure t o o k place along t h e interface of soil and c o n c r e t e as i n d i c a t e d o n figure 3 7 b . Rational

pier

formula

A rational pier formula can be derived b y e q u a t i n g t h e total uplifting force and t h e total w i t h h o l d i n g force acting o n a pier. 2 7 r r f u ( D - d ) = 7rrp + 27rrsd 2

then:

p = ~ [ f u ( D - d ) - sd]

where

ρ = unit dead load pressure

T o solve the above e q u a t i o n , it is necessary to assign values t o u, f, s, and D-d. T h e expansive soils u s u a l l y e n c o u n t e r e d in this area belong t o t h e categories of m e d i u m or high degree of expansion ( t a b l e 4 ) . F o r usual design purposes, it is possible t o assign values of t h e soil properties in the a b o v e e q u a t i o n and o b t a i n rational solutions. T h e following a s s u m p t i o n s are m a d e t o simplify t h e computation: 1. T h e soil is u n i f o r m for t h e full length of t h e pier. If shale b e d r o c k is e n c o u n t e r e d at t h e b o t t o m of the pier, t h e error will be on the safe side. 2. Surface w e t t i n g will affect only the u p p e r 5 feet of the pier. 3. Skin friction, s, of t h e soils s u r r o u n d i n g t h e pier in t h e u n w e t t e d z o n e is a b o u t 5 0 0 psf. 4 . T h e swelling pressure acting on the pier for soils w i t h high degree of e x p a n s i o n is a b o u t 1 0 , 0 0 0 psf and for soils w i t h m e d i u m degree of e x p a n s i o n it is a b o u t 5 , 0 0 0 psf. 5. T h e coefficient of uplift, f, b e t w e e n the pier and the soil is 0 . 1 5 .

83


Based o n t h e above simplified a s s u m p t i o n s , figure 3 8 was p r e p a r e d . F r o m t h e figure, it is possible for t h e s t r u c t u r a l engineer t o select t h e size of t h e pier, t h e dead load pressure, and t h e required t o t a l l e n g t h of t h e pier.

BELLED PIERS Piers drilled i n t o materials o t h e r t h a n b e d r o c k are often enlarged at t h e b o t t o m of t h e h o l e for t h e p u r p o s e of increasing t h e bearing area, t h u s increasing t h e t o t a l load-carrying c a p a c i t y . Such piers are c o m m o n l y referred t o as belled piers, or u n d e r r e a m e d piers. T h e ideal bell is in t h e shape of a frustum w i t h a vertical side at t h e b o t t o m . T h e vertical side m a y b e 6 t o 12 i n c h e s high. T h e sloping side of t h e bell should be at an angle at least 6 0 degrees w i t h t h e h o r i z o n t a l . Most drillers are capable of providing bells w i t h d i a m e t e r s equal t o three times t h e d i a m e t e r o f the shaft. T h e advantages and disadvantages of t h e belled system are p r e s e n t e d in t h e following t w o paragraphs. Advantage

of belled

piers

It is seen in figure 3 2 t h a t t h e uplift forces e x e r t e d on t h e belled pier system are as follows:

U=P + F where:

W

+ F

S

U = t o t a l uplifting force d u e t o t h e swelling of t h e soils s u r r o u n d i n g the pier shaft in t h e u n w e t t e d z o n e , (lbs.), Ρ = total vertical pressure exerted o n t h e pier, (lbs.), F

w

= t o t a l weight of t h e soil above t h e bell, as s h o w n on figure 3 2 , (lbs.),

F s = t o t a l shearing resistance along t h e assumed circular line of failure in t h e u n w e t t e d z o n e , d i a m e t e r of t h e t o p of frustum

2 R , (lbs.)

P r o b a b l y , t h e greatest advantage of t h e belled pier is t h a t t h e resistance against uplift will n o t be affected b y loss of friction in t h e z o n e unaffected b y w e t t i n g . As previously discussed u n d e r "Pier U p l i f t , " t h e w i t h h o l d i n g force d e p e n d s u p o n t h e skin friction of t h e pier in t h e z o n e unaffected b y w e t t i n g . If, for s o m e reason, such as t h e rise of g r o u n d w a t e r , t h e skin friction is lost, t h e n pier uplift is unavoidable. With belled piers, t h e total weight of t h e soil above t h e bell, F w, will n o t b e affected b y m o i s t u r e change. H e n c e , t h e r e is always an added factor of safety o f this system against uplift. In areas w h e r e t h e u p p e r soils are highly expansive, b e d r o c k is shallow, and t h e r e is a s t r o n g possibility of d e v e l o p m e n t of a perched w a t e r table c o n d i t i o n , t h e use of a belled pier s y s t e m should be favorably considered.

84


Uplift

Uplift

Original position ^^"'^ of pier bottom b

a

Figure 37. Soil failure plane resulting from pier uplift. Since a belled pier system relies entirely u p o n t h e anchorage of t h e lower p o r t i o n of the pier in t h e z o n e unaffected b y w e t t i n g against uplift, this t y p e of pier can b e used for c o l u m n s w i t h very light load. T h e m a g n i t u d e of dead load pressure exerted o n t h e pier is n o t a factor. Disadvantage

of belled

piers

T h e shaft of a belled pier m u s t be sufficiently large t o allow c l e a n o u t and inspection. T h e m i n i m u m shaft size is 2 4 inches, a l t h o u g h 3 0 inches is desirable. F o r straight-shaft piers w i t h a large shaft d i a m e t e r , t h e uplift pressure exerted o n t h e pier is several times higher t h a n if the smaller 12-ineh-diameter shaft is used. More r e i n f o r c e m e n t will be required in a belled pier t h a n in a small-diameter straight-shaft pier. If t h e r e i n f o r c e m e n t is i n a d e q u a t e , tension cracks can develop in t h e vicinity of t h e pier shaft and t h e bell. T h e m o s t p r o m i n e n t disadvantage of belled piers is the cost and t h e difficulty of inspection. Bells can be formed mechanically w i t h a special belling device. In hard b e d r o c k , a mechanical device m a y be unsuccessful. When it is necessary t o send a m a n d o w n the hole w i t h a j a c k h a m m e r to c o m p l e t e t h e bell, t h e cost can be prohibitive. In t h e Denver area where the cost of pier drilling is exceedingly c o m p e t i t i v e , it is estimated t h a t in a favorable drilling c o n d i t i o n , the cost of belling can easily exceed t h e cost of drilling an e x t r a 10 feet of pier shaft. T h e advantage of anchorage in a bell pier system can easily be offset b y drilling an additional 10 feet i n t o b e d r o c k with a straight-shaft pier. Cleaning a belled pier is m u c h m o r e difficult t h a n cleaning a straight-shaft pier. P r o p e r cleaning of a belled pier can only be accomplished b y sending a m a n d o w n t h e hole w i t h a shovel. With present safety regulations, this m e a n s t h a t all pier shafts will have t o be cased. T h e condition of t h e b o t t o m of the straight-shaft pier can b e inspected from t h e g r o u n d level with a t o r c h o r a m i r r o r , b u t t h e c o n d i t i o n of t h e bell c a n n o t be inspected from g r o u n d level. The

difficulty

of

belled

pier

construction

increases m a r k e d l y

where

the

materials

immediately overlying b e d r o c k are subject t o caving. It is nearly impossible t o bell in granular soil. T h e p r o b l e m is further c o m p l i c a t e d if ground w a t e r is e n c o u n t e r e d .


4

6

85

8

10 REQUIRED

12 TOTAL

14

16

18

20

PIER LENGTH - FEET

Figure 38. Rational pier design chart.

With d u e c o n s i d e r a t i o n of t h e cost, t h e advantages of t h e belled pier can be achieved b y the use of straight-shaft piers w i t h a larger p e n e t r a t i o n i n t o a zone unaffected b y w e t t i n g .

Isolation

of pier

uplift

Since t h e m o s t d a m a g i n g action in a drilled pier system is t h e uplifting pressure e x e r t e d o n t h e p e r i m e t e r of t h e pier, t h e simple solution appears t o be isolating t h e pier shaft from t h e uplifting forces. T h e obvious m e t h o d is t o drill an oversized pier hole and provide soft, compressible material a r o u n d t h e pier so t h a t t h e s u r r o u n d i n g soils c a n n o t exert uplifting pressure on t h e wall of t h e pier. However, b y so doing, t h e total load carrying capacity of t h e pier will be greatly r e d u c e d . Since m o s t piers rely heavily o n t h e skin friction t o carry t h e c o l u m n load, elimination of t h e skin friction necessitates t h e use of large bells for s u p p o r t t o justify t h e use of such a system. T h e s h o r t c o m i n g s of this system are as follows: 1. T h e load carrying capacity of t h e pier is r e d u c e d t o t h a t of a pad footing f o u n d a t i o n , 2. N o lateral resistance is available for t h e system, 3 . T h e a n n u l a r space m u s t be filled w i t h compressible m a t e r i a l such as s a w d u s t , ash, o r straw. T h e use of loose sand or pea gravel will b e satisfactory initially, b u t r e p e a t e d seasonal cycles of e x p a n s i o n and shrinkage will pack t h e filling and allow t h e s u r r o u n d i n g soils t o grip t h e pier shaft, and

86


4 . T h e annular space filled w i t h compressible material affords a free p a t h for w a t e r t o reach t h e b o t t o m of t h e pier and heave t h e entire pier from t h e b o t t o m . C o n s e q u e n t l y , t h e p u r p o s e of placing t h e pier in a z o n e unaffected b y wetting is defeated. T h e end result of using such isolated-shaft belled piers is n o different from using individual p a d s . This s h o r t c o m i n g can be partially corrected b y sealing t h e u p p e r 3 feet of t h e a n n u l a r space w i t h c o m p a c t e d clay. T h e process is costly and in m o s t cases n o t effective in preventing the seepage of w a t e r . O n e possible application of such a system is w h e r e t h e u p p e r expansive soils are underlain b y n o n e x p a n s i v e gravelly soils. A n oversized pier hole can be drilled t o t h e t o p of t h e gravel s t r a t u m , a small d i a m e t e r S o n o t u b e can t h e n be inserted i n t o the hole and t h e annular spaces filled w i t h sawdust, and lastly, t h e S o n o t u b e filled w i t h c o n c r e t e . With such subsoil c o n d i t i o n , t h e infiltration of w a t e r t o t h e b o t t o m of t h e pier hole will n o t cause uplift, as long as t h e pier shaft is free from c o n t a c t w i t h t h e expansive soils. Such systems have been used w i t h great success in a n o t o r i o u s l y t r o u b l e s o m e area w h e r e t h e thickness of the u p p e r swelling soil is a b o u t 20 feet. A n o t h e r a p p r o a c h has b e e n devised t o break t h e b o n d b e t w e e n the pier and t h e clay. T h e c o n c e p t was initiated in San A n t o n i o , Texas [ 4 3 ] w h e r e highly expansive clays are found. A pipe is i n t r o d u c e d i n t o the pier. T h e concrete-filled pipe is designed t o carry t h e c o l u m n load at t h e t o p . T h e outside of the pipe from the t o p d o w n to the b o t t o m of the p o t e n t i a l l y w e t t e d z o n e is coated w i t h a b i t u m i n o u s m a s t i c material. When t h e pier is lifted by swelling soils, the annulus of c o n c r e t e outside t h e c o a t e d section b r e a k s in tension, t h e m a s t i c coating breaks the b o n d , and the upward force is limited b y t h e viscosity of t h e mastic. Figure 39 illustrates this system.

FRICTION PIERS Since t h e p u r p o s e of a drilled pier f o u n d a t i o n is t o transfer t h e building load to a z o n e where t h e m o i s t u r e c o n t e n t will n o t be affected b y surface wetting, it is, therefore, unnecessary t o drill all piers i n t o b e d r o c k o r very h a r d f o r m a t i o n s . Where b e d r o c k is d e e p and t h e u p p e r overburden clays are expansive, a logical solution is t h e use of friction piers. In the design of a friction pier, a rational value should be assigned t o t h e skin friction.

Skin

friction It is generally recognized t h a t the skin friction b e t w e e n cohesive soils and the pier shaft

c a n n o t exceed t h e cohesion of t h e soil. Cohesion m a y be assumed to be equal t o one-half of t h e u n c o n f i n e d compressive s t r e n g t h of t h e soil. In stiff or hard clays, however, t h e b o n d b e t w e e n t h e c o n c r e t e and t h e soil m a y b e less t h a n cohesion. A c c o r d i n g t o T e n g [ 4 7 ] , the m a x i m u m design value for skin friction of all cohesive soils should b e limited t o a b o u t 1,000 (or u p t o

1,500) psf. U l t i m a t e pier shaft

resistance can b e calculated

as the sum of t h e u l t i m a t e shearing

resistance imposed b y t h e various strata w h i c h are in c o n t a c t w i t h t h e pier. T h e u l t i m a t e shaft


Figure 3 9 . Design of belled pier for relief of uplift due to expansion of upper clay layer. Note that the outer annulus of concrete is expected to break in tension near the bottom of the expansive clay layer. (Raba and Associates, Consulting Engineers, San Antonio, Tex.)

87

88


resistance of a pier drilled in uniform clay strata can be expressed in terms of the u n d r a i n e d shear strength S u and a r e d u c t i o n factor α as follows: S =n The

undrained

shear

strength

unconsolidated-undrained

of

r2aSu clays

can

be d e t e r m i n e d

in

the

triaxial test. F o r stiff clays, an u n c o n f i n e d

laboratory

from

the

compression test will

generally serve the p u r p o s e . O t h e r m o r e direct m e t h o d s in evaluating shear strength such as the use of a p r e s s u r e m e t e r inserted i n t o t h e drill h o l e at t h e desired d e p t h can be used. A vane shear test is n o t applicable for use in stiff clays b u t can be valuable in soft clays. Shear value can also be interpreted indirectly from field p e n e t r a t i o n resistance tests or c o n e p e n e t r a t i o n resistance data. T h e shear strength r e d u c t i o n factors a as derived from analysis of p r o t o t y p e field load tests in clays, shales, and tills, are listed in table 1 1 . T h e shear strength r e d u c t i o n factor, according t o W o o d w a r d , G a r d e n e r and Greer [ 4 3 ] is also influenced b y c o n s t r u c t i o n effects and t h e m o i s t u r e sensitivity of the s u p p o r t i n g materials. Water migrating from t h e c o n c r e t e t o t h e pier wall also reduces t h e shear strength. O t h e r elements such as t h e d u r a t i o n of e x p o s u r e of o p e n shafts can also be i m p o r t a n t factors. Design of friction

piers

T h e principle for t h e design of friction piers is given in a previous section u n d e r " R a t i o n a l Pier F o r m u l a . " T h e following considerations should be given: 1. F r i c t i o n piers generally bear o n stiff clays. With t h e use of small d i a m e t e r piers, the end bearing capacity of such piers can generally be neglected. 2. Sufficient field p e n e t r a t i o n resistance tests should b e p e r f o r m e d n o t only t o establish the p r o p e r friction value b u t also t o insure t h a t soft soils are n o t e n c o u n t e r e d within the length of t h e pier. 3. Since t h e u p p e r 5 feet of soil around t h e pier is subject t o surface w e t t i n g and uplifting, this length should be excluded in calculating t h e pier load capacity.

Table 11—. Shear strength reduction factors (a) for drilled piers. Material Properties Moisture content W n, percent

Plasticity Index I , percent

Stiff clay Stiff clay Massive shale Glacial till Stiff clay

23 25 15 12

35-55 20-60 7-16 2-16

. ..

. . ..

Stiff clay

19

36-46

Material

Shear strength, tons per sq. ft. 1.2 1.2 5.0 2.5 1.1 0.9 1.4

a 0.44 0.62 0.64* 0.64*+ 0.52 0.49 0.30

*Failure was not reached. +Sandy gravel with cobbles and approximately 50 percent silty clay, Ν > 45 blows per ft. (After Woodward, Gardner, and Greer)

Reference Whitaker and Cooke Reese and O'Neill Matich and Koziki Matich and Kozicki Woodward et al. Mohan and Jain

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4. F r i c t i o n piers should n o t be used at a site w h e r e g r o u n d w a t e r is high or w h e r e t h e r e is t h e possibility of future high g r o u n d - w a t e r c o n d i t i o n .

An e x a m p l e of typical friction pier design is as follows: Stiff clay t o d e p t h 4 0 feet, n o ground w a t e r

Data

e n c o u n t e r e d . Average p e n e t r a t i o n resistance

:

25 b l o w s p e r foot

Average unconfined compressive 4 , 0 0 0 psf

strength

2 0 feet

Pier length

Pier

capacity

Pier d i a m e t e r

12 inches

Shear s t r e n g t h r e d u c t i o n facto

0.5

:

r

U l t i m a t e skin friction

4,000

x0.5

1,000 psf T o t a l load carrying capacity = ( 2 0 - 5 ) x 1,000 x 3.14

4 7 . 1 kips

Using a factor of safety of 3 Design load

15.7 kips

W h e n designing for a d e a d load plus full live load, a factor of safety of 2 can b e used.

Uplifting

force

Withholding

force

T h e n design load

= 2 3 . 6 kips.

Swelling pressure

= 1 0 , 0 0 0 psf

Coefficient of uplift

= 0.15

T o t a l uplift = 3.14 χ 1 0 , 0 0 0 χ 0.15 χ 5

= 2 3 . 5 kips

U l t i m a t e skin friction

= 1,000 psf

U l t i m a t e w i t h h o l d i n g force = ( 2 0 - 5 ) x 1,000 x 3 . 1 4

= 47.1 kips

T h e above calculations indicate t h a t t h e design is safe as t h e w i t h h o l d i n g force is larger t h a n t h e uplifting force.

F A I L U R E O F T H E P I E R SYSTEM A p r o p e r l y designed drilled pier system involves t h e c o o r d i n a t i o n of t h e floor slab, grade b e a m , void space, r e i n f o r c e m e n t , e x p a n s i o n j o i n t , and floor system. A typical detail is s h o w n o n figure 4 0 . T h e grade b e a m and pier system offers t h e m o s t logical s o l u t i o n for lightly loaded s t r u c t u r e s founded

on expansive soils. However, if incorrectly designed, or incorrectly c o n s t r u c t e d , a

building w i t h a pier f o u n d a t i o n is j u s t as vulnerable, if n o t m o r e vulnerable, t o m o v e m e n t t h a n a building f o u n d e d o n spread footings.

90


Figure 40. Typical detail of grade beam and pier system. Considerable experience is required

to d e t e r m i n e the cause of cracking of a building

founded on piers. O f t e n t i m e s , the cracks are caused b y slab m o v e m e n t as discussed in c h a p t e r 6. Typical pier uplift m o v e m e n t generally takes place a short distance from t h e pier and has a 4 5 degree p a t t e r n . Cracks often are wider at t h e t o p and n a r r o w e r at t h e b o t t o m . Generally, the t y p e of cracking d e p e n d s u p o n the structural configuration of the building. Masonry walls and cinderblock walls are m o s t sensitive t o m o v e m e n t . C o n s e q u e n t l y , t h e first sign of pier m o v e m e n t will be reflected as cracks t h a t develop in the brick wall as s h o w n o n figures 41 and 4 2 . Basement walls are structurally m o r e resilient to differential m o v e m e n t t h a n m a s o n r y walls. When severe diagonal cracks a p p e a r in t h e b a s e m e n t as s h o w n o n figure 4 3 , pier uplifting can be considered a certainty.

DRILLE D PIE R FOUNDATION S

Figure 4 1 . Typical cracking caused by pier uplift.

Figure 4 2 . Heaving of piers of a three-story structure. Insufficient pier length is the cause.

91

92


Figure 43. Typical cracks developing in the basement wall immediately beneath the window well. Note that crack is wide at the top and narrow at the bottom. T h e m o s t c o m m o n errors in design are insufficient pier length, excessive pier diameter, or t h e absence of pier r e i n f o r c e m e n t . T h e m o s t c o m m o n errors in c o n s t r u c t i o n are excess c o n c r e t e on t o p of pier resulting in m u s h r o o m s at t h e t o p of t h e piers and t h e absence of, o r defective air space b e n e a t h t h e grade b e a m s . These are discussed in detail as follows: Excessive

pier

size

M a n y l a y m e n have t h e impression t h a t t h e larger t h e d i a m e t e r of t h e pier, t h e safer t h e building. Actually, t o e x e r t e n o u g h dead load pressure o n t h e pier, it is necessary t o use small d i a m e t e r piers in c o m b i n a t i o n w i t h long spans. T h e m o s t e c o n o m i c a l spacing of t h e piers is limited b y t h e a m o u n t of r e i n f o r c e m e n t in t h e grade b e a m s o r t h e e c o n o m i c a l size of t h e floor b e a m s . N o r m a l l y , piers should have a m i n i m u m spacing of 12 feet. Most small drill rigs in t h e R o c k y M o u n t a i n area are e q u i p p e d t o drill 12-inch-diameter pier holes. Auger sizes of 8 and 10 inches are also available. However, w i t h pier holes less t h a n 12 inches in d i a m e t e r , considerable difficulty can be e n c o u n t e r e d in cleaning t h e holes. A pier hole w i t h as little as 2 inches of loose soil at t h e b o t t o m will experience excessive s e t t l e m e n t at a later

93


d a t e . T h e use of 12-inch-diameter piers for residential and light c o m m e r c i a l c o n s t r u c t i o n is recommended.

insufficient

pier

length

As explained in t h e previous section, the stability of t h e pier against uplift d e p e n d s u p o n t h e a m o u n t of dead load pressure exerted o n t h e pier and t h e anchorage provided in t h e l o w e r p o r t i o n of the pier. If t h e length of t h e pier is short, t h e possibility of the soil at t h e b o t t o m of t h e pier b e c o m i n g w e t t e d is great, t h u s t h e skin friction along t h e pier providing t h e anchorage w o u l d b e lost and t h e pier w o u l d have t o d e p e n d u p o n dead load pressure alone t o resist uplifting. T h e dead load pressure for a lightly loaded building is n o t of sufficient m a g n i t u d e t o resist t h e uplifting. This is especially t r u e in t h e case of t h e interior, lightly loaded piers. T h e r e f o r e , t h e function of a s h o r t pier actually is n o different or m o r e desirable t h a n individual pad footings. This is illustrated on figure 4 4 . F o r short piers, t h e uplifting pressure is t h e sum of t h e swelling pressure acting o n t h e b o t t o m of t h e pier, plus t h e uplifting pressure acting u p o n t h e p e r i m e t e r of t h e pier. Failure of t h e pier system d u e t o insufficient pier length (fig. 4 5 ) is c o m m o n p l a c e . T h e practice of using piers of insufficient length is usually observed in areas w h e r e claystone shale is near g r o u n d surface and t h e engineer specifies only t h e d e p t h of p e n e t r a t i o n i n t o b e d r o c k . F o r e x a m p l e , it is c o m m o n in this area t o specify a m i n i m u m b e d r o c k p e n e t r a t i o n of 4 feet. In m a n y instances, this results in a pier w i t h a total length of only 4 feet. T h e r e is a good possibility of w e t t i n g of t h e entire length of such a pier and s u b s e q u e n t heaving of t h e pier.

Dead Dead

!

load

Swelling PIER

Swelling

Swelling p r e s s u r e on s u r f a c e of pier

t t

SHORT

HUH

\

\

load

pressure

pressure

pressure

FOUNDATION

PAD

FOUNDATION

Figure 44. Swelling pressure acting on short pier foundation and pad foundation.

pressure

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Figure 45. Typical example of short pier foundation (insufficient pier length). Pier drilled into highly weathered claystone. Note: Adjacent Sonotube placed for underpinning. It is i m p o r t a n t

t h a t t h e engineer also specify

t h e m i n i m u m total pier length in the

f o u n d a t i o n system to insure t h a t t h e piers are a n c h o r e d sufficiently d e e p in t h e u n w e t t e d zone of the bedrock. Uniform

pier

diameter

After t h e pier hole is drilled, and d u r i n g t h e placing of c o n c r e t e , excess c o n c r e t e is usually n o t removed from t h e t o p of t h e pier, resulting in a m u s h r o o m occurring at the t o p of t h e pier as indicated o n figures 4 6 and 4 7 . A t times, t h e m u s h r o o m has been k n o w n t o have a d i a m e t e r t h r e e times t h e d i a m e t e r of t h e pier. Soil b e n e a t h t h e grade b e a m s will exert direct uplifting pressure on t h e underside of t h e m u s h r o o m . F o r a 12-inch-diameter pier w i t h a 36-inch-diameter m u s h r o o m , the total area


95

GRADE BEAM

Reinforcement

Void beneath grade beam

Uplifting pressure exerted on mushroom of pier.

Figure 46. Effect of mushroom on the uplifting of the pier. subjected t o uplifting is 6.25 square feet. F o r a m o d e r a t e l y expansive soil having a swelling pressure of 1 0 , 0 0 0 psf, t h e t o t a l uplifting pressure exerted o n t h e m u s h r o o m is a b o u t 6.2 kips. This pressure alone will be sufficient t o lift a pier provided it is n o t a d e q u a t e l y a n c h o r e d . It is r e c o m m e n d e d t h a t S o n o t u b e s , or a similar p r o d u c t , b e used t o form t h e u p p e r p o r t i o n of t h e pier t o assure u n i f o r m pier d i a m e t e r . Pier

reinforcement Since t h e lower p o r t i o n of t h e pier is a n c h o r e d i n t o b e d r o c k b y skin friction, and t h e

uplifting pressure is acting u p o n t h e u p p e r w e t t e d p o r t i o n of t h e pier, tensile stress develops w i t h i n t h e pier. T h e m a x i m u m , a m o u n t of tensile force developed can roughly be calculated as follows: T = 27rrsd -P w h e r e : Τ = total tensile force in lbs, and Ρ = t o t a l dead load pressure exerted o n t h e pier in lbs.


96

Figure 47. Typical mushroom 26 inches in diameter on top of a 12-inch-diameter pier. F o r a 12-inch-diameter pier w i t h a 15-kip dead load pressure having 4 feet of u n w e t t e d length, and assuming t h a t t h e skin friction of c l a y s t o n e is 2 , 0 0 0 psf, t h e t o t a l possible uplifting force is 10.1 kips o r 89 psi. This stress can be t a k e n b y lightly reinforced piers; however, w i t h o u t r e i n f o r c e m e n t t h e pier will fail in tension. T h e location of t h e tension cracks is usually at t h e b o u n d a r y of t h e w e t t e d and u n w e t t e d p o r t i o n of t h e pier. This z o n e generally is located at least 3 feet b e l o w grade b e a m ; t h e r e f o r e , t h e n o r m a l d o w e l bars used in t h e piers will n o t provide t h e required resistance t o tension. Generally, 0.6 t o 1.0 p e r c e n t r e i n f o r c e m e n t is sufficient, b u t t h e r e are cases w h e n it is necessary t o use as m u c h as 7 p e r c e n t . R e i n f o r c e m e n t of t h e full length of t h e pier is essential t o avoid tensile failure. A typical tension crack in a pier is s h o w n on figure 48. Air

space T o prevent t h e lower soils from exerting uplifting pressure o n t h e grade b e a m s , it is essential

t h a t t h e r e b e n o c o n t a c t b e t w e e n soil and grade b e a m s . T h e required void space can be formed b y t h e use of sand, c a r d b o a r d , o r o t h e r similar material which can be removed after t h e grade b e a m

97


Figure 48. Tension crack developed at approximately 3 feet below grade beams. is p o u r e d . T h e m o s t convenient m e t h o d is b y t h e use of a void-forming c a r d b o a r d form k n o w n as " V e r t i c e l " . T h e c a r d b o a r d material

is w r a p p e d in plastic and has a d e q u a t e strength t o s u p p o r t

t h e c o n c r e t e b u t will d e t e r i o r a t e after t h e plastic is p u n c t u r e d as s h o w n on figure 4 9 . It is n o t necessary t o r e m o v e t h e c a r d b o a r d material after t h e c o m p l e t i o n of t h e grade b e a m s . T h e cardboard form material also p r o t e c t s t h e backfill soils from plugging u p t h e void space. Figure

98


Figure 49. Deterioration of Vertical (void-forming cardboard) beneath the grade beam.

50 indicates the l o c a t i o n of t h e c a r d b o a r d material. T h e thickness of air space provided b y t h e cardboard ranges from 3 t o 4 inches. It is assumed t h a t the a m o u n t of e x p a n s i o n of t h e soils b e n e a t h the grade b e a m will n o t exceed 3 inches; h o w e v e r , there have been installations where a 4-inch air space was c o m p l e t e l y closed b y highly expansive soils. In e x t r e m e cases, it m a y be necessary t o provide a 6-inch air space.

Pier

settlement If piers are drilled d e e p i n t o b e d r o c k t o provide t h e necessary anchorage in a swelling soil

area, pier s e t t l e m e n t should n o t pose a p r o b l e m . U n d e r n o r m a l pier load, t h e m a g n i t u d e of pier s e t t l e m e n t in c o m p e t e n t b e d r o c k should b e b e t w e e n 1/4 and 1/2 inches. However, t h e r e are cases w h e r e excessive s e t t l e m e n t of the pier has t a k e n place resulting in severe cracking of t h e building. These cases c o m m o n l y o c c u r in small projects w h e r e t h e r e is n o c o n s t r u c t i o n c o n t r o l and t h e pier driller d e t e r m i n e s t h e length of t h e pier and the a m o u n t of p e n e t r a t i o n . If the pier is n o t b o t t o m e d on b e d r o c k b u t instead o n the u p p e r stiff clays, excessive s e t t l e m e n t can occur. Stiff clays s o m e t i m e s have a strong resemblance t o claystone b e d r o c k , and a m i s t a k e n identification of t h e bearing soil occasionally occurs. A small a m o u n t of d r y or plastic c u t t i n g s o n t h e b o t t o m of a pier hole will n o t affect the bearing capacity of t h e pier. If, however, there are 1 or 2 inches of soft soils at the b o t t o m of t h e pier hole and c o n c r e t e is p o u r e d o n t h e soft m u d , t h e n excessive s e t t l e m e n t can occur. Such p h e n o m e n o n usually takes place in small-diameter piers (10 t o 12 inches in d i a m e t e r ) w h e r e t h e driller is u n a b l e t o remove t h e m u d a c c u m u l a t e d at the b o t t o m of the pier hole b y spinning the auger. S e t t l e m e n t of a single pier can usually be bridged b y t h e adjacent piers and is usually u n n o t i c e d . Bearing capacity of t h e pier is usually m u c h larger t h a n t h e design bearing value. An


99

Figure 50. The use of Verticel (void-forming cardboard) beneath the grade beam.

e x c e p t i o n is t h e c h i m n e y of a h o u s e . C h i m n e y f o u n d a t i o n pads are generally s u p p o r t e d b y t w o t o t h r e e piers. S e t t l e m e n t of o n e pier can cause visible lifting and separation of t h e c h i m n e y as s h o w n o n figure 5 1 . Void in pier

shaft

Voids in pier shafts have b e e n of great c o n c e r n t o f o u n d a t i o n engineers since t h e p r o b l e m developed in a major s t r u c t u r e in Chicago, 111. Voids o r discontinuities in t h e pier shaft often result w h e n a c o n c r e t e which is t o o stiff is used. In a d d i t i o n , voids are caused b y collapse of t h e

100


Figure 51. Separation between chimney and house caused by pier settlement.

casing, b y squeezing of t h e soft f o r m a t i o n , or b y hang-up of c o n c r e t e in t h e casing while being pulled. F o r t u n a t e l y , piers drilled in expansive soil areas often e n c o u n t e r stiff clays and the p r o b l e m of squeezing of t h e soft

formation

seldom takes place. However, cases are k n o w n where

reinforcing cage was inserted i n t o cased small-diameter piers w i t h stiff c o n c r e t e resulting in voids. Uplifting of foundation

walls

Theoretically, w i t h a carefully designed grade b e a m and pier s y s t e m , there should be n o m o v e m e n t of a building even u n d e r severe w e t t i n g c o n d i t i o n s . However, an i m p o r t a n t factor c o n t r i b u t i n g t o t h e m o v e m e n t of a building w h i c h is generally ignored is t h e possibility of uplifting pressure e x e r t e d o n t h e e x t e r i o r surface of t h e b a s e m e n t walls and grade b e a m s .


101

Assuming t h a t t h e soil has a swelling pressure of 1 0 , 0 0 0 psf and backfill is in c o n t a c t w i t h 8 feet of height of t h e e x t e r i o r wall, t h e n t h e c o n t a c t area b e t w e e n t h e soil and c o n c r e t e is 8 square feet per foot of wall. In e x t r e m e cases w i t h c o m p l e t e w e t t i n g , t h e uplfiting pressure along t h e face of t h e c o n c r e t e b a s e m e n t wall will be 10,000 χ 0.15 χ 8 = 1 2 , 0 0 0 lbs per r u n n i n g foot of t h e wall. With piers at 15-foot intervals, t h e uplifting pressure exerted o n each pier w o u l d be a b o u t 180 kips. T h e above calculation is based on an e x t r e m e case. In actual c o n d i t i o n s , such an uplifting pressure seldom occurs because t h e backfill a r o u n d t h e b a s e m e n t walls is loosely c o m p a c t e d and c o m p l e t e w e t t i n g of t h e backfill rarely takes place. However, swelling pressure acting along t h e face of t h e b a s e m e n t walls c a n n o t be ignored. All backfill material a r o u n d t h e b a s e m e n t walls, in expansive soil areas, should consist of n o n e x p a n s i v e soils t o m i n i m i z e t h e risk of wall uplift. Lateral pressure

on foundation

walls

Backfill against t h e b a s e m e n t wall n o t o n l y exerts uplifting pressure o n t h e wall, b u t also exerts full h o r i z o n t a l e x p a n s i o n pressure against t h e wall. This pressure equals at least t h e full swelling pressure of t h e soil. In an 8-foot b a s e m e n t backfilled w i t h expansive soils, and w i t h an e x p a n s i o n pressure of 1 0 , 0 0 0 psf, t h e a m o u n t of lateral pressure e x e r t e d o n t h e wall can r e a c h as high as 8 0 kips per r u n n i n g foot of t h e wall. It should be n o t e d t h a t t h e lateral pressure d u e t o swelling differs from active e a r t h pressure in t h a t it is of u n i f o r m i n t e n s i t y , d e p e n d i n g o n l y u p o n t h e d e p t h of w e t t i n g . Lateral m o v e m e n t of t h e b a s e m e n t walls is p r e v e n t e d at t h e t o p b y floor joists w h i c h are a n c h o r e d t o t h e wall b y m e a n s of w o o d e n sills and a n c h o r b o l t s and at t h e b o t t o m of floor slabs. T h e F H A Specifications call for t h e use of 1 /2-inch bolts e m b e d d e d n o t less t h a n 6 inches w i t h a m a x i m u m spacing n o t less t h a n 4 feet. T h e size and spacing of b o l t s is insufficient t o p r e v e n t lateral wall m o v e m e n t . Several h o u s e s were e x a m i n e d where t h e long side of t h e b a s e m e n t wall actually assumed a b o w shape w i t h m a x i m u m deflection in excess of 3 inches and all of t h e a n c h o r b o l t s were b e n t and p u s h e d in. T o r e d u c e such lateral m o v e m e n t , selected backfill material is desirable. In all cases, backfill material should b e n o n e x p a n s i v e and impervious. N o n e x p a n s i v e material will m i n i m i z e t h e expansive pressure e x e r t e d o n t h e wall, and at t h e same t i m e , well c o m p a c t e d , impervious backfill will p r e v e n t surface w a t e r from seeping t h r o u g h t h e backfill i n t o t h e f o u n d a t i o n soils. Rise of ground

water

T h e t h e o r y b e h i n d t h e drilled pier system is t h a t t h e r e must be sufficient d e a d load pressure and t h e pier m u s t be long e n o u g h so t h a t t h e lower p a r t of t h e pier is e m b e d d e d in a z o n e unaffected b y m o i s t u r e change. Assuming t h a t surface w a t e r will n o t p e n e t r a t e m o r e t h a n a b o u t 15 feet, a 20-foot-long, heavily loaded pier should theoretically b e free from a n y possible movement. T h e e x c e p t i o n being t h e rise of g r o u n d water w h e r e t h e o n c e dry p o r t i o n of soil s u r r o u n d i n g t h e pier b e c o m e s c o m p l e t e l y s a t u r a t e d . T h e skin friction used for pier w i t h h o l d i n g is n o w c o m p l e t e l y lost, and t h e pier will lift.

102


Rising of g r o u n d w a t e r can s o m e t i m e s cause expansion of soil in an otherwise relatively stable soil area. In t h e Cherry Creek D a m area of Denver, houses generally were f o u n d e d w i t h spread footings and had n o e x p a n s i o n p r o b l e m s for m a n y years. In 1 9 6 5 , during t h e area flood, t h e w a t e r level at Cherry Creek D a m reached an all time high and t h e g r o u n d - w a t e r level in the surrounding

area

rose. S u b s e q u e n t l y , m a n y

cases of cracked houses were r e p o r t e d .

Such

f o u n d a t i o n m o v e m e n t was directly a t t r i b u t e d t o t h e rise of g r o u n d water. REFERENCES [43] Woodward, R., Gardner, W. S. and Greer, D. M., "Drilled Pier Foundation," McGraw-Hill Book Company, 1972. [44] Parcher, J. V. and Liu, P. C , "Some Swelling Characteristics of Compacted Clays," Journal of the Soil Mechanics & Foundation Division, ASCE, Vol. 9 1 , pp. 1-17. [45] Mohan, D. and Chandra, S., "Frictional Resistance of Bored Piles in Expansive Clays," Geotechnique, Vol. XI, No. 4, pp. 294-301. [46] Seed, H. B., Mitchell, J. K. and Chan, C. K., "Studies of Swell and Swelling Pressure Characteristics of Compacted Clays," Highway Research Board Bulletin 313. [47] Teng, W. C , "Foundation Design," Prentice-Hall, Inc. 1962.

Chapter

5

FOOTING FOUNDATIONS

INTRODUCTION F o o t i n g f o u n d a t i o n s can be successfully placed o n expansive soil provided one or m o r e of t h e following criteria are m e t : 1. Sufficient dead load pressure is e x e r t e d on t h e f o u n d a t i o n , 2. T h e s t r u c t u r e is rigid e n o u g h so t h a t differential heaving will n o t cause cracking, or 3 . T h e swelling p o t e n t i a l of the f o u n d a t i o n soils can be eliminated o r r e d u c e d .

CONTINUOUS FOOTINGS T h e m o s t c o m m o n t y p e of f o u n d a t i o n for lightly loaded structures is the

continuous

footings. Local building codes s o m e t i m e s specify the m i n i m u m allowable w i d t h of footing as 2 0 inches w h i c h is n o t applicable for footings which are t o be placed on expansive soils. T o c o n c e n t r a t e sufficient dead load pressure o n expansive soils, t h e w i d t h of the footing should b e as n a r r o w as possible. It should be n o t e d t h a t c o n t i n u o u s spread footings c a n n o t be e x p e c t e d t o function well in highly expansive soil areas. T h e use of this system should be limited t o soils w i t h a low degree of e x p a n s i o n ; those having a swelling p o t e n t i a l of less t h a n 1 p e r c e n t and a swelling pressure of less t h a n 3 , 0 0 0 psf. Generally, t h e dead load pressure e x e r t e d o n a c o n t i n u o u s f o u n d a t i o n is low and in t h e following range:

Single story schools

2 , 0 0 0 to 4 , 0 0 0 lb. per ft.

Basement h o u s e

1,000 t o 1,500 lb. per ft.

Butler t y p e building

< 5 0 0 lb. per ft.

T o insure t h a t a d e a d load pressure of at least 1,000 psf is exerted on t h e soil, it will be necessary t o use very n a r r o w footings, in m o s t cases less t h a n 12 inches wide.

Wall

footings Engineers often specify t h e erection of b a s e m e n t walls directly o n t h e soil w i t h o u t t h e use

of footings. This reduces t h e bearing w i d t h t o a b o u t 9 inches and increases considerably t h e u n i t dead

load pressure e x e r t e d on the soils. Such a c o n c e p t is sound from the expansive soil


104

s t a n d p o i n t . However, care should b e exercised t o insure t h e rigidity of t h e system b y checking t h e following c o n d i t i o n s before c o n s t r u c t i o n begins: 1. D e t e r m i n e if t h e r e are any soft p o c k e t s in t h e excavation t h a t m a y i n t r o d u c e s e t t l e m e n t , 2. Insure t h a t t h e r e is sufficient c o n t i n u o u s r e i n f o r c e m e n t in t h e f o u n d a t i o n wall t o provide rigidity, and 3. M a k e sure t h a t t h e walls are p r o p e r l y restrained against e a r t h pressure. A n e x t r e m e case recently occurred w h i c h involved a wall bearing directly u p o n expansive soil. T h e u p p e r wall heaved and i m p a r t e d h o r i z o n t a l pressure t o t h e b a s e m e n t wall resulting in heavy b o w i n g of t h e wall even before t h e h o u s e was c o m p l e t e d (figs. 52 and 5 3 ) . Box

construction T h e use of heavy r e i n f o r c e m e n t in t h e f o u n d a t i o n wall can p r o t e c t t h e s t r u c t u r e from

cracking d u e t o differential heaving. T h e average height of a c o n c r e t e b a s e m e n t wall is 6 feet. Such walls can span an u n s u p p o r t e d length of at least 10 feet, and can therefore tolerate considerable differential m o v e m e n t w i t h o u t exhibiting cracks. Weak p o i n t s do a p p e a r at p o i n t s of d i s c o n t i n u i t y , such as d o o r s , d e e p w i n d o w s , and change of elevation. Box c o n s t r u c t i o n is based u p o n t h e principle t h a t t h e r e is n o d i s c o n t i n u i t y of s t r u c t u r e ; therefore, t h e r e are n o weak sections. Box c o n s t r u c t i o n is e c o n o m i c a l for s t r u c t u r e s having simple configurations. F o r split-level residential h o u s e s o r b a s e m e n t s with walk-out d o o r s , such c o n s t r u c t i o n is m o r e difficult. Consideration should t h e n be given t o t h e use of a c o n s t r u c t i o n j o i n t t o separate t h e s t r u c t u r e i n t o t w o o r m o r e u n i t s . Each u n i t will t h e n act i n d e p e n d e n t l y and differential m o v e m e n t can be confined t o t h e j o i n t s . S. Shraga and D. A m i r [ 4 8 ] r e p o r t e d t h e use of b o x c o n s t r u c t i o n in K i b u t z G a t , Israel, where t h e s t r u c t u r e consists of t w o reinforced c o n c r e t e b o x e s each a b o u t 2 2 b y 35 feet in dimension. T h e s t r u c t u r e did n o t exhibit damage after

17 years despite t h e considerable

differential m o v e m e n t s of u p t o 5 inches b e t w e e n t h e corners of individual b o x e s . Shraga and A m i r c o n c l u d e d t h a t b o x c o n s t r u c t i o n can structurally w i t h s t a n d m o v e m e n t and tension w i t h o u t cracking. Masonry bricks and cinder blocks c a n n o t w i t h s t a n d m o v e m e n t and should n o t be used for f o u n d a t i o n walls. T h e small saving derived from using m a s o n r y c o n s t r u c t i o n instead of c o n c r e t e f o u n d a t i o n walls m a y later result in heavy loss of p r o p e r t y in t h e event of f o u n d a t i o n m o v e m e n t . Reinforced b r i c k w o r k has b e e n widely used in S o u t h Africa. D. L. Webb [ 4 9 ] r e p o r t e d t h e use of r e i n f o r c e m e n t in t h e e x t e r n a l wall panels b e t w e e n j o i n t s t o resist b e n d i n g stresses and shear stresses resulting from f o u n d a t i o n m o v e m e n t . T h e a r r a n g e m e n t is s h o w n o n figure 54.

PAD F O U N D A T I O N S T h e pad f o u n d a t i o n system consists essentially of a series of individual footing pads placed o n t h e u p p e r soils and s p a n n e d b y grade b e a m s . T h e principle of a pad f o u n d a t i o n system is similar t o t h a t of a drilled pier f o u n d a t i o n in t h a t t h e load of t h e s t r u c t u r e is c o n c e n t r a t e d at several p o i n t s , t h e difference being t h a t t h e pads bear o n t h e u p p e r soils and skin friction is n o t involved.

FOOTING FOUNDATIONS

105

Swelling Pressure

SECTION

t 1

Side

Wall^

Basement

I

1 PLAN VIEW Figure 52. Plan and section of foundation wall bearing directly on expansive soil. Note pressure distribution.

106


Figure 53. Foundation wall bearing directly on expansive soil without footings. Heaving has pushed the side wall toward the basement wall resulting in heavy bowing of the basement wall. U n d e r t h e following c o n d i t i o n s , the use of a pad f o u n d a t i o n system can be advantageous: 1. Where b e d r o c k o r bearing s t r a t u m is deep and c a n n o t be e c o n o m i c a l l y reached b y drilled piers, 2. Where t h e w a t e r table or a soft layer exists preventing t h e use of a friction pier, 3 . Where t h e u p p e r soils possess m o d e r a t e swell p o t e n t i a l , and 4. Where t h e bearing capacity of t h e u p p e r soils is relatively high. Design By loading an expansive soil so t h a t the pressure e x e r t e d on the soil is greater t h a n the swelling pressure of t h e soil, heaving m o v e m e n t can be p r e v e n t e d . By using an individual pad f o u n d a t i o n system, it is theoretically possible t o exert any desirable dead load pressure. Actually, t h e capacity of t h e pad is limited b y t h e allowable bearing capacity of t h e f o u n d a t i o n soils. If a pad is f o u n d e d directly on b e d r o c k , m a x i m u m allowable soil pressure will n o t pose a p r o b l e m . However, if t h e pads are placed on stiff swelling clays, t h e m a x i m u m bearing capacity of t h e pad is limited b y the u n c o n f i n e d compressive strength of clay. Generally, t h e m a x i m u m bearing capacity should be a b o u t 5 , 0 0 0 psf. C o n s e q u e n t l y , the practical dead load pressure t h a t can be applied t o t h e pad is a b o u t 3 , 0 0 0 psf (considering the ratio of dead and live load t o be a b o u t 2 t o 3). Occasionally, pads f o u n d e d on clay are designed t o w i t h s t a n d a dead

107

FOOTING FOUNDATIONS

Figure 54. Section through externally reinforced brick wall. (After D. L. Webb)

load pressure as high as 5 , 0 0 0 psf. With this limitation, an individual pad f o u n d a t i o n system can only be used in those areas where t h e soils possess only a m e d i u m degree of e x p a n s i o n w i t h v o l u m e change—on t h e o r d e r of 1 t o 5 p e r c e n t and a swelling pressure in t h e range of 3 , 0 0 0 t o 5,000 psf. T o allow for t h e c o n c e n t r a t i o n of dead load pressure on t h e individual p a d s , a void space is required b e n e a t h the grade b e a m and should b e c o n s t r u c t e d in the same m a n n e r as grade b e a m s and pier system (fig. 5 5 ) . Figure 56 shows a grade b e a m and pad f o u n d a t i o n system t h a t failed because the dead load pressure was n o t sufficient t o prevent t h e heaving of t h e f o u n d a t i o n soils. Peck, H a n s o n & T h o r n b u r n stated in t h e second edition of F o u n d a t i o n Engineering t h a t , "Swelling can b e p r e v e n t e d only in a localized z o n e b e n e a t h t h e footings or piers w h e r e the stressed i n d u c t e d b y t h e f o u n d a t i o n are c o n c e n t r a t e d . " This is s h o w n o n figure 5 7 .

108


Figure 55. Grade beams and pads constructed with void space between pads.

A t a comparatively shallow d e p t h b e n e a t h t h e f o u n d a t i o n , t h e intensity of a d d e d stress is small and swelling m a y occur b e l o w this level, even if it is entirely prevented above. In t h e area b e t w e e n t h e footings, swelling is u n d i m i n i s h e d .

Deep

pads In areas w h e r e t h e layer of swelling soils is relatively t h i n , d e e p individual pads placed on

nonswelling soil can be e c o n o m i c a l l y used. A typical e x a m p l e is w h e r e 2 t o 3 feet of swelling clays are underlain b y sand and gravel o r b y nonswelling b e d r o c k such as granite or s a n d s t o n e . Pads placed as d e e p as 5 feet b e l o w t h e g r o u n d surface can b e economically used in areas where drill rigs are unavailable. Care should be exercised t o insure t h a t uplifting pressure will n o t b e exerted on t h e sides of t h e pad. T h e excavation should be larger t h a n t h e footing pad and t h e space b e t w e e n t h e c o n c r e t e and t h e soil filled w i t h loose backfill. T h e use of a d e e p pad system usually applies t o c o n s t r u c t i o n areas where the p r o b l e m soil ranges in thickness from 0 t o 5 feet. In such cases, it is desirable t o place all footing pads o n uniform nonswelling soils. In t h o s e parts of t h e world w h e r e hand labor is inexpensive and drilling e q u i p m e n t n o t readily available, t h e use of a d e e p pad system can be an advantage from a cost consideration.

FOOTING FOUNDATIONS

109

Figure 56. Typical crack which developed in the basement of house founded with grade beams and individual pads. Dead load pressure was not sufficient to prevent pad uplift.

Interrupted

footing

I n t e r r u p t e d footings are used in c o n j u n c t i o n w i t h a wall footing system. With f o u n d a t i o n walls bearing directly o n swelling soil, the m a x i m u m unit dead load pressure exerted on t h e soil is a b o u t 2 , 0 0 0 lbs. p e r ft. By placing a void space at intervals, t h e bearing area will be decreased, t h u s increasing t h e dead load pressure. In this m a n n e r , t h e dead load pressure exerted on t h e soil can b e easily d o u b l e d . This principle of i n t e r r u p t e d footings has b e e n successfully applied to t h e c o r r e c t i o n of cracked buildings having a c o n t i n u o u s footing f o u n d a t i o n . By i n t r o d u c i n g some void space b e n e a t h the footings, t h e dead load pressure can b e substantially increased, t h u s preventing further f o u n d a t i o n m o v e m e n t s . (See Case S t u d y IV for illustrations)

FOOTINGS ON SELECTED FILL T h e removal of n a t u r a l expansive soils and their r e p l a c e m e n t w i t h n o n e x p a n s i v e soil is t h e m o s t obvious m e t h o d of preventing structural d a m a g e d u e t o soil heaving. In a few cases, it m a y be possible t o c o m p l e t e l y r e m o v e t h e expansive strata, t h u s eliminating t h e heaving p r o b l e m . In m o s t cases, t h e expansive material e x t e n d s t o t o o great a d e p t h t o allow c o m p l e t e removal and


110

Figure 57. Diagram illustrating influence on swelling of high contact pressure beneath footing. If net pressure at base of footing is 8,000 Ib/sq ft and swelling pressure at zero volume change is 2,000 Ib/sq ft, swelling will be prevented within shaded areas only. (After Peck, Hanson & Thornburn)

backfill. T h e p r o b l e m is t h e n t o d e t e r m i n e t h e a m o u n t of excavation and t h e t y p e of backfill required to prevent heaving. Detail discussion is given in c h a p t e r 8 u n d e r "Soil R e p l a c e m e n t . " Probably t h e m o s t i m p o r t a n t single factor affecting t h e success of footings placed on selected

fill is the drainage c o n t r o l used during c o n s t r u c t i o n . If t h e excavation is w e t t e d

excessively before t h e p l a c e m e n t of t h e selected fill, t h e imbibed m o i s t u r e in t h e soil will cause t h e soil t o swell and heave and exert

pressure against t h e selected fill resulting in severe damage

t o t h e s t r u c t u r e . Many school buildings have been successfully placed on selected fill, b o t h for the entire system and for slabs alone. Coincidentally, t h e r e have also been failures w h e n such c o n s t r u c t i o n has b e e n used, mainly because t h e site was flooded during c o n s t r u c t i o n . F o r success in placing footings and slabs on selected fill, t h e following p r e c a u t i o n s should be observed: 1. T h e r e should be at least 3 feet of selected fill b e n e a t h t h e b o t t o m of footings and slabs. 2. T h e fill should e x t e n d b e y o n d the building line for a distance of at least 10 feet in every direction. 3 . T h e fill should consist of n o n e x p a n s i v e soil, preferably impervious and granular. 4. T h e fill should be c o m p a c t e d

t o at least 9 0 p e r c e n t standard P r o c t o r density

for

s u p p o r t i n g slabs and 100 p e r c e n t standard P r o c t o r density for s u p p o r t i n g footings. 5. Before t h e p l a c e m e n t of fill, care should be t a k e n t o avoid t h e excessive w e t t i n g of n a t u r a l soils.

111

FOOTING FOUNDATIONS

MAT FOUNDATION Mat

foundations,

sometimes

referred

to as structural slab-on-ground

or reinforced

and

stiffened slabs, are considered to be b o t h a load s u p p o r t i n g as well as a separating e l e m e n t . T h e slab receives and t r a n s m i t s all t h e structural load t o t h e underslab soils. T h e slab should be designed t o resist b o t h t h e positive and the negative m o m e n t . Positive m o m e n t includes t h a t induced by b o t h dead and live load pressure e x e r t e d o n t h e slab. Negative m o m e n t consists mainly of those pressures caused b y t h e swelling of the underslab soils. Since t h e swelling pressure in an expansive soil area can reach m a n y t h o u s a n d p o u n d s per square foot, negative m o m e n t consideration generally controls t h e design of t h e m a t f o u n d a t i o n . If all structural e l e m e n t s are t o be placed o n a stiffened slab, t h e n slab m o v e m e n t will n o t affect t h e stability of t h e s t r u c t u r e . However, there could be tilting of t h e m a t , b u t t h e p e r f o r m a n c e of t h e building would n o t be structurally affected. Such c o n c e p t i o n has been studied b y t h e "Building Research Advisory B o a r d " [ 5 0 ] . A s t u d y on the w o r k in t h e R o c k y M o u n t a i n areas indicates t h a t there are limitations o n the use of such a system as follows: 1. T h e success of such system so far is limited to m o d e r a t e swelling soil areas. 2. Configuration of building m u s t be relatively simple. 3 . T h e load exerted o n t h e f o u n d a t i o n m u s t be light. Past p e r f o r m a n c e has b e e n limited t o residential c o n s t r u c t i o n . 4. Single level c o n s t r u c t i o n is required. It would be difficult t o apply such c o n s t r u c t i o n t o b a s e m e n t houses w i t h an a t t a c h e d garage o r split level houses. Design T h e design of a m a t f o u n d a t i o n is generally based o n t h e following p a r a m e t e r s : 1. Slab d i m e n s i o n s , 2. T h e s u p p o r t index, and 3 . T h e dead and live load acting on the slab. F r o m the above p a r a m e t e r s , the designer m u s t develop a s t r u c t u r e capable of satisfying the shear, bending m o m e n t , and deflection c o n d i t i o n s . T h e first step in t h e design is t o d e t e r m i n e the s u p p o r t i n d e x . T h e s u p p o r t index is based u p o n t h e climatic rating, plasticity i n d e x , and length-to-width ratio of the f o u n d a t i o n . It is assumed t h a t t h e m o i s t u r e c o n t e n t in t h e soil is affected b y climatic c o n d i t i o n s , and the v o l u m e change of the soil is affected b y m o i s t u r e c o n t e n t . C o n s e q u e n t l y , b o t h t h e swelling and the s e t t l e m e n t of the soil will be affected b y climate. A s t u d y of w e a t h e r d a t a indicates t h a t the yearly annual p r e c i p i t a t i o n , d i s t r i b u t i o n of p r e c i p i t a t i o n , frequency of p r e c i p i t a t i o n , d u r a t i o n of precipitation, and a m o u n t of each precipitation all affect the consistency of climate. Based on d a t a o b t a i n e d from

122 w e a t h e r stations, t h e U. S. National Weather Service has developed

i n f o r m a t i o n which has been transformed i n t o frequency isolines on a m a p of t h e C o n t i n e n t a l United States as shown on figure 5 8 . F r o m figure 5 8 , the climate rating C instance, in C o l o r a d o , t h e climate rating is b e t w e e n 2 0 and 2 5 .

w

is selected. F o r


112

Figure 58. Climatic ratings C w for Continental United States, (after Federal Housing Administration)

T h e second major factor necessary for design is t h e s u p p o r t index. T h e s u p p o r t index is directly related t o t h e climate factor and the soil p r o p e r t i e s . Figure 59 shows t h e relationship of t h e various p r o p e r t i e s . T h e soil p r o p e r t i e s are related t o t h e A t t e r b e r g limits, percent swell in t h e P V C m e t e r , and swell i n d e x . O f these, t h e swell index is t h e m o s t reliable factor of t h e three for predicting p o t e n t i a l v o l u m e change of t h e f o u n d a t i o n soils. Soils w i t h identical plasticity index exhibit greatly varying swell p o t e n t i a l . Also, t h e PVC m e t e r is based o n testing soils in a r e m o l d e d state which can materially differ from t h a t in the u n d i s t u r b e d state. T o o b t a i n t h e swell i n d e x , the percentage swell for a specific soil s t r a t u m should be o b t a i n e d t h r o u g h swell tests using c o n v e n t i o n a l c o n s o l i d o m e t e r test e q u i p m e n t

on undisturbed

soil

samples and pressure c o r r e s p o n d i n g t o t h e in situ o v e r b u r d e n pressure plus the average of the t o t a l dead and live loads o n t h e slab. T h e u n d i s t u r b e d samples should be o b t a i n e d u n d e r soil m o i s t u r e c o n d i t i o n s representative of c o n d i t i o n s prevailing at the time of c o n s t r u c t i o n . With t h e swell i n d e x or t h e p e r c e n t of swell u n d e r specific loading c o n d i t i o n and the climate rating d e t e r m i n e d , t h e s u p p o r t i n d e x can t h e n be d e t e r m i n e d from figure 5 9 . T h e design of the stiffened slab section will b e based o n the value of t h e s u p p o r t i n d e x . R. L. L y t t o n and J. A. W o o d b u r n

[51]

of T e x a s A & M University have

performed

considerable research o n t h e design p r o c e d u r e for stiffened m a t s o n expansive clay. L y t t o n and

113

FOOTING FOUNDATIONS

ΙΟ I

20

30

I.I I

3.0

4.8

1.0

2.0 1

3.0

1

1

1 1 1

40 1 64 1 4.0 1

50

60

70

7.7

8.9 1

10.0 1

5.0 1

6.0

7.0

1

1

1

"Ρ

no I 80 1

9p

PI

12.0

PVC

9.0 Swell Index (%) 1

Figure 59. Support index C based upon criterion for soil sensitivity and climatic rating C w. (after Federal Housing Administration) W o o d b u r n d e t e r m i n e d t h e s u p p o r t index algebraically from t h e average f o u n d a t i o n pressure, subgrade m o d u l u s , m a x i m u m e x p e c t e d differential heave of t h e soil and t h e m o u n d e d area. L y t t o n and W o o d b u r n p r e p a r e d a n o m o g r a p h for d e t e r m i n i n g t h e s u p p o r t i n d e x as s h o w n o n figure 6 0 . With t h e s u p p o r t i n d e x d e t e r m i n e d , t h e design of t h e m a t f o u n d a t i o n is within the realm of a structural engineer. Typical m a t f o u n d a t i o n design is s h o w n o n figure 6 1 . Behavior Stiffened slab c o n s t r u c t i o n has been widely used in s o u t h e r n Texas w h e r e m o d e r a t e swelling soils are e n c o u n t e r e d . T h e so-called waffle slabs have b e e n in use in San A n t o n i o for m o r e t h a n 25 years and are also required for F.H.A.-sponsored c o n s t r u c t i o n in M o n t g o m e r y , A l a b a m a . In Denver, such f o u n d a t i o n system was first considered in 1 9 7 0 . S u b s e q u e n t l y , 5 2 houses were built using waffled slabs in West Field Park, Jefferson C o u n t y . T h e soils are generally stiff clays w i t h a plasticity i n d e x ranging from a low of 3.6 t o a high of 3 2 . 1 . T h e swell p o t e n t i a l ranged from a low of 1 to a high of 5.5 p e r c e n t u n d e r l o a d s ranging from 5 0 0 t o 1,000 psf. T h e soils are considered t o have m o d e r a t e swell p o t e n t i a l . In t h e same year, 12 h o u s e s were built using t h e stiffened slab system in Lake A r b o r Subdivision in N o r t h

Denver. T h e soils in t h e Lake A r b o r area possess m u c h higher swell

p o t e n t i a l t h a n t h a t of West Field Park, exhibiting swelling pressure as high as 10,000 psf. Typical design and c o n s t r u c t i o n details of these houses are given in figures 62 t h r o u g h 6 8 . A survey of these h o u s e s was m a d e in 1974 and their c o n d i t i o n was excellent. N o n e of these houses exhibited n o t i c e a b l e cracks. Differential elevation b e t w e e n t h e o p p o s i t e corners of t h e h o u s e in s o m e cases reached 1 inch, b u t distress, either in t h e interior or t h e e x t e r i o r , has n o t

114


Figure 60. Support index nomograph (After R. L. Lytton and J. A. Woodburn).

Figure 61. Typical mat foundation design. (Sheet 1 of 2)

t a k e n place. In t h e same Lake A r b o r area, some o t h e r houses have required r e p l a c e m e n t of their b a s e m e n t floor slab t h r e e times within 4 years. This indicates t h a t t h e stiffened slab system used for t h e 12 h o u s e s appears t o b e highly successful. In t h e Denver area d u r i n g 1970 t o 1 9 7 1 , t h e increased cost of using stiffened slabs r a t h e r t h a n c o n v e n t i o n a l pier and grade b e a m system was a b o u t 50 cents p e r square foot. This a m o u n t s t o an increase in cost of a b o u t $ 7 5 0 per h o u s e w h i c h is negligible w h e n c o m p a r e d t o t h e p r o b l e m s and rehabilitation costs e n c o u n t e r e d w h e r e t h e stiffened slab system had n o t b e e n used. As stated

in t h e opening r e m a r k s of the advisory b o a r d

[ 5 0 1 , " I t is recognized

that

experience and t h e state of engineering k n o w l e d g e are such t h a t precise answers t o m a n y of t h e p r o b l e m s posed m u s t , of necessity, be considered b e y o n d a t t a i n m e n t in t h e i m m e d i a t e foreseeable


SECTION Β

SECTION A

-ONE

3/β

β STRAND

SECTION D

SECTION C

TOTA L

3

TENDONS

IN F I R E P L A C E

TWO **5

BAR S

6"xl8" CURTAIN

SECTION

Ε

Figure 6 1 . Typical mat foundation design. (Sheet 2 of 2)

W AL

FOOTIN G FOUNDATION S

117

Figure 62.

Trenching the cross-beams.

Figure 63.

Placing reinforcement.

118


Figure 64. Post-tensioning.

Figure 65. Placing concrete.

FOOTING FOUNDATIONS

119

Figure 66. Completed mat.

Figure 67. Interior partitions.


120

Figure 68. Completed residence founded on mat foundation

foreseeable

future.

Nevertheless,

the

approach

recommended

herein

is considered

to

be

sufficiently valid t o w a r r a n t application n o w . " Based on o u r experience and p e r f o r m a n c e relating t o t h e Denver project, t h e stiffened slab system of c o n s t r u c t i o n can be successfully applied t o low t o m o d e r a t e swelling soil areas. Much research will be required

t o d e t e r m i n e and u n d e r s t a n d

t h e m a n y variables, especially the

relationship, of swelling characteristics w i t h s u p p o r t i n d e x . As discussed previously in " D e s i g n " , t h e s u p p o r t i n d e x should be related t o swelling pressure. T h u s , t h e loading c o n d i t i o n can be eliminated from the design as well as t h e climatic rating.

REFERENCES [48] Shraga, S., Amir, D., and Kassiff, G., "Review of Foundation Practice for Kibbutz Dwelling in Expansive Clay." Proceedings of the Third International Conference on Expansive Soils, 1973. [49] Webb, D. L., "Foundations and Structural Treatment of Buildings on Expansive Clay in South Africa," Second International Research and Engineering Conference on Expansive Clay Soils, Texas A & M Press, 1966. [50] "Criteria for Selection and Design of Residential Slabs-on-Ground," Building Research Advisory Board. [51] Lytton, R. L. and Woodburn, J. Α., "Design and Performance of Mat Foundations on Expansive Clay," Proceedings of the Third International Conference on Expansive Soils, 1973.

Chapter

6

SLABS ON EXPANSIVE SOILS

INTRODUCTION Slab-on-ground c o n s t r u c t i o n , w h e n o n expansive soils, is a very difficult aspect t o c o n t r o l . In t h e category of slabs are interior floor slabs, e x t e r i o r sidewalks or a p r o n s , and p a t i o slabs. Generally, floor slabs d o n o t s u p p o r t any appreciable live load, and t h e dead load actually exerted o n t h e slab is small. C o n s e q u e n t l y , m o v e m e n t of t h e slab is t o be e x p e c t e d w h e n t h e underslab m o i s t u r e c o n t e n t increases, and it should be designed accordingly. T h e m o v e m e n t of slabs n o t only presents unsightly cracks b u t , in m o s t cases, also directly affects t h e stability of the s t r u c t u r e .

SLAB-ON-GROUND C o n c r e t e slabs, placed directly on t h e g r o u n d , are m u c h less expensive t h a n s t r u c t u r a l floor slabs o r "crawl s p a c e " t y p e c o n s t r u c t i o n . This is especially t r u e w h e r e b a s e m e n t c o n s t r u c t i o n is involved. Since 1940, m o s t of t h e residential h o u s e s , school buildings, industrial, and w a r e h o u s e structures call for t h e use of slab-on-ground c o n s t r u c t i o n . It was n o t until t h e discovery of t h e expansive soil p r o b l e m t h a t engineers began t o q u e s t i o n t h e w i s d o m of using slab-on-ground construction. Types of

slab-on-ground

Slab-on-ground, s o m e t i m e s referred t o as slab-on-grade, are c o n c r e t e slabs placed directly o n the ground

w i t h little consideration given t o their structural r e q u i r e m e n t s . These slabs are

c o n s t r u c t e d b o t h w i t h or w i t h o u t r e i n f o r c e m e n t . T h e unreinforced slabs are generally c o n s t r u c t e d in residential houses o r w h e r e light floor load is e x p e c t e d . T h e limits of t h e length of t h e unreinforced slab are based u p o n the a m o u n t of shrinkage cracking c o n t r o l desired. N o r m a l l y , shrinkage cracks are controlled

b y designed

weakened plane j o i n t s . A lightly reinforced slab is n o r m a l l y reinforced w i t h t e m p e r a t u r e c o n t r o l as a p r i m e design factor. T h e Portland C e m e n t Association [ 5 2 ]

r e c o m m e n d e d t h e use of a 4-inch-thick slab

reinforced w i t h 6 x 6 - 1 0 / 1 0 mesh or N o . 3 bar at 24 inches on c e n t e r each w a y for slabs placed in m o d e r a t e l y swelling soil areas. F o r high swelling soil areas, the Association r e c o m m e n d e d the use of 6 x 6 - 6/6 m e s h or N o . 3 b a r at 18 inches on center each w a y . T h e choice b e t w e e n an unreinforced slab and a lightly reinforced slab d e p e n d s u p o n t h e subsoil c o n d i t i o n s as well as t h e loading c o n d i t i o n s . R e i n f o r c e m e n t in t h e slab will r e d u c e t h e

122


opening of t e m p e r a t u r e cracks b u t will n o t prevent cracking of t h e slab caused b y heaving of t h e underslab soils. In 1 9 6 8 , a r e p o r t concerning residential slab-on-ground c o n s t r u c t i o n was prepared by the Building Research Advisory Board [ 5 0 ] for use b y t h e Federal Housing A d m i n i s t r a t i o n which provided criteria for the selection and design of residential floor slabs. T h e r e p o r t r e c o m m e n d e d t h e use of unreinforced c o n c r e t e slabs for firm, n o n e x p a n s i v e soils. N o m i n a l r e i n f o r c e m e n t was recommended

where

the

subgrade

may

undergo

slight m o v e m e n t .

Both

reinforced

and

unreinforced slabs are considered to have the limiting function of separating t h e g r o u n d from living space. Slab-on-ground c o n s t r u c t i o n o n expansive soil will always pose a cracking and heaving p r o b l e m unless t h e subgrade soils are treated or replaced. In commercial buildings such as warehouses and storage areas, w h e r e floor loads as high as 3 , 0 0 0 psf are e x p e c t e d , special design will be required, n o t only from t h e s t a n d p o i n t of expansive soils, b u t also to m a i n t a i n t h e structural integrity

of t h e building. M i n o r floor cracking of slab-on-ground c o n s t r u c t i o n is

difficult if n o t impossible to prevent. Slab

movement In expansive soil areas, floor m o v e m e n t is invariably associated w i t h the increase of m o i s t u r e

c o n t e n t of t h e underslab soils. T h e source of w a t e r t h a t enters i n t o t h e underslab soils can generally be associated w i t h t h e following: 1. Rise of g r o u n d w a t e r , usually perched water, can cause excessive swelling. Heaving of slabs, in excess of 6 inches, is n o t u n c o m m o n as s h o w n on figure 6 9 . Water m a r k s and severe floor cracks indicate t h e e x t e n t of d a m a g e . 2. B r o k e n utility lines often c o n t r i b u t e w a t e r t o t h e underslab soils. Water and sewer lines buried in expansive soils are subject t o stress. Differential heaving can break pipes and cause leakage. Such leakage can c o n t i n u e for a long period of time w i t h o u t being d e t e c t e d . In o n e case, t h e c o n t r a c t o r neglected t o c o n n e c t interior sewer line to t h e street sewer, and this fault w e n t u n d e t e c t e d until extensive damage had taken place. Figure 7 0 shows floor m o v e m e n t in a boiler r o o m . T h e floor drain t o t h e boiler r o o m b e c a m e plugged resulting in severe slab heaving from uplift. 3. A m o s t c o m m o n source of m o i s t u r e entering t h e underslab soils is derived from irrigation, lawn watering, and r o o f d o w n s p r o u t s . Surface w a t e r enters the loose backfill and causes a wetting condition. The above sources of w a t e r t h a t e n t e r t h e underslab soils are t h e obvious ones. Moisture migration d u e t o t h e r m a l differential as m e n t i o n e d in c h a p t e r 2 can also cause d a m a g e t o slab-on-ground w i t h o u t the observance of free water. F l o o r cracking caused b y swelling soils m u s t be differentiated from t h a t caused by shrinkage of c o n c r e t e . In an expansive soil area, soil heaving is unjustly blamed for all cracks t h a t develop in a floor. F l o o r cracks d u e t o heaving generally take place along t h e bearing wall as shown in figure 6 9 . In t h e absence of j o i n t s , shrinkage cracks can t a k e place at a p p r o x i m a t e equally spaced intervals. F o r c o n c r e t e floors covering a large area, the Portland C e m e n t Association r e c o m m e n d s t h e installation of c o n t r o l j o i n t s at intervals of a p p r o x i m a t l e y 20 feet. Isolation j o i n t s separating


•

Figure 69. Differential slab heaving of up t o 12 inches in a newly completed basement.

123

124


Figure 70. Heaving of a floor slab in a boiler room. Source of water derived from inadequate floor drain system. Uplift is 2 inches. concrete slabs from c o l u m n s , footings, or walls t o p e r m i t b o t h h o r i z o n t a l m o v e m e n t d u e t o v o l u m e changes and vertical m o v e m e n t caused b y differential s e t t l e m e n t or heaving are also recommended. Curling of c o n c r e t e slabs in large floor areas d u e t o i m p r o p e r curing is n o t u n c o m m o n . C o n c r e t e curling has a strong resemblance t o uplift of slabs d u e t o heaving of u n d e r s l a b soils. Underslab

gravel

C o n v e n t i o n a l slab c o n s t r u c t i o n uses 4 inches of gravel b e n e a t h all c o n c r e t e floors. A widely accepted t h e o r y pertaining t o slab-on-ground c o n s t r u c t i o n in expansive soil areas is that w a t e r from a single source, such as from a b r o k e n pipe or from an i m p r o p e r l y located d o w n s p o u t , will travel w i t h o u t resistance t h r o u g h o u t t h e gravel bed and saturate t h e entire area u n d e r n e a t h the slab. Therefore, m o r e extensive damage t o t h e floor will take place w h e n t h e gravel is used. T o d a t e , this t h e o r y has n o t b e e n proven. T h e use of gravel b e n e a t h t h e slab allows t h e uniform d i s t r i b u t i o n of floor load and the uniform curing of c o n c r e t e , t h u s reducing t h e shrinkage cracks and s o m e t i m e s t h e curling of c o n c r e t e . T h e m a i n advantage of using gravel b e n e a t h t h e floor slab, however, is t o p r o t e c t t h e building from t h e rise of g r o u n d water. If a perched w a t e r c o n d i t i o n develops b e n e a t h a b a s e m e n t which has n e i t h e r a subdrainage system n o r a gravel bed b e n e a t h t h e slab, there is n o easy m e t h o d of removing t h e water. T h e installation of a subdrainage system inside or outside of a c o m p l e t e d building is a major u n d e r t a k i n g . If, however, free draining gravel has b e e n previously installed

125


b e n e a t h the slab, it m a y only be necessary to install a s u m p p u m p in t h e b a s e m e n t as the w a t e r will flow t h r o u g h t h e gravel t o w a r d the s u m p . In any event, t h e advantages of providing gravel b e n e a t h t h e slab far exceeds any possible disadvantages.

STIFFENED SLABS Slab-on-ground c o n s t r u c t i o n c a n n o t be safely used in an area w h e r e the subsoil possesses high swell p o t e n t i a l . F o r m a n y years, b o t h s t r u c t u r a l and soil engineers a t t e m p t e d t o devise an economical floor system w h i c h would c o m b a t t h e p r o b l e m of swelling soil. U n f o r t u n a t e l y , such a system has n o t b e e n devised t o d a t e . T h e systems n o w include t h e s t r u c t u r a l floor slab, t h e raised floor system, and the h o n e y c o m b system. Structural

floor

slabs

T h e best m e t h o d t o prevent floor m o v e m e n t is t o c o n s t r u c t a s t r u c t u r a l slab s u p p o r t e d o n each side b y grade b e a m s and provide a void b e n e a t h t h e slab t o prevent c o n t a c t b e t w e e n t h e soil and t h e slab. T h e s h o r t c o m i n g s of this system lie n o t only in t h e cost of c o n s t r u c t i o n , w h i c h is much

more

expensive

than

the

conventional

slab-on-ground

m e t h o d , b u t also u p o n

the

construction technique. T h e m o s t c o n v e n i e n t c o n s t r u c t i o n m e t h o d is t o provide a crawl space b e n e a t h t h e slab. This can be readily provided in major structures such as schools and office buildings. T h e crawl space provides access for inspection, can be ventilated, and can also serve as a convenient area for utility pipes and c o n d u i t s . Either t i m b e r o r c o n c r e t e floors can be used in this t y p e of construction. O f t e n t i m e s , it is n o t possible t o c o n s t r u c t a crawl space, and t h e s t r u c t u r a l floor m u s t t h e n be c o n s t r u c t e d w i t h only a few inches of air space b e t w e e n t h e slab and g r o u n d . This is typical w h e r e a structural slab is to be c o n s t r u c t e d in a b a s e m e n t area. T h e p r o b l e m w i t h this t y p e of c o n s t r u c t i o n is t h a t of providing a forming material t o allow t h e placing of c o n c r e t e . T h e use of Verticel, J-void, o r o t h e r forming material similar t o t h a t used b e n e a t h the grade b e a m s in t h e pier f o u n d a t i o n is satisfactory. F o r m i n g materials are costly and t h e r e is n o assurance t h a t t h e material will c o m p l e t e l y d e t e r i o r a t e b e n e a t h t h e slab, t h e r e b y allowing the build-up of uplifting pressure. O n e possible alternative is t h e use of balloons as a forming material w h i c h could b e deflated after t h e c o n c r e t e has reached its initial set. C o m m e r c i a l prestressed, hollow core, flat slabs are available in sufficient length to span 20 feet, t h e r e b y eliminating t h e need for void forming material. T h e use of prestressed slabs in large q u a n t i t i e s can prove t o be e c o n o m i c a l . Raised floor

system

T h e Portland C e m e n t Association [ 5 2 ] has a p p r o a c h e d t h e p r o b l e m of t h e c o n s t r u c t i o n of a structural floor slab on expansive soils b y utilizing a c o n c r e t e floor raised above grade b y intersecting c o n c r e t e ribs f o r m e d in a waffle p a t t e r n .


126

T h e raised boxes)

upon

a

floor

system is c o n s t r u c t e d by placing Verticel or J-void (waxed c a r d b o a r d

level

subgrade.

The

spaces

between

the

boxes

contain

reinforcing

and

form-supporting concrete ribs. T h e actual floor slab, containing wire fabric, is placed over the s u p p o r t i n g ribs and c a r d b o a r d b o x e s in a m o n o l i t h i c c o n c r e t e p l a c e m e n t . A typical plan and cross-section is s h o w n on figure 7 1 . T h e spacing of the ribs and the thickness of the slab d e p e n d s u p o n the swelling p o t e n t i a l of t h e surface soils and t h e dead load imposed on t h e waffle s t r u c t u r e . T h e advantage of such a system is t h a t it offers a clear, rational a p p r o a c h for t h e structural engineers, and the formed voids provide a m e a n s of relieving u p w a r d swelling pressure. T h e system can also i n c o r p o r a t e utility r o u t e s , such as heating and cooling, t h r o u g h the floor. T h e disadvantage of such a floor system is t h e inability t o e x e r t sufficient dead load pressure u p o n the ribs t o c o u n t e r a c t the swelling pressure. This t y p e of c o n s t r u c t i o n is also expensive. In a d d i t i o n , t h e floor area m u s t be very finely graded

to provide a level base for t h e void forming material so t h a t

uniform

thicknesses result. This grading is an additional cost.

i

' TYPICAL

1

1 ' ,.' 32 SQ. BOXES

1

—

EVERY 3 6 " ON CENTER EA CH WAY , 2 1/2 si_AB

—

I

SECTION

1 FLOOR

A

1 Values and dimensions are i l l u s t r a t i v e purposes only.

PLAN

for

Figure 71. Raised concrete floor system (after Portland Cement Association).

Honeycomb The

system development

of

the

honeycomb

system

was based

upon

the

assumption

that

comparatively slight m o v e m e n t s of some clays reduce or relieve swelling pressures [ 5 3 ] . The f o u n d a t i o n consists of longitudinally split S o n o t u b e s t h a t are placed w i t h t h e openings t o w a r d the soil as s h o w n o n figure 7 2 , t h e b o t t o m 2 inches of the space b e t w e e n S o n o t u b e s being filled w i t h sand. T h e S o n o t u b e forms stand u p well during placing of the c o n c r e t e b u t disintegrate after being w e t t e d . After t h e t u b e distintegrates, the sand runs o u t from u n d e r the joists. It was theorized t h a t as t h e clay swells, it could e x p a n d i n t o these openings and reduce the swelling pressure. T h e system has been tried in a few limited cases in the Denver area with d o u b t f u l success.


127

8"

8" SECTION

A-A

Figure 72. Typical honeycomb form system.

F L O A T I N G SLABS A

floating

slab refers t o a slab-on-ground c o n s t r u c t i o n in which t h e slabs are totally

separated from t h e grade b e a m a n d building s t r u c t u r e . T h e o r e t i c a l l y , t h e slab is capable of moving i n d e p e n d e n t l y w i t h o u t being in c o n t a c t w i t h the s u r r o u n d i n g s t r u c t u r e .

Slip

joints Interior floor slabs should be totally separated from t h e grade b e a m s and interior c o l u m n s

t o allow for free slab m o v e m e n t . If the slab is n o t separated from t h e grade b e a m , heaving of t h e slab can t r a n s m i t pressure t o t h e grade b e a m and, in t u r n , lift t h e piers. In practice, the slab is separated from the grade b e a m s b y t h e use of asphalt felt e x p a n s i o n j o i n t s . When the backfill exerts lateral pressure o n t h e grade b e a m , the expansion j o i n t is u n d e r compression and part of the uplift pressure b e n e a t h the slab is t h e n t r a n s m i t t e d t o t h e grade b e a m . In nearly all b a s e m e n t buildings which have been subjected t o uplifting, the central p o r t i o n of the slab raised while t h e area along the p e r i m e t e r of t h e grade b e a m remained essentially in place. In m a n y cases, cracks appeared a b o u t 2 feet from and parallel t o t h e grade b e a m , as s h o w n on figure 7 3 .

128


Figure 73. Floor cracks parallel to the foundation wall resulting from lack of slip joints.

Theoretically, if the swelling pressure of the underslab soil is 5 , 0 0 0 psf, as m u c h as 1 0 , 0 0 0 p o u n d s per linear foot of uplifting pressure can be t r a n s m i t t e d t o t h e grade b e a m . Naturally, as soon as t h e slab cracked, t h e uplifting pressure was relieved. However; t h e initial uplift force is s o m e t i m e s sufficient t o cause heavy d a m a g e . A n improved c o n s t r u c t i o n m e t h o d is t h e installation of a lubricated slip j o i n t b e t w e e n the grade b e a m and the slab as s h o w n on figure 7 4 . This installation involves t h e use of t w o 1/8-inch m a s o n i t e strips w i t h silicone l u b r i c a n t b e t w e e n t h e m . This t y p e of j o i n t system is n o t affected b y lateral pressure t h e r e b y allowing free slab m o v e m e n t . S o m e architects prefer t o e x t e n d the floor slab i n t o the e x t e r i o r f o u n d a t i o n wall as s h o w n o n figure 7 5 . T h e result is o b v i o u s ; heaving of the floor slab n o t only p r o d u c e s floor cracks, b u t also tilts t h e e x t e r i o r wall, causing great structural d a m a g e . Figure 76 indicates t h e results of faulty design.


129

2 - l / 8 " x 9 " continuous tempered masonite. Coat smooth sides with silicone lubricant. Tape smooth sides together. Provide temporary support by taping to wall. Do N. not nail.

Figure 74. Typical slip joint detail between slab-on-ground and foundation wall.

Exterior slabs Exterior patio slabs can also transmit swelling pressure to the structure. Conventional practice calls for the exterior slab to be tied in with the grade beam by the use of dowel bars. In this manner, full swelling pressure is transmitted into the foundation walls. Such type design is not recommended. See also "Aprons" for a subsequent discussion of similar problems. Figure 77 shows a typical case where the patio slab has transmitted pressure through dowel bars to the foundation wall causing considerable damage. In another case, the exterior sidewalk slab was extended about half an inch into the brick course for aesthetic reasons (fig. 78). This resulted in the flaking and damaging of the brick wall, as shown on figure 79. Oftentimes, the patio slabs are tied into the top of the foundation wall with dowel bar as shown on figure 80. Heaving of the patio slab can cause severe cracking of the upper structure without any sign of movement being apparent in the basement portion of the structure. Partition wall The single largest factor that causes damage to structures founded on expansive soils is partition walls that bear directly on a slab. When the floor slab heaves, everything resting on the

130


τ

Figure 75. Tilting of exterior wall caused by slab heaving and improper slip joints.

131


Figure 76. Exterior brick course buckled due to heaving of interior floor slab.

floor will rise. T h e a m o u n t of floor heaving d e p e n d s u p o n t h e swelling p o r t e n t i a l of t h e underslab soils as well as t o t h e degree of w e t t i n g . Slab heaving ranging from a fraction of an inch t o as m u c h as 12 inches has been observed. T h e i t e m s affected b y floor heaving are: Stud walls, Sheet rock, Wall paneling, Cinder block p a r t i t i o n s , Staircase walls, D o o r frames, Water lines, F u r n a c e d u c t s , and Shelves and b o o k c a s e s

132


Brick Course

Floor Joist

Dowel bar . Patio Slab

t u t 4

-Swelling Pressure

Basement Wall

Figure 77. Patio slab dowelled into wall. Crack appeared parallel to wall. Magnitude of swelling pressure transmitted is estimated to be about 10,000 pounds per running foot.

A n y o n e , or several, of t h e above i t e m s can i m p a r t pressure t o the u p p e r floor joist or b e a m s ; initially, d o o r s will bind followed b y t h e occurrence of severe cracking. Almost every building investigated suffered some degree of d a m a g e d u e to t h e uplifting of t h e slab-bearing partition walls. F o r stud wall c o n s t r u c t i o n , it is relatively easy t o provide slip j o i n t s in t h e system so the wall is free t o m o v e w i t h o u t exerting pressure on t h e u p p e r s t r u c t u r e s . A typical detail of such c o n s t r u c t i o n is s h o w n o n figure 8 1 . The slip j o i n t s can either be installed at the t o p (floor s u p p o r t e d ) or at t h e b o t t o m ( h u n g p a r t i t i o n wall). T h e disadvantage of using a floor-supported wall is t h a t w h e n t h e wall lifts, vertical cracking will o c c u r b e t w e e n t h e partition wall and t h e exterior walls as s h o w n on figure 8 2 . Figure 83 indicates t h e architectural detail of a school building in which t h e load-bearing walls are s u p p o r t e d b y piers and t h e interior m a s o n r y walls are placed o n t h e slabs. Heaving of t h e slab-on-ground has resulted in severe cracking of the slab-bearing p a r t i t i o n wall while t h e walls s u p p o r t e d b y t h e grade b e a m s and piers remain stable as s h o w n on figure 8 4 .


Figure 78. Flaking of brick course caused by slab heaving.

134


Figure 79. Sidewalk heaving caused flaking of the brick course as a direct result of the slab extending into the brick course.

F r e q u e n t l y , the stud walls have been p r o p e r l y provided with slip j o i n t s ; b u t sheet rock, applied on b o t h sides of t h e s t u d s , resting on t h e floor as s h o w n on figure 85 negates the slip joint installation. Sheet r o c k is capable of t r a n s m i t t i n g sufficient pressure t o t h e floor j o i n t or ceiling resulting in great d a m a g e . A similar situation occurs w h e n b a s e m e n t walls are paneled. T h e studs m a y be free from the floor, b u t t h e paneling bears directly on t h e floor. If uplifting occurs, it m a y result in t h e p o p p i n g of t h e paneling as s h o w n on figure 8 6 . Staircase walls are t h e m o s t frequently neglected w h e n it comes to providing p r o p e r uplift precautions. When instructions are t o provide slip j o i n t s to all slab bearing walls, the staircase is usually neglected. O n e single 2 x 4

can exert great uplift pressure and t h u s damage the u p p e r

s t r u c t u r e , and t h e force m a y even extend to t h e second level in the case of split level buildings as s h o w n on figure 8 7 . Figure 88 indicates a p r o p e r l y formed slab-bearing partition wall with slip j o i n t s at t h e bottom.


135

Figure 80. Patio slab attached to basement wall by dowel bar. Swelling pressure results in damage to brick course.

Door frames and

utilities

D o o r frames should be h u n g from the t o p and n o t s u p p o r t e d on slabs. Slab heaving can t r a n s m i t high intensity pressure t h r o u g h t h e d o o r frame t o t h e u p p e r s t r u c t u r e s . A very c o m m o n distress in residential houses is t h e separation of garage d o o r frames from m a s o n r y walls. This is essentially caused by heaving of t h e garage slab w h i c h results from failure t o provide a grade beam, across the e n t r a n c e for the garage d o o r opening. Figure 89 indicates a c o m m o n sight of garage floor slab heaving in a swelling soil area. T h e central p o r t i o n of t h e slab heaved; however, t h e edges remained in place because t h e y were restrained b y t h e d o o r frame. Figure 9 0 indicates t h e crushing and d i s t o r t i o n of furnace d u c t s resulting from heaving of a b a s e m e n t slab. Figure 91 shows t h e severe b e n d i n g of w a t e r line also caused b y floor heaving. Such distress generally brings i m m e d i a t e alarm t o t h e h o m e o w n e r . If t h e utility lines above t h e slab are being d a m a g e d , t h o s e below t h e slab can also be seriously d a m a g e d .


FLOOR SUPPORTED PARTITION WALL

HUNG PARTITION WALL

Figure 81. Detail of slip joints used in a partition wall. (After Jorgensen and Hendrickson, Inc.)


137

Figure 82. The cracking of slab bearing partition wall at the junction of the exterior wall.

Aprons C o n c r e t e sidewalk slabs a r o u n d a building will prevent surface w a t e r from entering t h r o u g h t h e backfill and i n t o t h e f o u n d a t i o n soils. However, t h e c o n c r e t e apron should n o t be doweled i n t o the f o u n d a t i o n wall for t h e reasons previously discussed. C o n c r e t e sidewalks or a p r o n s will heave and crack. Heaving of c o n c r e t e walks can s o m e t i m e s result in t h e drainage being directed toward t h e building, allowing surface w a t e r t o e n t e r t h r o u g h


138

\Z2

PROJECT 2l'-IO"

0

CLASS 30-/0

CRACKED SLAB BEARING WALL

Figure 83.

Architectural detail of school building.

RM.


Figure 84. Heaving of interior slab-bearing partition. Note the Figure 85. Buckling of stud wall due t o floor heaving,

139

uncracked wall, on the right, is supported by structural grade beams.

140

3

/

.

%

χη

„

1

ι.Ι

ι

ri

ι

Figure 87. Staircase 2x4 s rest directly on floor. Note bow of ~, , 0 2x4 s due to floor heaving.


Λ

Figure 86. Buckling of wall paneling due t o floor heaving.


Figure 88. Properly formed slab-bearing partition wall w i t h slip joint at b o t t o m .

Figure 89. Typical garage floor heaving at the central portion.

141

! 42

e

ig

D j sn t foo wr artR Jt p g eo iudper uto

floor heaving,


Figure 90. Crushing of furnace duct caused by heaving of basement floor slab.


Figure 93. Sidewalk slab heaving transmits pressure through slab bearing timber frame t o the building causing heavy structure cracking.

143

Figure 92. Typical heaving of sidewalk.

144


t h e j o i n t b e t w e e n t h e a p r o n and t h e wall. F r e q u e n t expansion j o i n t s will be necessary t o prevent excessive cracking of t h e slab. Figure 9 2 shows t h e heaving of a c o n c r e t e slab. A n o t h e r serious m i s t a k e is t o place t h e posts of a p a t i o on a c o n c r e t e walk as s h o w n on figure 9 3 . As t h e c o n c r e t e walk heaves, it t r a n s m i t s pressure to t h e s t r u c t u r e t h r o u g h

the

c o n n e c t i n g girders causing building d a m a g e .

REFERENCES [521 "Recommended Practice for Construction of Residential Concrete Floors on Expansive Soils," (Vol. II), Portland Cement Association, Los Angeles, California. [531 Means, R. E., "Buildings on Expansive Clay," Quarterly of the Colorado School of Mines, Vol. 54, No. 4, 1959.

Chapter

7

MOISTURE CONTROL

INTRODUCTION Terzaghi stated t h a t , " W i t h o u t any w a t e r t h e r e would be n o use for soil m e c h a n i c s . " Terzaghi had only limited k n o w l e d g e of swelling soils; however, his s t a t e m e n t can be accurately applied t o t h e b e h a v i o r of e x p a n s i o n soils. Ever since t h e a c k n o w l e d g m e n t of expansive soil p r o b l e m s , engineers have b e e n a t t e m p t i n g to isolate w a t e r from t h e f o u n d a t i o n structure.. It is a relatively simple u n d e r t a k i n g t o remove free w a t e r w h i c h m a y seep i n t o a building f o u n d a t i o n b y providing a d e q u a t e surface drainage and p r o p e r l y installed subdrainage systems. However, it is difficult t o isolate t h e m i g r a t i o n of m o i s t u r e from an e x t e r i o r l o c a t i o n t o a covered area. V a p o r barriers, b o t h h o r i z o n t a l and vertical, have b e e n used w i t h only a limited degree of success in impeding m o i s t u r e migration. F u r t h e r research is necessary in b o t h t h e field and l a b o r a t o r y t o establish a practical and e c o n o m i c a l m e t h o d of controlling m o i s t u r e migration.

HORIZONTAL MOISTURE BARRIERS H o r i z o n t a l m o i s t u r e barriers can be installed a r o u n d a building in the form of m e m b r a n e s , rigid paving, or flexible paving. T h e p u r p o s e of t h e h o r i z o n t a l barriers is t o prevent excessive i n t a k e of surface m o i s t u r e . T h e use and effectiveness of these m o i s t u r e barriers are discussed below. Membranes A widely used h o r i z o n t a l m o i s t u r e barrier is a c o m b i n a t i o n of a p o l y e t h y l e n e m e m b r a n e extending beyond

t h e limits of backfill and loose gravel placed o n t o p of t h e m e m b r a n e .

S o m e t i m e s a plank is installed along the edge of the m e m b r a n e . T h e p u r p o s e of such installation is t o prevent surface w a t e r from seeping t h r o u g h t h e backfill i n t o t h e building and to prevent the g r o w t h of w e e d s . Figure 9 4 indicates the typical design. It should be realized t h a t the d r y soils b e n e a t h an impervious m e m b r a n e will, in t i m e , b e c o m e w e t regardless of the presence of such m e m b r a n e because evaporation can n o longer take place. T h e thickness of t h e p o l y e t h y l e n e m e m b r a n e

ranges from

a b o u t 4 t o 2 0 mils. T h e

m e m b r a n e tears easily and eventually develops holes. Surface w a t e r p o n d i n g in a depressed area will in time leak t h r o u g h t h e holes and edges of the m e m b r a n e and e n t e r t h e soil b e n e a t h , while evaporation and drying of t h e soil b e n e a t h t h e m e m b r a n e is impossible. Even in t h e case of a perfect impervious m e m b r a n e , m o i s t u r e migration d u e to t h e r m a l transfer as explained in c h a p t e r 2 will i n t r o d u c e additional m o i s t u r e to the f o u n d a t i o n soils.

146


Foundation wall

Figure 94. Impervious membrane along exterior walls.

T h u s , it appears t h a t t h e q u e s t i o n a b l e advantage of using a m e m b r a n e around the building is t o increase t h e time required for m o i s t u r e p e n e t r a t i o n and m a k e t h e m o i s t u r e d i s t r i b u t i o n m o r e u n i f o r m . In t h e course of several years, t h e backfill soil b e n e a t h a m e m b r a n e will be totally s a t u r a t e d . By lifting t h e m e m b r a n e , it is easy to find t h a t t h e soil has a m o i s t u r e c o n t e n t greater t h a n t h e plastic limit. Concrete

aprons

T h e installation of concrete aprons or sidewalks has been found effective in controlling m o i s t u r e fluctuation. T h e advantage of using c o n c r e t e aprons r a t h e r t h a n plastic m e m b r a n e s is t h a t t h e former offers a positive barrier t o water. Obviously, within reason, the wider the c o n c r e t e apron t h e m o r e p r o t e c t i o n it offers t o t h e building. Paving t h e entire non-building area is impractical and unsightly. Nonetheless, it has b e e n observed t h a t f o u n d a t i o n m o v e m e n t d u e t o expansive soils seldom takes place in gasoline service stations where t h e ground surface is extensively covered. M o h a n and R a o [ 5 4 ] installed a 4-foot-wide c o n c r e t e apron a r o u n d distressed buildings f o u n d e d on black c o t t o n soils, w h i c h proved effective in controlling m o v e m e n t . T h e y claim that t h e function of t h e a p r o n is t o m o v e the marginal m o i s t u r e variation away from the building. While t h e use of concrete aprons a r o u n d t h e e x t e r i o r of the building m a y prove beneficial, care should be exercised in obtaining an effective seal b e t w e e n the aprons and the f o u n d a t i o n walls. Swelling soils can heave an apron so t h a t surface drainage is t o w a r d the building rather t h a n away. With p o o r l y constructed j o i n t s , w a t e r will e n t e r t h e j o i n t and seep into the f o u n d a t i o n soil,

147

MOISTURE CONTROL

t h u s an a p r o n can cause m o r e damage t h a n g o o d . In t h o s e areas w h e r e c o n c r e t e a p r o n s are used, c o n s t a n t care and m a i n t e n a n c e is required. Asphalt

membranes

As early as 1 9 3 3 , t h e T e x a s Highway D e p a r t m e n t used asphalt m e m b r a n e s t o p r e v e n t surface w a t e r from entering t h e expansive clay subgrade [ 5 5 ] . T h e c o n c r e t e p a v e m e n t was placed u p o n an asphalt m a t 4 8 feet wide. This m e m b r a n e installation m a y retard swelling b u t will n o t prevent it. Research c o n d u c t e d constructed

from

b y t h e Asphalt

Institute [56]

advocates t h a t asphalt

membranes

catalytically b l o w n asphalt can be effective in preventing m o i s t u r e

from

intruding i n t o subgrade soils. Specifications for catalytically b l o w n asphalt c e m e n t are given in table 12. V a n L o n d o n [ 5 7 ] used t h e 50-60 p e n e t r a t i o n asphalt m e m b r a n e t o c o m p l e t e l y envelop t h e highway e m b a n k m e n t . T h e p u r p o s e of t h e m e m b r a n e was t o m a i n t a i n a c o n s t a n t m o i s t u r e content

in

the embankment

soil, t h u s preventing v o l u m e t r i c change of t h e fill

material.

S u b s e q u e n t m o i s t u r e d e t e r m i n a t i o n s of t h e enclosed e m b a n k m e n t indicated very little change in moisture

content

over t h e

10-year period

s u b s e q u e n t t o c o n s t r u c t i o n , and t h e

pavement

remained in a stable c o n d i t i o n . A n o t h e r t y p e of asphalt m e m b r a n e consisted of prefabricated

asphalt sheets, less t h a n

one-half inch thick, 3 t o 4 feet w i d e , and u p t o 2 0 feet long. Such material can b e c o n v e n i e n t l y handled and easily placed.

Table 12.—Specification for catalytically-blown asphalt cement. Test Designation Flash point Softening point Penetration, 77° F. 100 gms., 5 sec. Penetration, 32° F., 200 gms., 60 sec. Penetration, 115° F., 50 gms., 5 sec. Ductility, 77° F. (5 cm per min) cm Loss on heating 325° F. in 5 hrs. Penetration of residue, 77° F. (100 gms., 5 sec. compared to original) percent Solubility in C C I 4 , percent

Test Method ASTM

50-60 Penetration Grade

D D

92 36

D

5

50-60

D

5

30 Min.

D

5

120 Max.

D

113

3.5 Min.

D

6

1.0 Max.

D

165

425° F. Min. 175°F.-225°F.

60.0 Min. 97.0 Min.

Asphalt used as a membrane shall be 50-60 penetration grade. This material shall be prepared by the catalytic blowing of petroleum asphalt. The use of iron chlorides or compounds thereof will not be permitted. The asphalt shall be homogeneous, free of water and shall not foam when heated to 347° F. It shall meet the above tabulated requirements.

148


Asphalt m e m b r a n e s can be used t o cover t h e surface of expansive soils so t h a t nonexpansive fill can b e placed o n t o p of t h e m e m b r a n e s . This will m i n i m i z e the infiltration of surface w a t e r into t h e underslab soils. Where slab-on-ground c o n s t r u c t i o n is required, such t r e a t m e n t can be very advantageous. T h e a m o u n t of asphalt c e m e n t required to c o n s t r u c t a m e m b r a n e , according t o t h e Asphalt I n s t i t u t e , is a b o u t 1.3 gallons per square y a r d . T h e use of asphalt m e m b r a n e s in c o n n e c t i o n w i t h t h e c o n s t r u c t i o n of a swimming pool in an expansive soil area is particularly desirable. F u r t h e r research and field observation will be required.

VERTICAL MOISTURE BARRIERS Vertical m o i s t u r e barriers have been used a r o u n d t h e p e r i m e t e r of t h e building t o cut off t h e source of w a t e r t h a t m a y e n t e r t h e underslab soils. Theoretically, vertical barriers should b e m o r e effective t h a n h o r i z o n t a l barriers in minimizing seasonal drying and shrinking of the p e r i m e t e r f o u n d a t i o n soils, as well as m a i n t a i n i n g long term uniform m o i s t u r e c o n d i t i o n s b e n e a t h t h e covered area.

Installation Buried vertical barriers m a y consist of p o l y e t h y l e n e m e m b r a n e , c o n c r e t e , or o t h e r d u r a b l e , impervious materials. T h e p a t h of m o i s t u r e migration w h e n using a vertical barrier is s h o w n on figure 9 5 . As seen from figure 9 5 , t h e installation of a vertical barrier prevents edge w e t t i n g d u e t o lateral m o i s t u r e migration w i t h i n t h e d e p t h t o which t h e m e m b r a n e e x t e n d s . However, over a period of t i m e , rainfall, melting snow, and lawn irrigation w a t e r will a c c u m u l a t e near the b o t t o m

Rainfall and yard watering

Depth of seasonal moisture change

Most migration "blocked" by membrane. With time, some moisture migrates below membrane depth.

Moisture content relatively constant, but can still absorb moisture with time.

Figure 95. Path of moisture migration blocked by vertical barrier. (After Woodward-Clyde-Sherard and Associates).

MOISTURE CONTROL

149

of t h e m e m b r a n e and t h e m o i s t u r e will be sucked i n t o t h e mositure-deficient soils b e n e a t h t h e building. T h u s , t h e same degree of w e t t i n g of t h e f o u n d a t i o n soil could result w i t h or w i t h o u t t h e use of a barrier, b u t w o u l d o c c u r over a longer p e r i o d of t i m e . By installing a m o s i t u r e barrier, t h e p o t e n t i a l for damage w o u l d b e less because of t h e slower rate of m o s i t u r e migration and t h e m o r e u n i f o r m m o i s t u r e c o n t e n t of t h e soil at any particular t i m e . Vertical m o i s t u r e barriers should be installed t o a d e p t h equal t o or greater t h a n t h e d e p t h of seasonal m o i s t u r e change. A s t u d y in S o u t h Africa [ 5 8 ] has s h o w n t h a t w e t t i n g of soils b e n e a t h a h o u s e occurred t o a d e p t h of at least 24 feet in a fissured-clay profile. Most of this w e t t i n g was a t t r i b u t e d t o lateral migration of m o i s t u r e from seasonal rains, r a t h e r t h a n from capillary rise. T h e following should be considered in t h e installation of a vertical m o i s t u r e barrier:

1. Vertical m o i s t u r e barriers c a n n o t be effectively installed a r o u n d b a s e m e n t s t r u c t u r e s . 2. Vertical m o s i t u r e barriers should be installed 2 t o 3 feet from t h e p e r i m e t e r f o u n d a t i o n t o p e r m i t m a c h i n e excavation of t h e t r e n c h for t h e m e m b r a n e . 3. T h e vertical barrier is s o m e t i m e s a t t a c h e d t o a h o r i z o n t a l barrier t o prevent w e t t i n g b e t w e e n t h e vertical barrier and t h e building. 4. E i t h e r c o n c r e t e or p o l y e t h y l e n e m e m b r a n e can b e used. T h e m e m b r a n e should be of sufficient thickness and durability t o resist p u n c t u r e s d u r i n g backfilling of t h e trench. 5. It is also possible t o use semi-hardening, impervious slurries installed in a n a r r o w t r e n c h . Theoretically, vertical m o i s t u r e barriers have a distinct advantage over h o r i z o n t a l m o i s t u r e barriers. However, in view of the high cost involved in t h e installation of a vertical m o i s t u r e barrier, especially w h e r e great d e p t h is required, it is d o u b t f u l t h a t such an installation is of sufficient m e r i t t o w a r r a n t t h e e x p e n s e . Backfill A n i m p o r t a n t e l e m e n t involved in building c o n s t r u c t i o n w h i c h is usually slighted is t h e backfill a r o u n d a building. When p r o p e r l y c o n s t r u c t e d , backfill serves t h e same p u r p o s e as d o vertical m o i s t u r e barriers. This is especially t r u e in b a s e m e n t building w h e r e p r o p e r l y c o m p a c t e d backfill can prevent surface w a t e r from entering t h e f o u n d a t i o n soils. U n f o r t u n a t l e y , backfill is seldom c o m p a c t e d p r o p e r l y . M a n y builders choose t o p u s h t h e loose soil i n t o t h e excavation w i t h n o further c o m p a c t i o n effort. O t h e r s a t t e m p t t o consolidate t h e backfill b y p u d d l i n g . It is obvious t h a t p r o p e r c o m p a c t i o n of t h e backfill c a n n o t b e achieved b y such processes. When i m p r o p e r l y c o m p a c t e d , almost all of t h e backfill along t h e f o u n d a t i o n walls is in a loose state. Surface w a t e r can t h e n e n t e r t h e backfill and seep freely i n t o t h e f o u n d a t i o n soils. T h e result of s e t t l e m e n t of loose backfill is s h o w n o n figure 9 6 . C o m p a c t i o n of backfill in restricted areas, such as in utility t r e n c h e s , c a n n o t b e p e r f o r m e d b y large c o m p a c t i n g e q u i p m e n t . These areas should b e c o m p a c t e d , b y h a n d - o p e r a t e d vibrating plates, in t h i n lifts of n o t m o r e t h a n 4 inches. Because of p o o r c o m p a c t i v e effort, s e t t l e m e n t of backfill a r o u n d a building as well as s e t t l e m e n t of utility t r e n c h e s is t h e rule, r a t h e r t h a n t h e exception.

150


Figure 96. Typical case of loose backfill around the building. Loose backfill allows surface water to enter the foundation soils. Note depressions which trap water.

O n e recently advertised

hydraulic-operated

c o m p a c t o r claimed

to have the ability to

c o m p a c t backfill in a t r e n c h , in one effort, from t h e ground surface. Such a claim appears t o be false while, in fact, t h e c o m p a c t o r has p r o b a b l y a lift c o m p a c t i o n capacity of no m o r e t h a n 12 inches. T h e so-called p u d d l i n g process has been widely used b y c o n t r a c t o r s in small j o b s w i t h the assumption t h a t t h e soils will consolidate w i t h o u t c o m p a c t i o n . N o t only does such practice m a k e it impossible t o o b t a i n the required density b u t it is s o m e t i m e s d a n g e r o u s . In cohesive backfills, puddling inevitably leads t o weakening and softening of the soil and to future loss of stability and subsidence. In u n i f o r m l y granular soils, puddling will cause t h e collapse of the e x t r e m e l y loose, unstable z o n e associated w i t h bulking. Excess puddling, w h i c h frequently results w h e n a hose is left discharging i n t o t h e backfill overnight, can easily i n t r o d u c e w a t e r i n t o the f o u n d a t i o n soils and b e n e a t h t h e slab, resulting in great d a m a g e . In expansive soil areas, such d a m a g e will be reflected in t h e s t r u c t u r e for m a n y years. By using impervious clay c o m p a c t e d t o 85 percent or m o r e of standard P r o c t o r density at o p t i m u m m o i s t u r e c o n t e n t , t h e backfill acts as a very effective vertical m o i s t u r e barrier. Such barriers are m o r e effective t h a n m e m b r a n e s .

MOISTURE CONTROL

151

SUBSURFACE DRAINAGE T h e purposes of a subsurface drainage system are as follows: 1. I n t e r c e p t t h e gravity flow of free w a t e r , 2. L o w e r t h e g r o u n d w a t e r or perched w a t e r , and 3 . Arrest t h e capillary m o i s t u r e m o v e m e n t and m o v e m e n t of m o i s t u r e in t h e v a p o r state. In tercep ting d rains Intercepting drains are effective in minimizing t h e w e t t i n g of t h e f o u n d a t i o n soils w h e r e t h e w e t t i n g is d u e t o t h e gravity flow of free w a t e r in a subsurface pervious layer such as a layer of gravel or fissured clay. This is s h o w n o n figure 9 7 . T o insure t h e i n t e r c e p t i o n of free w a t e r , t h e drain m u s t be c o m p l e t e l y filled w i t h gravel and t h e t r e n c h should be d e e p e n o u g h t o reach t h e water-bearing layer. Intercepting drains are m o s t effective w h e n located along t h e t o e of a slope w h e r e g r o u n d w a t e r leaves t h e d e e p strata and w h e r e it m a y emerge to t h e surface. When a s t r u c t u r e is located near an irrigation d i t c h or canal w i t h a leakage p r o b l e m , t h e installation of an i n t e r c e p t i n g drain will p r o t e c t against t h e infiltration of seepage w a t e r . I n t e r c e p t i n g drains are also widely used for improving slope stability and preventing landslides.

INTERCEPT GRAVITATIONAL FREE WATER. Figure 97. Typical function of an intercepting drain. (After Woodward-Clyde-Sherard & Associates).

Perched

water

As m e n t i o n e d previously u n d e r "Slabs on Expansive Soils," a perched w a t e r table c o n d i t i o n can develop in areas w h e r e b e d r o c k is shallow. Surface w a t e r a c c u m u l a t e d from yard irrigation will n o t p e r m e a t e t h e b e d r o c k and can create a local perched w a t e r c o n d i t i o n . A perched w a t e r table can also be created b y a relatively i m p e r m e a b l e s t r a t u m of small areal e x t e n t and b y a z o n e of aeration above t h e main b o d y of g r o u n d w a t e r [ 5 9 ] as s h o w n o n figure 9 8 . Where b e d r o c k is situated at a slight d e p t h b e n e a t h t h e g r o u n d surface, a perched w a t e r table c o n d i t i o n m a y develop d u e t o t h e following: 1. T h e u p p e r soils are relatively pervious and surface w a t e r is capable of seeping t h r o u g h t h e u p p e r soils e n c o u n t e r i n g relatively low resistance.


152

Ground surface

Perched water tables

Water table Unconfined aquifer Figure 98. Perched aquifers. (After Todd).

2. T h e lower b e d r o c k is impervious and will n o t allow t h e infiltration of water. However, t h e r e are seams and fissures in t h e b e d r o c k w h i c h provide passage for w a t e r . A large v o l u m e of w a t e r is capable of flowing in the fissures of t h e b e d r o c k . 3. With surface irrigation and precipitation, t h e surface w a t e r t e n d s t o seep t h r o u g h t h e u p p e r soil and a c c u m u l a t e on t o p of b e d r o c k . Part of t h e a c c u m u l a t e d w a t e r will flow on t o p of t h e b e d r o c k and p a r t of it will run t h r o u g h t h e fissures of t h e b e d r o c k . When a d e e p b a s e m e n t is c o n s t r u c t e d , t h e b a s e m e n t excavation can cut t h r o u g h the fissures of b e d r o c k and t h e w a t e r can a c c u m u l a t e in t h e low b a s e m e n t area, creating a perched w a t e r table c o n d i t i o n . Wells tapping perched aquifers yield only t e m p o r a r y or small a m o u n t s of w a t e r . However, such w a t e r can e n t e r b a s e m e n t s and cause considerable d a m a g e . T h e installation of a subsurface drainage system a r o u n d t h e p e r i m e t e r of t h e lower level of a s t r u c t u r e can p r o t e c t against infiltration from perched w a t e r . T o be effective, an intercepting drain should be installed at least 2 feet below t h e floor and should lead t o a suitable o u t l e t w h e r e w a t e r can be removed by gravity or b y a s u m p p u m p . Peripheral

drains

Figure 9 9 indicates t h e suggested location of peripheral drains. These drains can be installed around either t h e interior or exterior of t h e building. T h e subdrainage system is effective in minimizing general w e t t i n g of t h e f o u n d a t i o n soils, w h i c h o c c u r n o t only because of gravitational flow of free w a t e r , b u t also because of m o i s t u r e migration [ 6 0 ] . As explained in c h a p t e r 2, m o i s t u r e migration includes capillary m o i s t u r e m o v e m e n t in t h e liquid state and m o v e m e n t of m o i s t u r e in t h e vapor state d u e t o t e m p e r a t u r e differential. Where t h e w a t e r table is d e e p ,

MOISTURE CONTROL

153

Foundation Wall

Grade down 0 . 5 % to sump

Drain Trench

•5 ï

Sump 3 deep can be located at any convenient location in the basement.

Grade beam or Foundation wall J / 2 " Expansion joint material

4 Void Material

Concrete Pier

SECTION A A Figure 99. Typical sub-drain detail.

154


capillary

action

and v a p o r

transfer

are p r o b a b l y

t h e major

causes

of w e t t i n g

of t h e

moisture-deficient soils in a covered area. T h e possible p l a c e m e n t of a subdrain t o i n t e r r u p t m o i s t u r e m o v e m e n t is s h o w n o n figure 1 0 0 . It should b e noted t h a t , for a subdrainage system t o b e effective in preventing m o i s t u r e m o v e m e n t discussed above, it m u s t b e designed as a capillary b r e a k ; a n d additionally, t h e vapor pressure in t h e drain should b e at a lower value t h a n t h e v a p o r pressure in t h e f o u n d a t i o n soils. T h e gravel used t o fill t h e subdrain t r e n c h should have a gradation b e t w e e n 3 / 4 and 2 inches in size w i t h p e r c e n t o f fines less t h a n 5. Positive o u t l e t s should b e provided for t h e subdrainage s y s t e m . If a gravity o u t l e t is n o t possible, t h e drain should b e discharged t o a s u m p w h e r e w a t e r can b e removed b y p u m p i n g . In some instances, it m a y b e permissible t o c o n n e c t t h e subdrain t o t h e gravel b e d b e n e a t h t h e street

MOISTURE MOVEMENT WITHOUT SUBDRAIN

Legend Gravitational flow of free water (in shrinkage cracks and fissures)

Saturated soil at surface

Capillary or vapor movement in soil mass.

SUBDRAIN INTERRUPTS MOISTURE MOVEMENT Figure 100. Interruption of moisture movement by subdrain. (After Woodward-Clyde-Sherard & Associates)

MOISTURE CONTROL

155

sewer line. However, because of t h e small gradient differential b e t w e e n sewer line i n t a k e and subdrain o u t l e t , such in a r r a n g e m e n t is n o t usually satisfactory. Experience indicates t h a t t o be fully effective, t h e peripheral drain should be placed at least 12 inches b e l o w t h e floor level, preferably 24 inches.

SURFACE DRAINAGE T h e ground surface a r o u n d a building should b e graded so t h a t surface w a t e r will drain away from t h e s t r u c t u r e in all directions. This usually is n o t accomplished d u e t o negligence, cost, limited p r o p e r t y size and o t h e r reasons. In m a n y cases, t h e area a r o u n d the building had b e e n p r o p e r l y graded after c o n s t r u c t i o n , b u t t h e grade was later changed t o improve t h e a p p e a r a n c e of t h e landscape. As a result, it is n o t u n c o m m o n t o find buildings w i t h surface drainage d i r e c t e d toward t h e f o u n d a t i o n walls. Moisture change at t h e p e r i m e t e r of t h e building apears t o be t h e m o s t significant c o n t r i b u t o r t o d a m a g e . T h e r e f o r e , b y improving t h e drainage, a beneficial effect is inevitable. Sprinkling

system

Lawn sprinkling systems often create f o u n d a t i o n soil p r o b l e m s . L a w n sprinkling systems should be installed at least 10 feet from t h e building. Nozzles of t h e sprinkling system should never be directed t o w a r d a building. An a u t o m a t i c timing device should be provided for all sprinkling systems so t h a t excessive watering is avoided. Before use, all o u t l e t s in t h e system should b e pressure checked t o d e t e c t t h e presence of a n y possible o p e n o u t l e t u n d e r g r o u n d w h e r e w a t e r could flow u n c h e c k e d for a long period of t i m e w i t h o u t arousing suspicion.

Vegetation F r o m an architectural s t a n d p o i n t , it is pleasing t o have shrubs and flower beds p l a n t e d adjacent t o buildings. However, since it is necessary t o irrigate flower b e d s and shrubs, t h e excess w a t e r will p e n e t r a t e t h r o u g h t h e loose backfill i n t o t h e f o u n d a t i o n soils. E x p e r i e n c e indicates t h a t in practically every investigation of a cracked building, shrubs and flower b e d s were l o c a t e d adjacent t o t h e building. Figure 101 indicates a typical case w h e r e drainage away from t h e building is o b s t r u c t e d b y t h e paved walk and i m p r o p e r c o m p a c t i o n results in a depression along t h e building creating a p o n d i n g c o n d i t i o n . S h r u b s planted along a wall is typical of m a n y school buildings and residential houses. M a n y studies have s h o w n t h a t large bushes and trees can cause differential drying [611 of t h e f o u n d a t i o n soils and result in d a m a g e t o t h e building from shrinkage. Most of t h e d a m a g e caused b y shrinkage takes place in non-swelling or low-swelling soil areas. It is d o u b t f u l w h e t h e r large tress will pose a p r o b l e m in high-swelling soil areas. Nevertheless, it is good practice t o p l a n t trees and shrubs at least 10 feet from a s t r u c t u r e .

156


Figure 101. Plantation strip around a school building allowing the ponding of water between the sidewalk and the foundation wall. Roof

drain R o o f d o w n s p o u t s m u s t be directed away from a s t r u c t u r e so t h a t w a t e r will n o t seep i n t o

the

foundation

soils. T h e

downspouts

should

e x t e n d well b e y o n d

t h e p e r i m e t e r of

the

f o u n d a t i o n and should discharge to an area w h e r e t h e surface drainage is a d e q u a t e to carry off t h e w a t e r rapidly and prevent any possible p o n d i n g of w a t e r . If necessary, t h e w a t e r from d o w n s p o u t s should b e carried in a closed pipe or lined d i t c h to t h e street. M a n y m o d e r n buildings are c o n s t r u c t e d w i t h o u t d o w n s p o u t s , and w a t e r from t h e r o o f drains freely t h r o u g h the loose backfill i n t o t h e f o u n d a t i o n soils. S o m e t i m e s an o p e n c o u r t y a r d is c o n s t r u c t e d in t h e central p o r t i o n of a building. T h e c o u r t y a r d is usually covered w i t h lawn, flower beds, and trees. Such a c o u r t y a r d c o n s t i t u t e s a major drainage p r o b l e m

because surface w a t e r will b e u n a b l e t o drain unless an a d e q u a t e

subsurface drainage system is provided.

MOISTURE CONTROL

Interior

157

plumbing

Interior p l u m b i n g , including sewer and w a t e r lines, should be carefully checked for leakage. Sewer lines laid b e n e a t h t h e b a s e m e n t are subjected t o stress w h e n t h e s u r r o u n d i n g soils e x p a n d , and in e x t r e m e cases, shearing stress has caused pipe breakage resulting in flooding. In o n e instance, while investigating a cracked h o u s e , it was found t h a t t h e plug on the drain trap b e n e a t h t h e s h o w e r stall was missing. As a result, all t h e w a t e r from t h e shower drained i n t o t h e crawl space area for a period of at least 3 years. Leakage from w a t e r lines is less frequent. S o m e t i m e s leakage is found n e a r t h e w a t e r m e t e r w h i c h generally is situated n e a r t h e sidewalk in front of t h e h o u s e . Leaking w a t e r follows t h e loose backfill a r o u n d t h e w a t e r pipe i n t o the f o u n d a t i o n soils causing d a m a g e .

REFERENCES [54] Mohan, D. and Rao, B. G., "Moisture Variation and Performance of Foundations in Black Cotton Soils in India," Moisture Equilibria and Moisture Change in Soils Beneath Covered Areas, Australia, Butterworth. [55] "Report of Committee on Warping of Concrete Pavements," Highway Research Board Proceedings, Vol. 25, 1945. [56] "Asphalt Membranes and Expansive Soils," Information Service No. 145 (IS-145) May 1968. [57] Van London, W. J., "Waterproofing Value of Asphalt Membranes in Earth Fills for Gulf Freeway," Proceedings, Association of Asphalt Paving Technologists, Vol. 22, 1953. [58] Collins, L. E., "Some Observations on the Movement of Buildings on Soils in Vereening and Odendaalsrus," Symposium on Expansive Clays, South African Institution of Civil Engineers. [59] Todd, D. K., "Ground Water Hydrology," John Wiley & Sons, Inc., 1959. [60] "Remedial Methods Applied to Houses Damaged by High Volume Change Soils," Woodward-Clyde-Sherard & Associates, Oakland, California, 1968. [61] Hammer, M. J. and Thompson, Ο. B., "Foundation Clay Shrinkage Caused by Large Trees," Journal ASCE, Soil Mechanics and Foundation Division, Vol. 92, No. SM 6, Nov. 1966.

Chapter

8

SOIL STABILIZATION

INTRODUCTION In t h e o r y , t h e swelling p o t e n t i a l of an expansive clay can be m i n i m i z e d or c o m p l e t e l y eliminated b y o n e of t h e following m e t h o d s : 1. F l o o d t h e in-place soil t o acheive swelling prior t o c o n s t r u c t i o n , 2. Decrease t h e d e n s i t y of t h e soil b y c o m p a c t i o n c o n t r o l , 3 . Replace t h e swelling soils w i t h nonswelling soils, 4. Change t h e p r o p e r t i e s of expansive soils b y chemical injection, or 5. Isolate t h e soil so t h e r e will be n o m o i s t u r e change. Isolation of t h e soil has b e e n extensively discussed in c h a p t e r 7.

PREWETTING A n old established c o n c e p t a m o n g engineers and c o n t r a c t o r s as well as l a y m e n in dealing w i t h swelling soils is p r e w e t t i n g . As explained in c h a p t e r 2 , m o i s t u r e can migrate from a moderate-depth

w a t e r table t o an u p p e r moisture-deficient soil b y m e a n s of capillary rise.

Moisture migration can also take place from a h i g h - t e m p e r a t u r e area t o a l o w - t e m p e r a t u r e area b y m e a n s of t h e r m o - o s m o s i s or o t h e r m e c h a n i s m s . N o r m a l l y this m o i s t u r e evaporates at t h e surface and m o i s t u r e equilibrium is m a i n t a i n e d in t h e soil. T h e presence of covered areas, such as floor slabs, p a v e m e n t s , o r similar s t r u c t u r e s which inhibit this evaporation increases t h e m o i s t u r e c o n t e n t of t h e f o u n d a t i o n soil w i t h resultant swell. T h e p r e w e t t i n g t h e o r y is based o n t h e a s s u m p t i o n t h a t if soil is allowed t o swell b y w e t t i n g prior t o c o n s t r u c t i o n and if t h e high soil m o i s t u r e c o n t e n t is m a i n t a i n e d , t h e soil v o l u m e will remain essentially c o n s t a n t , achieving a no-heave state and therefore structural damage will n o t occur. Ponding T h e present p r e w e t t i n g practice usually involves direct flooding or p o n d i n g of t h e building area. T h e f o u n d a t i o n and floor area is flooded b y c o n s t r u c t i n g a small e a r t h b e r m a r o u n d t h e outside of t h e f o u n d a t i o n

t r e n c h e s t o i m p o u n d t h e water. A n o t h e r practice includes first

p r e w e t t i n g the f o u n d a t i o n t r e n c h e s , t h e n placing t h e f o u n d a t i o n w h i c h is used as a dike t o flood t h e floor area. In s o m e cases, where t h e m o i s t u r e c o n t e n t at footing d e p t h is stable, it is possible t o place c o n c r e t e footings and utilize t h e m as dikes so t h a t only the floor area is pre w e t t e d .

160

FOUNDATIONS ON EXPANSIVE SOILS The effect of ponding or flooding on the moisture content at various depths has been

investigated by the Texas Highway Department [ 6 2 ] . A section of Interstate Route 35 north of Waco, Texas was chosen for the experiment. The subgrade was ponded and the moisture content at various depths was taken. The moisture variation at specific depths beneath the ponding area is shown on figure 102. The following observations were made: 1. The moisture content achieved a significant penetration of only 4 feet below the pond during a period of 24 days. 2. To obtain desirable moisture distribution at greater depths, ponding should extend approximately 30 days. Experience in Southern California [63] indicates that pre wetting moderately expansive soils to a condition of 85 percent saturation at a depth of 2-1/2 feet is often satisfactory. In the case of highly expansive soils, prewetting to as much as 3 feet may not be sufficient. For slab-on-ground construction, after completing of the prewetting treatment, the ground surface must be kept moist until the slab is placed. A gravel or sand bed 4 to 6 inches thick should be placed over the subgrade prior to the prewetting period. The gravel layer prevents the clay from drying and shrinking. The prewetting operation must not be at the discretion of a contractor or owner. The treatment should be based upon an engineering investigation and evaluation of the site, subsoil condition, swelling potential, climatic condition, foundation system, and prior local experience. The moisture content profile should be checked frequently by tests in the field to assure that the desired results are achieved.

Figure 102. Subgrade moisture movement on IH35. (After McDowell) McLennan Co., Texas.

SOIL STABILIZATION

161

Practice Ponding or sprinkling, t o increase t h e soil m o i s t u r e t o a degree t h a t will prevent harmful heaving u p o n s u b s e q u e n t w e t t i n g , has been used in the c o n s t r u c t i o n of t h e San Luis Drain on the San Luis Unit of t h e Bureau of R e c l a m a t i o n ' s Central Valley project in California. Bara [ 6 4 ] claimed t h a t if t h e dense clays w i t h a particular liquid limit could be e x p a n d e d t o densities at or above critical n a t u r a l density-liquid limit reference line, a stable or n e a r u l t i m a t e m o i s t u r e c o n d i t i o n w o u l d have been a p p r o a c h e d and future v o l u m e changes w o u l d be small. The liquid limit versus d r y d e n s i t y relationship is s h o w n o n figure 1 0 3 . A soil w i t h liquid limit of 70 intercepts

at 9 0 pcf d e n s i t y . Similarly, t h e w a t e r content-liquid

limit

relationship was developed for soil liquid limit ranging b e t w e e n 4 0 and 100 as s h o w n on

the

reference

line

figure

104. Moisture c o n t e n t s above t h e reference line in figure 104 would assure t h a t the densities were on the non-critical side of t h e reference line in figure 103. Figure 104 indicates t h a t clays at liquid limit 4 0 require only a b o u t 2 3 p e r c e n t m o i s t u r e , while t h o s e near liquid limit 100 require at least 37 p e r c e n t m o i s t u r e before t h e y are considered t o be relatively n o n e x p a n s i v e in situ. Large

scale e x p e r i m e n t s of flooding of f o u n d a t i o n

soil for building sites have

been

c o n d u c t e d in Vereeniging, S o u t h Africa [ 6 5 ] . Here, t h e effect of w e t t i n g was accelerated b y a grid of vertical 4-inch-diameter wells each 20 feet d e e p . A t the end of 9 6 d a y s , over 9 0 p e r c e n t of

Figure 103. Clays encountered along San Luis Drain, (after Bara).

162


LIQUID LIMIT (%) 40 40 J

50

70 1

80

1

60 1

1

90 1

I

I

I

I

I

100 1

cc

10

0 I

Figure 104. Minimum water content required for soil liquid limit, (after Bara).

t h e m a x i m u m surface heave h a d t a k e n place. It is c o n c l u d e d b y t h e a u t h o r s t h a t t h e acceleration of heave b y flooding is a feasible p r e - c o n s t r u c t i o n p r o c e d u r e for light s t r u c t u r e s . E. J. Felt [ 6 6 ] discusses a p r e w e t t i n g project in w h i c h t h e soil m o i s t u r e c o n t e n t did n o t increase appreciably after t h e first m o n t h of p r e w e t t i n g . F o r 5 m o n t h s thereafter, soil swelling c o n t i n u e d . It was suggested t h a t t h e first infiltration of w a t e r was p r o b a b l y t a k e n b y seams and fissures present in t h e clay and, t h e r e f o r e , full soil e x p a n s i o n did n o t occur. As t i m e passed, t h e w a t e r moved from the fissures i n t o the b l o c k y soil mass, and swelling t o o k place t h r o u g h o u t t h e mass of t h e soil and n o t merely along a seepage p a t h . A t a housing project near Austin, Texas, t h e expansive soil b e n e a t h t h e f o u n d a t i o n was p r e w e t t e d b y filling t h e f o u n d a t i o n t r e n c h w i t h water. After 6 weeks of soaking, t h e w a t e r was p u m p e d o u t , t h e f o u n d a t i o n placed on t h e wet soil, t h e trenches again filled w i t h water, and the soil k e p t w e t t e d thereafter. This h o u s e heaved b o t h during and after c o n s t r u c t i o n . It was c o n c l u d e d b y D a w s o n [ 6 7 ] t h a t it is e x t r e m e l y difficult t o s a t u r a t e high plasticity clays within a reasonable period of t i m e . E x p a n s i o n of partially saturated clays will c o n t i n u e after c o m p l e t i o n of t h e s t r u c t u r e . Evaluation Most highway engineers strongly endorse t h e use of p r e w e t t i n g t o m i n i m i z e subgrade heaving. In view of t h e past experience and actual case studies, it is d o u b t f u l if p r e w e t t i n g can be successfully used w i t h lightly loaded s t r u c t u r e s . T h e effective migration of m o i s t u r e , d e p t h of p e n e t r a t i o n , t i m e required for s a t u r a t i o n , and swelling of partially saturated soils is n o t fully u n d e r s t o o d . Prewetting practice is m u c h m o r e complicated t h a n assumed b y m o s t l a y m e n . A

SOIL STABILIZATION

163

great a m o u n t of research will b e required before c o m p l e t e evaluation of the p r e w e t t i n g practice can be m a d e . S o m e of t h e disadvantages of t h e p r e w e t t i n g m e t h o d are as follows: 1. Allowing t h e p o n d i n g w a t e r t o migrate i n t o the lower moisture-deficient soils. E x p e r i e n c e indicates t h a t in a covered area, t h e m o i s t u r e c o n t e n t of the u n d e r s l a b soil seldom decreases. Wet soil will i n d u c e swelling. After t h e swelling has reached its m a x i m u m p o t e n t i a l , m o i s t u r e migrates t o the lower moisture-deficient soil and induces further swelling. This p r o c e d u r e can c o n t i n u e for as long as 10 years. 2. F r o m a c o n s t r u c t i o n s t a n d p o i n t , t h e t i m e required for p r e w e t t i n g can be critical. A m o i s t u r e c o n d i t i o n of less t h a n s a t u r a t i o n is often a d e q u a t e t o inhibit objectionable uplift. T h e length of p o n d i n g t i m e required is usually a b o u t 1 t o 2 m o n t h s . Even this length of time m a y be objectionable as being t o o great. 3 . It is highly q u e s t i o n a b l e if a u n i f o r m m o i s t u r e c o n t e n t can be o b t a i n e d in pre w e t t e d areas. Water can o n l y seep i n t o t h e stiff clay t h r o u g h fissures, and c o n s e q u e n t l y , u n i f o r m d i s t r i b u t i o n of m o i s t u r e c o n t e n t is n o t likely t o take place. As a result, differential heaving can be critical even after a p r o l o n g e d period of p r e w e t t i n g . 4 . E x p e r i m e n t s indicate t h a t p o n d i n g w a t e r can effectively p e n e t r a t e t h e soil t o a d e p t h of 4 feet within a reasonable t i m e . Such d e p t h is insufficient t o provide a balanced m o i s t u r e z o n e for t h e c o n s t r u c t i o n of i m p o r t a n t s t r u c t u r e s . 5. While p r e w e t t i n g m a y prove t o be a possible m e t h o d of stabilizing t h e soil b e n e a t h t h e floor slab, p a v e m e n t , or canal lining, it is d o u b t f u l t h a t footing f o u n d a t i o n s can be placed o n p r e w e t t e d soil. In saturated c o n d i t i o n s , t h e bearing capacity of a stiff clay can be reduced t o a very low value, less t h a n 1,000 psf, which p r o h i b i t s t h e use of c o n v e n t i o n a l footing f o u n d a t i o n s . While p r e w e t t i n g m a y play an i m p o r t a n t role in t h e c o n s t r u c t i o n of slabs, it is d o u b t f u l t h a t this m e t h o d will be an i m p o r t a n t c o n s t r u c t i o n t e c h n i q u e for building f o u n d a t i o n s o n expansive soils.

COMPACTION C O N T R O L T h e a m o u n t of swelling t h a t occurs w h e n a structural fill is e x p o s e d t o additional m o i s t u r e d e p e n d s u p o n t h e following: 1. T h e c o m p a c t e d dry d e n s i t y , 2. T h e m o i s t u r e c o n t e n t , 3 . T h e m e t h o d of c o m p a c t i o n , and 4 . T h e surcharge load. T h e last t w o r e q u i r e m e n t s are n o t critical in actual c o n s t r u c t i o n . T h e m e t h o d of c o m p a c t i o n is generally limited b y available e q u i p m e n t . F o r lightly loaded slabs, t h e surcharge load is usually very small.

164


Placement condition As early as 1959, Dawson [67] suggested that highly expansive soils be compacted to some minimum density rather than to a maximum density. Holtz and Gibbs [68] show the influence of density and moisture on the expansion of a compacted expansive clay, as shown on figure 105.

IVS

/

1

V b

/ &.*

S %/V

*J%*

MOISTURE CONTENT • PERCENT OF DRY WEIGHT Figure 105. and Gibbs).

Percentage of expansion for various placement conditions when under unit psi load. (After Holtz

V T

165

SOIL STABILIZATION

It can be seen t h a t expansive clays e x p a n d very little w h e n c o m p a c t e d at low densities and high m o i s t u r e b u t e x p a n d greatly w h e n c o m p a c t e d at high densities and low m o i s t u r e s . Gizienski and Lee [ 6 9 ] show t h a t w h e n their test soil was c o m p a c t e d at a b o u t 4-1/2 p e r c e n t above o p t i m u m , which is 10-1/2 p e r c e n t , t h e swell was negligible for any degree of c o m p a c t i o n . T h e m a i n reason m o i s t u r e c o n t e n t is i m p o r t a n t is t h a t m o i s t u r e c o n t e n t can generally result in low density fill; n o t t h a t high m o i s t u r e c o n t e n t will r e d u c e swelling. T h e controlling e l e m e n t is density. C o m p a c t i n g stiff clay at 4 t o 5 p e r c e n t above o p t i m u m is very difficult. T h e process of r e c o m p a c t i n g swelling clays at m o i s t u r e c o n t e n t s slightly above t h e i r n a t u r a l m o i s t u r e c o n t e n t and at a low density should be an excellent a p p r o a c h . Referring t o c h a p t e r 2, it was established t h a t t h e swelling pressure of clay is i n d e p e n d e n t of t h e surcharge pressure, initial m o i s t u r e c o n t e n t , degree of s a t u r a t i o n , thickness of s t r a t u m , and increases only w i t h t h e increase of initial dry d e n s i t y . F o r i n s t a n c e , w i t h reference t o figure 2 8 and table 10, b y decreasing t h e dry density of a typical expansive clay from 109 t o 100 pcf, t h e swelling pressure decreases from 1 3 , 0 0 0 t o 5 , 0 0 0 psf and t h e swelling p o t e n t i a l decreases from 6.7 t o 4.2 p e r c e n t . All of this can be accomplished w i t h o u t changing the m o i s t u r e c o n t e n t . T h e m a i n advantage of using this a p p r o a c h is t h a t t h e swelling p o t e n t i a l can be r e d u c e d w i t h o u t t h e adverse effects caused b y i n t r o d u c i n g excessive m o i s t u r e i n t o t h e soil. Figure 22 indicates t h a t t o decrease t h e swelling p o t e n t i a l from 6.7 t o 4.2 p e r c e n t , an increase of m o i s t u r e c o n t e n t of a b o u t 5 p e r c e n t will be required. T h e s h o r t c o m i n g s of p r e w e t t i n g m e t h o d s m e n t i o n e d

in t h e preceding section can b e

eliminated b y c o m p a c t i o n c o n t r o l . Excess w a t e r will n o t be present in t h e soil; t h e r e f o r e , t h e r e will n o t be migration of m o i s t u r e t o t h e u n d e r l y i n g moisture-deficient soils and long w a i t i n g periods, prior t o c o n s t r u c t i o n , will be unnecessary. A reasonably good bearing capacity can be assigned t o t h e low density soil. With m o d e r n c o n s t r u c t i o n t e c h n i q u e s , it is possible t o scarify, pulverize, and r e c o m p a c t t h e n a t u r a l soil effectively w i t h o u t substantially increasing the c o n s t r u c t i o n costs. Design L e o n a r d K r a y n s k i of W o o d w a r d , Clyde & Associates suggests t h e following design p r o c e d u r e on compaction control: 1. A d e q u a t e m i x should be p r e p a r e d for three P r o c t o r cylinders at each m o i s t u r e c o n t e n t . T h e cylinders are t o be c o m p a c t e d using three different efforts, such as 1 2 , 4 0 0 ft.-lb. p e r cu. ft. ( S t a n d a r d A A S H O ) , 2 3 , 0 0 0 ft.-lb. p e r cu. ft., and 5 6 , 2 0 0 ft.-lb. p e r cu. ft. (Modified

A A S H O ) . T h u s , a t o t a l of 12 t o 15 samples will be a d e q u a t e t o

define

m o i s t u r e - d e n s i t y curves as s h o w n on figure 106. 2. F r o m each c o m p a c t e d s a m p l e , a 2-inch-diameter core m a y be e x t r a c t e d and t e s t e d in t h e c o n s o l i d o m e t e r for swell. T h e samples are subjected t o 144-psf surcharge pressure, t h e n submerged in w a t e r and allowed t o swell. F r o m t h e m e a s u r e d p e r c e n t e x p a n s i o n , curves of equal swell were p l o t t e d as s h o w n on figure 107. 3 . F r o m a s t u d y of t h e s e results, a m o i s t u r e c o n t e n t of 19 t o 2 3 p e r c e n t and a d r y d e n s i t y ranging from 9 6 t o 102 pcf were selected as design specifications. Using t h e p l a c e m e n t c o n d i t i o n s , t h e average swell u n d e r a surcharge load of 144 psf is p r e d i c t e d t o be 5

166


Figure 106. Preparation of specimens for earthwork specifications. (after Woodward-Clyde and Associates)

percent with maximum swell potential of less than 8 percent. Such average and maximum swell is considered to be acceptable for the proposed type of construction. 4. The required depth of compaction depends upon the degree of expansion and the magnitude of the imposed loads. Generally, 1 to 5 feet of compacted material will be adequate with the range of 2 to 3 feet being the most commonly used.

SOIL REPLACEMEN T A simple and easy solution for slabs and footings founded on expansive soils is to replace the foundation soil with nonswelling soils. Experience indicates that if the subsoil consists of more than about 5 feet of granular soils (SC-SP), underlain by highly expansive soils, there is no danger of foundation movement when the structure is placed on the granular soils. The mechanics and the path of surface water seeping through the upper granular soils and into the expansive soils is not clear. It is concluded 1}hat either seepage water has never reached the expansive soils, or the heaving of the lower expansive soils is so uniform that structural movement is not noticeable. This is not true in the case of man-made fill. For economic reasons, the extent of the selected fill must be limited to a maximum of 10 feet beyond the building line. Therefore, the possibility of edge wetting exists. A guideline has not been established as to the thickness

SOIL STABILIZATION

0

167

5

10 Moisture Content

15

20

25

30

(%)

Figure 107. Determination of fill placement moisture and density, (after Woodward-Clyde and Associates)

r e q u i r e m e n t for t h e selected fill. A m i n i m u m of 3 feet should always be insisted u p o n , a l t h o u g h 5 feet is preferred. This t h i c k n e s s refers t o thickness of selected fill b e n e a t h t h e b o t t o m of t h e footings or b o t t o m of floor slabs. T h e p e r t i n e n t r e q u i r e m e n t s c o n c e r n i n g soil r e p l a c e m e n t

are t h e t y p e of r e p l a c e m e n t

material, t h e d e p t h of r e p l a c e m e n t , and t h e e x t e n t of r e p l a c e m e n t . Type of

material

Obviously, t h e first r e q u i r e m e n t for the r e p l a c e m e n t soil is t h a t it be n o n e x p a n s i v e . All granular soils ranging from GW t o SC in t h e Unified Soil Classification System may! fulfill t h e n o n e x p a n s i v e soil r e q u i r e m e n t . However, for clean, granular soils such as GW and SP, surface w a t e r can travel freely t h r o u g h t h e soil and cause w e t t i n g of t h e lower swelling soils. In the o t h e r e x t r e m e , SC material w i t h a high percentage of plastic clay s o m e t i m e s will e x h i b i t swelling p o t e n t i a l . T h e following criteria have been used w i t h a certain degree of success:


168 Liquid limit,

Percent m i n u s

percent

N o . 2 0 0 sieve

G r e a t e r t h a n 50 30 -

50

15 -

30

10 -

40

5-50

Less t h a n 3 0

It is b e c o m i n g increasingly difficult t o locate materials, fulfilling the above r e q u i r e m e n t s , in expansive soil areas such as M e t r o p o l i t a n Denver. If necessary, t h e r e q u i r e m e n t for imperviousness can be forfeited. A n y selected fill will be satisfactory provided t h e material is n o n e x p a n s i v e . Also, swell tests are t h e only positive m e t h o d of d e t e r m i n i n g t h e expansiveness of t h e material. When in d o u b t , such tests should be c o n d u c t e d r a t h e r t h a n relying o n plasticity tests. A great deal of emphasis has been given t o t h e possibility of blending granular soil w i t h the on-site swelling soils, t h u s reducing t h e a m o u n t of i m p o r t e d fill required. T h e o r e t i c a l l y , such a m e t h o d is reasonable; b u t in practice it is difficult t o i n c o r p o r a t e granular soil w i t h stiff, dry expansive clays. Disc h a r r o w s and p l o w s will be required t o break the clay i n t o reasonably sized clods. Such an u n d e r t a k i n g will p r o b a b l y be as expensive as using t h e lime stabilization m e t h o d . Depth

of

replacement

T h e d e p t h of influence is a m o s t c o m p l i c a t e d q u e s t i o n t h a t m u s t be answered w h e n dealing w i t h soil t r e a t m e n t b e n e a t h the slabs o r footings. T o w h a t d e p t h should t h e n a t u r a l soil be r e c o m p a c t e d ? H o w m a n y feet of overexcavation will be required? H o w m a n y cubic yards of nonexpansive soil will have t o be i m p o r t e d ? These q u e s t i o n s c a n n o t b e intelligently answered until t h e a m o u n t of m o v e m e n t t h a t will o c c u r b e n e a t h t h e slabs o r footings can be assessed. Theoretically, t h e a m o u n t of uplift can b e evaluated from t h e d a t a derived from swell tests and pressure d i s t r i b u t i o n m e t h o d s .

Gizienski and Lee

[701 evaluated t h e theoretically c o m -

p u t e d uplift derived from l a b o r a t o r y test d a t a and t h e actual m e a s u r e m e n t t a k e n from a small scale field test. T h e y found t h a t t h e actual heave in t h e field was only one-third of t h a t estimated from t h e results of l a b o r a t o r y tests. T h e C o l o r a d o Highway D e p a r t m e n t established curves which show the relationship b e t w e e n t o t a l swell and t h e d e p t h below t h e surface of t h e subgrade [ 7 1 ] . Studies have s h o w n t h a t t h e swelling can t a k e place d o w n t o a d e p t h of as m u c h as 50 feet. Also, 6 0 p e r c e n t of t h e swell in m a n y of t h e C o l o r a d o subgrade clays can o c c u r d o w n t o a 20-foot d e p t h . While b o t h t h e t h e o r e t i c a l a p p r o a c h and actual m e a s u r e m e n t concerning d e p t h of influence are urgently n e e d e d , t h e following should be p o i n t e d o u t : 1. T h e p o t e n t i a l vertical rise of a soil mass, say 10-by 10-by 3-feet, (such as t h a t used in Gizienski's e x p e r i m e n t ) u n d e r u n i f o r m s a t u r a t i o n c o n d i t i o n s , can be less t h a n t h a t of t h e same mass subject t o local w e t t i n g only. Uniform wetting t e n d s t o equalize heaving. 2. T h e r e is a definite gain in placing t h e s t r u c t u r e on a nonexpansive soil cushion. Even if t h e d e e p seated soils swell, the m o v e m e n t will be m o r e u n i f o r m , and c o n s e q u e n t l y , m o r e tolerable.

SOIL STABILIZATION

169

3 . T h e d e p t h of selected fill should never be less t h a n 3 6 inches and preferably 4 8 inches. T h e swelling p o t e n t i a l of t h e soil b e n e a t h the fill is very i m p o r t a n t as d e n s i t y and m o i s t u r e c o n d i t i o n s change at various l o c a t i o n s . It should be n o t e d t h a t w i t h 4 feet of fill plus t h e weight of c o n c r e t e , a u n i f o r m pressure of a b o u t 6 0 0 psf is applied t o t h e surface of expansive soils. F o r m o d e r a t e l y swelling soil, such surcharge load can be i m p o r t a n t in preventing p o t e n t i a l heave. 4 . T h e failure of t h e soil r e p l a c e m e n t m e t h o d generally occurs during c o n s t r u c t i o n . If t h e subgrade o r o p e n excavation b e c o m e s w e t t e d excessively before the p l a c e m e n t of the fill, the trapped

w a t e r will cause heaving. In such case, d e t r i m e n t a l heaving will o c c u r

regardless of thickness of the selected fill. T h e soils engineer should have the o p p o r t u n i t y of supervising t h e p l a c e m e n t of fill, or such a scheme should n o t be a d o p t e d . 5. T h e

thickness

of

the

imported

fill can be r e d u c e d if a c o m b i n a t i o n

of t h e soil

r e c o m p a c t i o n and soil r e p l a c e m e n t m e t h o d s is used. T h e n a t u r a l soil is scarified and r e c o m p a c t e d as described u n d e r " C o m p a c t i o n C o n t r o l " for a thickness of a b o u t 2 feet, t h e n a n o t h e r 2 feet of selected c o m p a c t e d fill placed. T h e c o m b i n e d thickness of 4 feet should be a d e q u a t e to c o n t r o l heaving. 6. T h e degree of c o m p a c t i o n of the selected fill d e p e n d s u p o n t h e t y p e of s u p p o r t i n g s t r u c t u r e . F o r s u p p o r t i n g slabs, 9 0 p e r c e n t of s t a n d a r d

P r o c t o r density should

be

a d e q u a t e . F o r s u p p o r t i n g footings, a degree of c o m p a c t i o n of 9 5 t o 100 p e r c e n t should be achieved. Extent

of

replacement

T h e m a i n reason t h a t an artificially selected fill cushion is less effective t h a n a n a t u r a l granular soil b l a n k e t is t h a t in natural c o n d i t i o n s , t h e b l a n k e t e x t e n d s over a large area, m u c h larger t h a n in t h e artificial c o n d i t i o n . In an artificial fill s i t u a t i o n , it is always possible for surface w a t e r t o seep i n t o t h e deep-seated expansive soil at the p e r i m e t e r of the fill. T h e r e f o r e , t h e larger t h e area of r e p l a c e m e n t , t h e m o r e effective t h e fill. Figure 108 shows t h e suggested e x t e n t of r e p l a c e m e n t for b o t h b a s e m e n t and n o n b a s e m e n t c o n d i t i o n s . With this a r r a n g e m e n t , t h e possibility of surface water entering t h e f o u n d a t i o n soil is greatly r e d u c e d . T h e t y p e of material used for backfill should be t h e same as used for t h e underslab selected fill. Evaluation With present t e c h n o l o g y o n expansive soils, soil r e p l a c e m e n t is t h e best m e t h o d t o use in obtaining a stabilized f o u n d a t i o n soil. T h e following are t h e evaluations of soil r e p l a c e m e n t method: 1. It is possible t o c o m p a c t t h e replaced n o n e x p a n s i v e soil t o a high degree of c o m p a c t i o n , t h u s enabling t h e material t o s u p p o r t either heavily loaded slabs or footings. S u c h capability c a n n o t be o b t a i n e d b y t h e p r e w e t t i n g m e t h o d . Also, w i t h t h e c o m p a c t i o n c o n t r o l m e t h o d , a high degree of c o m p a c t i o n o n expansive soils is n o t desirable, a n d , c o n s e q u e n t l y , t h e load carrying capacity is limited.

170


Overexcavated and replaced with nonexpansive f i l l .

-Ground surface

7 Min.

-Drilled Pier Building Lines

NON BASEMENT CONDITION

f— Drilled Pier Building Lines

DEEP BASEMENT CONDITION Figure 108. Suggested extent of fill replacement.

2 . T h e cost of soil r e p l a c e m e n t is relatively inexpensive w h e n c o m p a r e d t o chemically treating t h e soil. N o special c o n s t r u c t i o n e q u i p m e n t , such as disc h a r r o w , spreader, o r mixer

will be required. T h e c o n s t r u c t i o n

can be carried o u t w i t h o u t

delay as is

e n c o u n t e r e d in t h e p r e w e t t i n g m e t h o d . 3 . T h e granular soil cushion also serves as an effective barrier against t h e rise of g r o u n d w a t e r o r perched water. 4 . With t h e e x c e p t i o n of a s t r u c t u r a l floor slab (suspended floor), soil r e p l a c e m e n t provides t h e safest a p p r o a c h t o slab-on-ground c o n s t r u c t i o n . 5. T o guard against u n e x p e c t e d

c o n d i t i o n s which might cause heaving, it is strongly

suggested t h a t floating slab c o n s t r u c t i o n be used. Slip j o i n t s m u s t be provided for all

SOIL STABILIZATION

171

slab-bearing p a r t i t i o n walls so there is n o c h a n c e of slab m o v e m e n t disturbing t h e structure. 6. Surface drainage a r o u n d

t h e building m u s t b e p r o p e r l y m a i n t a i n e d

so there is n o

o p p o r t u n i t y for w a t e r t o e n t e r t h e expansive soils b e n e a t h t h e selected fill.

LIME S T A B I L I Z A T I O N T h e use of lime t o stabilize subgrade soil has been k n o w n t o engineers all over t h e world for a long time. F o r centuries, t h e Chinese have used lime as a stabilizing agent in f o u n d a t i o n soils. M o d e r n engineering rejected t h e use of lime—in preference t o cement—because t h e c e m e n t a t i o n reaction of lime requires m a n y m o n t h s and t h e gain in s t r e n g t h is m u c h smaller t h a n with c e m e n t . Since s t r e n g t h is n o t a r e q u i r e m e n t , lime is a favorable agent t o r e d u c e the swelling p o t e n t i a l of f o u n d a t i o n soils. Most of t h e lime stabilization projects were carried o u t b y t h e highway d e p a r t m e n t s of various states. F o r i n s t a n c e , t h e T e x a s State Highway D e p a r t m e n t used nearly 1/2 million t o n s of lime for stabilization in 1 9 6 9 . A l t h o u g h t h e success of lime-treated subgrade is q u e s t i o n a b l e in m a n y instances, t h e use of t h e lime stabilization m e t h o d has been steadily increasing. Reaction It is generally recognized t h a t t h e a d d i t i o n of lime t o expansive clays will r e d u c e the plasticity of t h e soil and, h e n c e , its swelling p o t e n t i a l . T h e chemical reaction occurring b e t w e e n lime and soil is q u i t e c o m p l e x . T h e stabilization a p p a r e n t l y occurs as t h e result of t w o processes. In o n e process, a base exchange occurs w i t h t h e strong calcium ions of lime replacing t h e weaker ions such as s o d i u m o n t h e surface of t h e clay particle [12].

Also, additional

non-exchanged calcium ions m a y be adsorbed so t h a t t h e t o t a l ion density increases. T h e net result is a low base-exchange capacity for t h e particle w i t h a resulting lower v o l u m e change potential. In t h e o t h e r process, a change of soil t e x t u r e t h r o u g h flocculation of t h e clay particles takes place w h e n lime is m i x e d w i t h clays. As t h e c o n c e n t r a t i o n of lime is increased, t h e r e is a r e d u c t i o n in clay c o n t e n t and a c o r r e s p o n d i n g increase in t h e p e r c e n t a g e o f coarse particles. T h e r e a c t i o n results in r e d u c t i o n of shrinkage and swell and i m p r o v e d w o r k a b i l i t y . W. G. H o l t z

[73]

found

t h a t lime drastically reduces t h e plasticity i n d e x

and

drastically raises t h e shrinkage limit of m o n t m o r i l l o n i t i c clays, as s h o w n on figure 1 0 9 . Application T h e a m o u n t of lime required t o stabilize t h e expansive soils ranges from 2 t o 8 p e r c e n t b y weight. T h e recently c o m p l e t e d

Dallas-Fort W o r t h Regional A i r p o r t

[74]

claims t o

have

u n d e r t a k e n t h e w o r l d ' s largest lime stabilization project, c o n s u m i n g a b o u t 3 0 0 , 0 0 0 t o n s of lime. T h e subsoil consists of 8 t o 16 feet of expansive clay w i t h a p o t e n t i a l vertical expansion equivalent t o 10 p e r c e n t of t h e layer thickness. T h e clays are underlain b y shale of t h e Eagle Ford Formation.

172


PORTERVILLE (US BR.

CLAY

FT.

(U.S.BR.

S

PERCENT LIME ADMIXTURE

HOUSTON

(From SD.

BY WEIGHT

3

WEATHERED

Oota )

40

PERCENT

LIME

ADMIXTURE

BY WEIGHT

BLACK CLAY C.

PIERRE

McDowell ) 'SHALE'

MONTGOMERY

(From Rood s 8 Streets )

2

THOMPSON, S D .

Data)

4

5

COUNTY

CLAY,

ILLINOIS

Thompson )

(From

MR.

LIME

ADMIXTURE

6

PERCENT LIME ADMIXTURE • BY WEIGHT

PERCENT

BY WEIGHT

Figure 109. Effect of lime on plastic characteristics of montmorillonitic clays. (After Holtz).

T h e thickness of t h e t r e a t m e n t ranged from 9 inches for t a x i w a y s and r u n w a y s t o 18 inches for a p r o n s . F o r stabilization, 6 t o 7 p e r c e n t of lime was required. T h e stiff clay subgrade was b r o k e n d o w n with a disc h a r r o w t o m a x i m u m sized clods of 4 t o 6 inches. Lime was applied in slurry consisting of one part lime t o t w o p a r t s w a t e r b y weight. T h e slurry was applied t o t h e subgrade at 4 0 t o 6 0 p o u n d s pressure using w a t e r t r u c k s . T h e application rate was sufficient t o p r o d u c e , within t h e stabilized layer, a d r y lime c o n t e n t of 6

173

SOIL STABILIZATION

p e r c e n t . E x p e r i e n c e w i t h this project indicated t h a t t h e lime t r e a t m e n t n o t o n l y t r a n s f o r m e d t h e soil t o a nonswelling, friable m i x t u r e , b u t also i m p r o v e d the s t r u c t u r a l capacity of t h e t r e a t e d layer. In i n t e r s t a t e highway c o n s t r u c t i o n in Florida, O k l a h o m a , and o t h e r states lime stabilization was used t o a large e x t e n t . In O k l a h o m a [ 7 5 1 , a d e e p plowing t e c h n i q u e was used. T h e subgrade was overexcavated 2 feet, t h e n d e e p p l o w e d b y ripper-type e q u i p m e n t for an additional 2 feet. Lime was t h e n a d d e d , and t h e d e e p plowing o p e r a t i o n was c o n t i n u e d until a good m i x was o b t a i n e d . After

c o m p a c t i o n , t h e 2 feet of soil which had been removed was replaced in

6-inch-thick layers, m i x e d w i t h lime, and c o m p a c t e d . T h e a m o u n t of lime used was a b o u t 3 p e r c e n t b y weight. T h e successful use of mixing lime in expansive soils for highway and airport c o n s t r u c t i o n is encouraging, a l t h o u g h t h e d e p t h of t r e a t m e n t required and t h e results of t h e t r e a t m e n t on a long term basis has n o t b e e n evaluated. Mixing lime in f o u n d a t i o n soils t o r e d u c e swelling has n o t b e e n serious considered in t h e past. It appears t h a t , w i t h t h e k n o w l e d g e gained from airport and h i g h w a y c o n s t r u c t i o n using lime, t r e a t m e n t of underslab soils w i t h lime deserves m o r e a t t e n t i o n . This is especially t r u e in t h e case of large w a r e h o u s e s or school buildings w h e r e t h e floor covers a large area and a structural floor slab is n o t feasible d u e t o t h e high cost. By overexcavating t h e site b o t h in d e p t h (3 t o 4 feet) and area and replacing t h e soil in c o m p a c t e d layers having a d e q u a t e lime t r e a t m e n t , a stable slab can b e e x p e c t e d . With t h e present d a y limited k n o w l e d g e of lime stabilization, footing f o u n d a t i o n s should n o t be placed o n treated expansive soils. Pressure

injection

T h e pressure injection m e t h o d of lime stabilization has b e e n used in J a c k s o n , Mississippi, in Calexico, California, and in T u c s o n , Arizona [ 7 7 1 . T h e m e t h o d consists of pressure injecting lime-water slurry i n t o t h e soil t h r o u g h closely spaced drill holes as s h o w n on figure 110. T h e drilled holes were 5 feet d e e p , located adjacent t o t h e building, and on 3-foot centers. In J a c k s o n , Mississippi, w h e r e t h e soils b e n e a t h 2 0 0 houses were t r e a t e d , it was r e p o r t e d t h a t an estimated

10 p e r c e n t of t h e t r e a t e d soils had t o b e r e t r e a t e d , and 1 p e r c e n t received t h r e e

treatments. Q u e s t i o n s arise concerning t h e lime pressure-injection m e t h o d and t h e e x t e n t of lime migration i n t o t h e swelling soils. Expansive clays are generally stiff and practically impervious. Lime slurry will disperse from t h e injection p o i n t t h r o u g h r o o t holes, fissures in clay, and desiccation cracks. T h e e x t e n t of such migration is p r o b a b l y limited. A d d i t i o n a l i n f o r m a t i o n o n the swelling p o t e n t i a l or swelling pressure of t h e soils u n d e r t r e a t m e n t in J a c k s o n , Calexico, and T u c s o n is n o t k n o w n , b u t it is very possible t h a t t h e a m o u n t of swell in these areas is mild. L. K. Davidson [ 7 6 ] stated in 1965 t h a t t h e results of l a b o r a t o r y studies s h o w t h a t lime d o e s diffuse i n t o a soil-water s y s t e m . F o r t h e e x p e r i m e n t a l c o n d i t i o n s , the rate of diffusion was very slow and given b y t h e e q u a t i o n : L = 0.081 t* Where: L = lime p e n e t r a t i o n d i s t a n c e , in. t = time, days


174

• t z -

DRILL

OR TRENCH THROUGH

DRILLED HOLES OR PRESSURE INJECTION POINTS

CONCRETE .

TYPICAL PLAN

INJECT SLURRY USING TWO PIPE SYSTEM OUTER PIPE 3 / 4 INCH DIAMETER, POINTED AT BOTTOM AND PERFORATED IN LOWER FOOT WITH 1/8" HOLES-, INNER PIPE IS 1/4" IN. DIAMETER. THE PIPES ARE JETTED IN THE GROUND. CONTINUE TO INJECT SLURRY IN EACH HOLE UNTIL SLURRY COMES OUT OF GROUND AROUND ΓΗΕ PIPE. REPORTED INJECTION PRESSURES AT NOZZLE ARE IN THE RANGE OF 2 0 0 TO 4 0 0 PSI TYPICAL SLURRY PROPORTIONS • 5 0 SACKS HYDRATED LIME ( 5 0 LBS / S A C K ) TO 9 0 0 OF WATER, Co ( 0 H ) 2 CONTENT LIME AVERAGES 9 5 % . LIME AND WATER ARE MIXED IN A BLENDING TANK PRIOR INJECTION.

GALLON S 3 / 4 " DIAMETER INJECTION HOLES 3 FOOT CENTERS (TYPICAL)

OF IN SLURRY TO

AT

Figure 110. Lime stabilization - pressure injection method. (Calexico, Calif. & Jackson, Miss.).

Using this formula results in a p e n e t r a t i o n distance of 1.5 inches in 1 year. It is t h e conclusion from b o t h l a b o r a t o r y and field e x p e r i e n c e t h a t lime m i g r a t i o n i n t o expansive soils is e x t r e m e l y slow. T h e rate of m i g r a t i o n can p r o b a b l y be increased b y i n t r o d u c i n g large q u a n t i t i e s of w a t e r t o carry t h e lime slurry. T h e r e is t h e p o t e n t i a l danger of triggering an excessive a m o u n t of swelling in t h e d e e p seated soils. Woodward-Clyde-Sherard

& Associates [ 7 7 1 , in their investigation, c o n c l u d e d t h a t

the

success of lime t r e a t m e n t is p r o b a b l y because of m o i s t u r e barrier effects r a t h e r t h a n because of any widespread changing of soil p r o p e r t i e s .

SOIL STABILIZATION

175

CHEMICAL STABILIZATION Besides the use of lime, o t h e r stabilize expansive soils. C e m e n t

chemicals,

and

b o t h organic and inorganic, can be used t o

fly ash have b o t h b e e n used in the l a b o r a t o r y

with

successful results. Of course, the cost of c e m e n t is considerably m o r e t h a n t h a t of lime. Fly ash is s o m e t i m e s a d d e d t o t h e soil-lime m i x t u r e t o increase p o z z o l a n i c reaction. O t h e r inorganic chemicals such as s o d i u m silicate, calcium h y d r o x i d e , s o d i u m chloride, calcium chloride, and p h o s p h o r i c acid have b e e n used t o stabilize expansive soil. Most of these chemicals are effective u n d e r l a b o r a t o r y c o n d i t i o n s , b u t their application in t h e field is very difficult. T h e r e is n o s u p p o r t i n g evidence t h a t any of t h e chemicals has e c o n o m i c a l l y w o r t h w h i l e benefits [ 7 8 ] .

Cement

stabilization

The hydration

p r o d u c t s of p o r t l a n d c e m e n t include calcium silicate h y d r a t e s , calcium

a l u m i n a t e h y d r a t e s , and h y d r a t e d lime. During h y d r a t i o n , p o r t l a n d c e m e n t releases a large a m o u n t of lime. It is believed t h a t t h e base e x c h a n g e and c e m e n t i n g action of p o r t l a n d c e m e n t w i t h clay is similar t o t h a t of lime. In a d d i t i o n t o t h e above actions, t h e i n c o r p o r a t i o n of p o r t l a n d c e m e n t in clay increases t h e s t r e n g t h of t h e m i x t u r e . T h e resulting p r o d u c t c o m m o n l y k n o w n as soil-cement is familiar t o m o s t soil engineers. T h e a c t i o n of c e m e n t o n clay minerals is t o r e d u c e t h e liquid limit, plasticity i n d e x , and p o t e n t i a l v o l u m e change, and t o increase t h e shrinkage limit and shear strength [ 7 9 ] . Spangler and Patel

[80]

reported

o n the l a b o r a t o r y t r e a t m e n t of an expansive Iowa

g u m b o t i l w i t h p o r t l a n d c e m e n t . T h e a d d i t i o n of 2 p e r c e n t and 4 p e r c e n t of p o r t l a n d c e m e n t considerably reduced t h e p o t e n t i a l v o l u m e change of t h e soil. J o n e s [ 7 2 ] a d d e d 2 t o 6 p e r c e n t of p o r t l a n d c e m e n t t o t h e expansive Porterville clay of California w h i c h resulted in the p r o n o u n c e d r e d u c t i o n of v o l u m e change characteristics. T h e effect of c e m e n t and of lime was a b o u t the same in reducing soil e x p a n s i o n , b u t t h e c e m e n t r e d u c e d t h e shrinkage of air-dried specimens a b o u t 25 t o 50 p e r c e n t m o r e t h a n did the lime. T h e mixing and dispersing m e t h o d s for c e m e n t are nearly identical t o t h o s e for lime. T h e difficulties of u n i f o r m l y i n t r o d u c i n g p o r t l a n d c e m e n t i n t o very fine-grained soils are generally greater t h a n w i t h lime because it is less soluble. B o t h c e m e n t and lime have b e e n used in h i g h w a y c o n s t r u c t i o n for m o d i f y i n g t h e swelling p r o p e r t y of t h e subgrade soil. T h e use of c e m e n t and lime t o stabilize underslab soil in buildings is seldom r e p o r t e d . T h e r e appears t o b e a great p o t e n t i a l for using c e m e n t t o modify

the

underslab soils. With 2 t o 6 p e r c e n t c e m e n t i n c o r p o r a t e d in t h e clay, t h e resulting soil-cement m i x t u r e acts as a semi-rigid slab. If t h e deep-seated soil e x p a n d s , t h e swelling effect t e n d s t o d i s t r i b u t e u n i f o r m l y , t h u s reducing damage caused b y differential heaving. S u c h c o n s t r u c t i o n is particularly favorable for the t r e a t m e n t of a large w a r e h o u s e floor w h e r e a crack-free, level floor is essential and t h e use of a s t r u c t u r a l floor slab is e c o n o m i c a l l y prohibitive. D u e t o lack of strength, t h e use of lime c a n n o t provide a semi-rigid e l e m e n t b e n e a t h t h e slab.


176

A great deal of research and field s t u d y will b e required before c e m e n t stabilization can be economically applied. A n effective application m e t h o d , either b y m i x i n g or b y slurry injection, m u s t b e perfected before t h e scheme can b e considered in practice. Organic

compound

Organic

compounds

stabilize

expansive

soils

by

waterproofing,

by

retarding

water

a d s o r p t i o n , or b y hardening t h e soil w i t h resins. Organic c o m p o u n d s such as Arguard 2 H T or 4-Terf-Butylpyrocatechol have b e e n used w i t h a limited degree of success. Davidson and Glab [ 8 1 ] , in l a b o r a t o r y investigation of highly plastic Iowa subgrade soils, have s h o w n t h a t certain organic c o m p o u n d s which furnish large organic cations w h e n dissolved in w a t e r have considerable p r o m i s e as a d m i x t u r e s t o increase t h e stability of such soils. T h e y found t h a t w a t e r solutions of chemical a d m i x t u r e s of this t y p e decreased plasticity, shrinkage, and swelling of plastic soil samples. A p r o p r i e t a r y liquid k n o w n as Fluid 7 0 5 , 7 0 6 , and 7 0 7 was i n t r o d u c e d b y Soil T e c h n o l o g y C o r p o r a t i o n in Denver, C o l o r a d o . T h e fluid was m i x e d w i t h swelling clays and tested in t h e l a b o r a t o r y for physical characteristics, swelling p o t e n t i a l , and p e r m e a b i l i t y . E x p a n s i o n tests were performed on t h r e e r e m o l d e d specimens of Denver clay shale. One specimen was treated w i t h distilled w a t e r , the second w i t h p r o p r i e t a r y fluid 7 0 5 , and t h e third w i t h p r o p r i e t a r y fluid 7 0 6 . A surcharge load of 100 psf was applied t o each specimen. T h e specimens were saturated w i t h distilled w a t e r and t h e a m o u n t of e x p a n s i o n d e t e r m i n e d . T h e specimen t r e a t e d w i t h fluid 7 0 5 did n o t e x p a n d . T h e specimen t r e a t e d w i t h fluid 7 0 6 was m o d e r a t e l y expansive. T h e specimen t r e a t e d w i t h distilled w a t e r was highly expansive. Figures 1 1 1 , 112, and 113 give t h e test results and t h e change of A t t e r b e r g limits from high plasticity t o nonplastic. T h e p e r m e a b i l i t y tests were p e r f o r m e d o n specimens comprised of a m i x t u r e of 15 p e r c e n t clay and 85 p e r c e n t silica sand b y weight. T h e sand used was Silica Sand N a t u r a l Grain, supplied b y the O t t a w a Silica C o m p a n y , O t t a w a , Illinois. T h e clay used was Aquagel supplied b y t h e Baroid Division of t h e N a t i o n a l Lead C o m p a n y , H o u s t o n , Texas. C o n s t a n t head p e r m e a b i l i t y tests were p e r f o r m e d o n t w o remolded specimens of clay and silica sand using distilled w a t e r in o n e test and t h e p r o p r i e t a r y Fluid 7 0 7 in t h e o t h e r . T h e material was found t o be impervious t o distilled w a t e r during t h e 34-day testing period. The. coefficient

of permeability of t h e p r o p r i e t a r y Fluid 7 0 7 t r e a t e d soil was d e t e r m i n e d t o be

a p p r o x i m a t e l y 6 feet p e r year. Permeability test is i m p o r t a n t , as in actual application the fluid m u s t be able t o migrate i n t o t h e soil. T h e ability of t h e fluid t o p e r m e a t e in t h e impervious soil is encouraging. T h e first large-scale e x p e r i m e n t o n t h e use of t h e p r o p r i e t a r y fluid t o o k place in D e c e m b e r 1974 in Denver, C o l o r a d o . Specially designed e q u i p m e n t as s h o w n o n figures 114 and 115 was used. T h e m a c h i n e could hydraulically b o r e 1-1/2-inch-diameter holes, t h r e e at a t i m e , t o a d e p t h of m o r e t h a n 10 feet in stiff clay and claystone shale. T h e auger was advanced b y a pressure of 3 0 0 t o 5 0 0 psi. T h u s , t h e holes could be advanced a t o t a l of 10 feet in less t h a n a half m i n u t e . T h e holes were spaced 3 6 inches apart and in highly impervious soil t h e spacing was r e d u c e d t o 18 inches. P r o p r i e t a r y fluid was i n t r o d u c e d i n t o t h e holes u n d e r a pressure of a b o u t 10 psi.

SOIL STABILIZATION

177

PLACEMENT CONDITIONS' Dry Density = 76.3 pcf Moisture Content = 1 4 . 0 % Atterberg LimitsLiquid Limit = 6 8 % Plasticity Index = 17 %

^ 3 0 . 8 % Expansion at 100 psf when wetted

ο ο

ιοο

ιοοο

APPLIED PRESSURE (psf) Figure 111. Swell-consolidation test results on remolded sample of Denver clay shale treated with distilled water only.

T h e t r e a t m e n t was i n t e n d e d t o e x t e n d for a d e p t h of at least 6 feet. It was i n t e n d e d t o reduce t h e plasticity i n d e x of the expansive clays from a b o u t 4 0 t o 10 p e r c e n t and t h e swelling p o t e n t i a l from m o d e r a t e swelling t o nonswelling. U n d i s t u r b e d soil samples were t a k e n before and after t r e a t m e n t

t o d e t e r m i n e t h e effectiveness of t h e application. T h e results were n o t as

e x p e c t e d . B o t h t h e plasticity index and t h e swell p o t e n t i a l did n o t significantly r e d u c e . Valuable experience was gained from t h e e x p e r i m e n t , however, some of which follows: 1. T h e holes should have a m a x i m u m spacing of 12 inches. 2. T h e fluid m u s t b e applied u n d e r a pressure of n o t less t h a n 2 5 0 psi. 3 . Pressure gages should be provided t o indicate a pressure d r o p w h e n t h e fluid flows i n t o t h e seams and fissures in t h e clay. T h e auger should t h e n be advanced t o avoid t h e fissures.

It is believed t h a t w i t h further s t u d y o n field application and m e c h a n i c a l i m p r o v e m e n t , t h e above m e t h o d will eventually find an i m p o r t a n t place in t h e realm of chemical stabilization.

178


PLACEMENT CONDITIONS' Dry Density = 8 1 . 4 pcf Moisture Content = 1 1 . 5 % Atterberg Limits = Non-plastic

No Expansion when wetted

8

100

1000

APPLIED PRESSURE (psf) Figure 112. Swell-consolidation test results on remolded sample of Denver clay shale treated with Fluid 705.

SOIL STABILIZATION

179

PLACEMENT CONDITIONS' Dry Density = 7 8 . 6 pcf Moisture Content = 1 2 . 2 % Atterberg Limits = Non-plastic

2 Ο CO

3.7% Expansion at 1 0 0 p s f when wetted

2

X

2 Ο

<

Ο ο

CO

ο ο

100

1000

APPLIED PRESSURE (psf) Figure 113. Swell-consolidation test results on remolded sample of Denver clay shale treated with Fluid 706.

180

Equipment used for chemical injection.

Figure 115.

Injection heads bored hydraulically.


Figure 114.

SOIL STABILIZATION

181

REFERENCES McDowell, C , "Remedial Procedures Used in the Reduction of Detrimental Effect of Swelling Soils," Texas Highway Department. "Recommended Practices for construction of Residential Concrete Floors on Expansive Soil" Portland Cement Association Vol. II, Los Angeles, California, 1970 Bara, J. P., "Controlling the Expansion of Desiccated Clays During Construction," Second International Research Conference on Expansive Clay Soils, August, 1969. Blight, G. E., and Wet, J. Α., "Acceleration of Heave of Structures on Expansive Clay," Moisture Equilibria and Moisture Changes in the Soils Beneath Covered Areas. Felt, E. J., "Influence of Vegetation on Soil Moisture Content and Resulting Soil Volume Changes," Proceedings, Third International Conference on Soil Mechanics and Foundation Engineering, Zurich, Vol. I, 1953. Dawson, R. F., "Modern Practices Used in the Design of Foundations for Structures on Expansive Soils," Quarterly, Colorado School of Mines, Vol. 54, No. 4, 1959. Holtz, W. G., and Gibbs, H. J. "Expansive Clay-Properties and Problems," Quarterly of Colorado School of Mines, Vol. 54, No. 4, October, 1959. Gizienski, S. F. and Lee, L. J., "Comparison of Laboratory Swell Tests to Small Field Tests," Concluding Proceedings, International Research and Engineering Conference on Expansive Clay Soils, Texas A and M Press. Gizienski, S. F. and Lee, L. J., "Comparison of Laboratory Swell Tests to Small Scale Field Tests," International Research and Engineering Conference on Expansive Soils, 1965. "Lime Shaft and Lime Tilled Stabilization of Subgrades in Colorado Highways," Interim Report 1967, Planning and Research Division, Dept. of Highways, State of Colorado. Jones, C. W., "Stabilization of Expansive Clay with Hydrated Lime and with Portland Cement," Bulletin, Highway Research Board, No. 193, 1958. Holtz, W. G., "Volume Change in Expansive Clay Soils and Control by Lime Treatment," Proceedings of the Second International Research and Engineering Conference on Expansive Soils, Texas A & M Press, 1969. Kelly, J. E., "Lime Stabilization of Expansive Clays at the Dallas-Fort Worth Airport," Proceedings of Workshop on Expansive Clays and Shale in Highway Design and Construction. Thompson, M. R., "Lime Stabilization: Deep Flow Style," Road and Streets, March 1969. Davidson, L. K., Demirel, T., and Hardy, R. L., "Soil Pulverization and Lime Migration in Soil-Lime Stabilization," Highway Research Board, 1965. "Remedial Methods Applied to Houses Damaged by High Volume-Change Soils," Woodward-Clyde-Sherard & Associates, FHA Contract H-799. Gromko, G. J., "Review of Expansive Soils," Journal of the Geotechnical Engineering Division, June 1974. Croft, J. B., "The Influence of Soil Mineralogical Composition on Cement Stabilization," Geotechnique, London, England, Vol. 17, June, 1967. Spangler, M. G. and Patel, Ο. H., "Modification of a Gumbotil Soil by Lime and Portland Cement Admixtures," Proceedings, Highway Research Board, Vol. 29, 1949. Davidson, D. T. and Glab, J. E., "An Organic Compound as a Stabilization Agent for Two Soil Aggregate Mixtures," Proceedings Highway Research Board, Vol. 29, 1949.

Chapter

9

INVESTIGATION OF FOUNDATION MOVEMENT

INTRODUCTION Investigating t h e cause of f o u n d a t i o n m o v e m e n t of an existing building and prescribing remedial measures requires careful field investigation, exhaustive l a b o r a t o r y testing, and m a n y years of experience. In some respects, this is similar t o t h e t r e a t m e n t of a p a t i e n t . I n q u i r y of t h e p a t i e n t ' s m e d i c a l record, a physical e x a m i n a t i o n , and a l a b o r a t o r y diagnosis will b e necessary t o diagnose t h e cause of t h e sickness. Prescription and t r e a t m e n t will b e relatively simple once t h e cause of illness has b e e n d e t e r m i n e d . As in t h e case of a d o c t o r , n o e x a m i n a t i o n and testing can replace e x p e r i e n c e , and e x p e r i e n c e can o n l y be o b t a i n e d b y trial and error. In t h e past 2 0 years, t h e a u t h o r has had t h e o p p o r t u n i t y t o s t u d y m o r e t h a n 1,200 cases of cracked buildings in t h e States of C o l o r a d o and W y o m i n g . These cases include residences, school buildings, offices, w a r e h o u s e s , swimming pools, a p a r t m e n t buildings, religious s t r u c t u r e s , and p a v e m e n t s . Most of t h e cracked buildings were t h e result of f o u n d a t i o n m o v e m e n t caused b y swelling soils.

HISTORY STUDY T h e first step in t h e investigation of a building is t o o b t a i n c o m p l e t e i n f o r m a t i o n pertaining t o t h e building. U n f o r t u n a t e l y , such i n f o r m a t i o n is o f t e n t i m e s absent and it is necessary t o u n c o v e r m u c h of t h e required i n f o r m a t i o n b y soil e x p l o r a t i o n . Foundation

information

Effort should b e m a d e t o o b t a i n t h e existing i n f o r m a t i o n f o u n d a t i o n relative t o t h e soil. F o r buildings erected before 1 9 6 0 , such i n f o r m a t i o n is generally s k e t c h y . Soil tests o n individual sites have b e o m e a r e q u i r e m e n t after 1 9 6 0 . F r o m t h e soil test d a t a , it will b e possible to d e t e r m i n e t h e following: 1. T y p e of f o u n d a t i o n , 2. Design criteria, 3 . Water table c o n d i t i o n , 4 . T y p e of f o u n d a t i o n soils, 5. Moisture c o n t e n t of f o u n d a t i o n soils, and 6. Swelling p o t e n t i a l of f o u n d a t i o n soils.

184


S o m e t i m e s , t h e subsoil investigation is n o t c o n d u c t e d for a specific building b u t for a general area. In such case, t h e subsoil i n f o r m a t i o n has only limited use. Care should be exercised t o locate t h e building u n d e r investigation t o t h e nearest test h o l e , so t h a t it is possible t o d e t e r m i n e as closely as possible t h e subsoil c o n d i t i o n b e n e a t h t h e building. T h e above soil test d a t a can be invaluable t o w a r d finding t h e cause of s t r u c t u r e m o v e m e n t . The second step is t o check the f o u n d a t i o n plan. Again, such i n f o r m a t i o n m a y n o t be available, either because t h e drawing is lost, or t h e c o n c e r n e d p a r t y d o e s n o t w a n t t o p r o d u c e it. T h e foundation plan will reveal if t h e r e c o m m e n d a t i o n s given in t h e soil r e p o r t have been followed. These are: 1. T h e dead load pressure exerted on t h e footings o r piers, 2. T h e size of footings or piers, 3 . T h e length of t h e piers, 4 . Pier r e i n f o r c e m e n t , e x p a n s i o n j o i n t , d o w e l bars, underslab gravel, and o t h e r details, and 5 . Subdrainage system. If t h e above i n f o r m a t i o n is available, t h e investigation will b e greatly simplified. This is similar t o t h e case w h e r e t h e c o m p l e t e m e d i c a l record of a p a t i e n t is at t h e disposal of t h e examining d o c t o r . It is also necessary t o e x a m i n e t h e qualifications of t h e designer, w h e t h e r the design is m a d e b y a registered professional engineer or b y t h e c o n t r a c t o r . If t h e above i n f o r m a t i o n is n o t available, it will b e necessary t o e x p o s e t h e f o u n d a t i o n system b y excavation. In t h e case of a n o n b a s e m e n t building, excavation can be easily m a d e outside of t h e building adjacent t o t h e grade b e a m . In t h e case of b a s e m e n t c o n s t r u c t i o n , it will be necessary t o break t h e c o n c r e t e slab t o reach t h e f o u n d a t i o n . It w o u l d b e a difficult j o b t o expose t h e entire length of t h e pier, b u t m a n y times it is advisable t o e x a m i n e t h e pier t o ascertain a p r o b l e m such as uplift. Logs k e p t b y t h e driller are s o m e t i m e s available. In such cases, a c o m p l e t e i n f o r m a t i o n of the pier system will be a p p a r e n t . This will also provide i n f o r m a t i o n o n t h e d e p t h of p e n e t r a t i o n into b e d r o c k , for b o t h i n t e r i o r and e x t e r i o r piers, as well as t h e w a t e r table c o n d i t i o n . Movement

data

Effort should be m a d e t o o b t a i n chronological d a t a o n t h e building m o v e m e n t , items such as w h e n t h e building was c o m p l e t e d , w h e n t h e first o c c u p a n t m o v e d in, and w h e n the first crack appeared. All i n f o r m a t i o n o b t a i n e d from t h e o w n e r should be carefully scrutinized for its validity. If t h e o w n e r i n t e n d s t o sue t h e builder t o recover his damages, h e t e n d s t o exaggerate his findings. However, w i t h careful i n t e r r o g a t i o n and k e e n observation, the actual s t o r y can be revealed. When examining t h e e x t e r i o r of t h e building, it is helpful to d e t e r m i n e the lawn watering practice, t h e setting of t h e a u t o m a t i c sprinkling s y s t e m , and t h e c o n d i t i o n of t h e backfill. Most owners d e n y excessive irrigation of t h e lawn and flower b e d s . In t h e interior of t h e building, p r i m a r y i n f o r m a t i o n can b e o b t a i n e d in t h e b a s e m e n t area. Water m a r k s or effloresce o n t h e wall usually tell t h e s t o r y of seepage water. A c o m p l e t e record on seepage w a t e r should be o b t a i n e d ; t h e first a p p e a r a n c e of w a t e r in t h e b a s e m e n t , the l o c a t i o n


185

of seepage, t h e a m o u n t of observed w a t e r , and w h e t h e r seepage has t a k e n place after heavy precipitation. Also i m p o r t a n t is t h e p e r f o r m a n c e of utility lines. Has t h e r e b e e n p l u m b i n g difficulty experienced in t h e past years? Has t h e floor drain b e e n plugged? In o n e instance, investigation revealed t h a t t h e interior h o u s e sewer was never c o n n e c t e d to t h e street sewer, b u t e m p t i e d i n t o t h e underslab soils. T h e defect was n o t discovered until an o d o r was d e t e c t e d in t h e b a s e m e n t . In a n o t h e r case, t h e b a s e m e n t s h o w e r drain was n o t c o n n e c t e d t o t h e sewer line. F o r years, t h e error remained u n d e t e c t e d until the crawl space was e n t e r e d and an excessive w e t t i n g c o n d i t i o n discovered. It is n o t always possible t o d e t e r m i n e t h e site c o n d i t i o n s during c o n s t r u c t i o n , b u t if such i n f o r m a t i o n is secured b y an observant o w n e r , it can u n l o c k m a n y m o v e m e n t puzzles. T h e r e are instances w h e r e t h e soils were flooded during c o n s t r u c t i o n , and heaving m o v e m e n t t o o k place even before t h e building was c o m p l e t e d . Investigation of partially c o m p l e t e d h o u s e revealed t h a t t h e b a s e m e n t was covered w i t h m o r e t h a n 2 feet of s n o w w h i c h t h e c o n t r a c t o r had failed t o remove before enclosing t h e s t r u c t u r e . F o r drilled pier f o u n d a t i o n s , it is d a n g e r o u s t o allow t h e s u r r o u n d i n g soils t o b e c o m e w e t t e d before t h e application of dead load pressure. Pier uplift can begin before the placing of t h e f o u n d a t i o n c o n c r e t e . In t h e i n f o r m a t i o n gathering process, all hearsay should b e screened. Stories such as an u n d e r g r o u n d river r u n n i n g u n d e r t h e s t r u c t u r e , t h e building is sliding d o w n h i l l , t h e b e n t o n i t e in t h e soil has pulled t h e building apart and o t h e r s should be dismissed as hearsay b y an experienced engineer, and only substantive evidence considered.

DISTRESS STUDY T h e first sign of f o u n d a t i o n m o v e m e n t , for s t r u c t u r e s f o u n d e d o n expansive soils, is t h e cracking of t h e floor slab. This is generally followed b y d o o r s b i n d i n g , w i n d o w s sticking, and cracks appearing in t h e e x t e r i o r and interior walls and even in t h e ceiling. Crack

pattern F o u n d a t i o n m o v e m e n t s are reflected as cracks. Cracks caused b y swelling soils have t h e

same general p a t t e r n as s e t t l e m e n t cracks, a l t h o u g h swelling cracks are generally w i d e at t h e t o p and n a r r o w at t h e b o t t o m . T h e same crack p a t t e r n s can develop from s e t t l e m e n t . However, in t h e m o s t severe s e t t l e m e n t cases, diagonal cracks are usually associated w i t h a series of h o r i z o n t a l cracks as s h o w n o n figure 116. In t h e R o c k y M o u n t a i n area, t h e p r o b l e m caused b y expansive soil is well k n o w n , n o t o n l y t o t h e soil engineer,

b u t o f t e n t i m e s even t o t h e l a y m a n . A t t h e first

sign of cracking, t h e i m m e d i a t e r e a c t i o n is t h a t t h e p r o b l e m is caused b y swelling soils. F i g u r e 117 indicates a severe crack t h a t developed in a twin-tee s t r u c t u r a l slab. T h e building is f o u n d e d w i t h drilled piers, and t h e e x t e r i o r walls are in excellent c o n d i t i o n . These cracks had n o t h i n g t o do with foundation movement. It is n o t always t r u e t h a t f o u n d a t i o n m o v e m e n t of a specific p o r t i o n of a s t r u c t u r e is responsible for certain cracks appearing in t h e i m m e d i a t e vicinity of t h a t m o v e m e n t .

The

186


Figure 116. Typical settlement cracks; crack pattern varies f r o m horizontal t o diagonal which is quite different f r o m heaving cracks.

Figure 117.

Floor cracks due t o shrinkage of twin-tee topping.

187


Figure 118. Vertical cracks beneath the beam pocket caused by lifting of I-beam.

structural a r r a n g e m e n t of a building, especially t h a t of a h o u s e , is c o m p l e x . M o v e m e n t of one p o r t i o n of t h e building can cause cracks t o appear at t h e o p p o s i t e end of t h e building. It is always p r u d e n t to explain t h e cause of m o v e m e n t in a general sense and treat and s t u d y the m o v e m e n t as a unit. T h e following crack analysis can serve as a guide: 1. Diagonal cracks b e l o w e x t e r i o r w i n d o w s or above e x t e r i o r d o o r s generally indicate footing or drilled pier f o u n d a t i o n m o v e m e n t . 2. If such cracks a p p e a r only in t h e e x t e r i o r brick course b u t n o t on t h e interior d r y wall, the cracks can be caused b y e x t e r i o r p a t i o slab heaving. 3 . Hairline cracks appearing above interior d o o r s and closets could be caused b y plaster shrinkage or t i m b e r shrinkage are n o t necessarily f o u n d a t i o n m o v e m e n t . 4 . Vertical cracks below t h e I-beam in t h e b a s e m e n t c o n c r e t e wall can be caused b y the lifting of t h e I-beam, resulting in tension cracks as s h o w n in figure 1 1 8 . 5. Separation of t h e w i n d o w frame from t h e brick course as s h o w n o n figure 119 generally indicates

differential

heaving. Such m o v e m e n t

has a strong resemblance t o

m o v e m e n t . Actually, almost all lateral separation is caused b y differential heaving.

lateral

188


Figure 119. Separation of window frame from brick course. Stress

build-up M o v e m e n t of interior structural m e m b e r s can result in stress build-up in t h e s t r u c t u r e . T h e

m o s t c o m m o n instance is t h e uplift of t h e I-beam caused b y t h e uplift of steel pipe c o l u m n s . When t h e I-beam lifts, t h e joist system in t h e u p p e r level is d i s t u r b e d , d o o r s stick and closets c a n n o t b e o p e n e d . T h e o w n e r generally planes t h e d o o r only t o find t h a t it fails t o open and close again after a period of time. T h e I-beam in t h e b a s e m e n t is c o m m o n l y s u p p o r t e d b y t w o t o t h r e e steel pipe c o l u m n s . When t h e o n e pipe c o l u m n f o u n d a t i o n heaves, t h e o t h e r pipe c o l u m n is usually rendered idle and can be shaken loose b y h a n d . Pipe c o l u m n s are provided w i t h a screw jack at t h e t o p . T h e situation can be c o r r e c t e d , at least t e m p o r a r i l y , b y lowering t h e screw j a c k and revealing the I-beam. T h e d o o r s are t h e n able t o b e o p e n e d and closed freely again.


189

Stress build-up caused b y slab bearing p a r t i t i o n walls has b e e n discussed u n d e r "Slabs on Expansive Soils." O w n e r s s o m e t i m e s r e p o r t t h a t t h e cracks in their buildings are subject t o o p e n i n g and closing and a t t e m p t t o correlate the m o v e m e n t t o seasonal climate change. This t h e n leads t o the t h e o r y t h a t t h e subsoil has u n d e r g o n e cycles of d r y i n g and w e t t i n g . In m o s t cases, t h e opening and closing of t h e cracks are caused b y t h e shifting of t h e l o c a t i o n of stress c o n c e n t r a t i o n . When a new crack appears, t h e stress d i s t r i b u t i o n is altered, and this will t e m p o r a r i l y close an old crack. Careful observation will indicate t h a t t h e t o t a l n u m b e r of cracks appearing in a building is c o n s t a n t l y increasing and seldom decreasing.

INVESTIGATION Subsoils T o definitely define

t h e cause of f o u n d a t i o n m o v e m e n t and t o r e c o m m e n d

remedial

measures, it is necessary t o d e t e r m i n e t h e subsoil c o n d i t i o n s and w a t e r table. Test holes should be drilled adjacent t o t h e building and sufficient samples should be t a k e n for t h e d e t e r m i n a t i o n of t h e swelling characteristics and m o i s t u r e c o n t e n t of t h e soil. A t least o n e test hole should be drilled r e m o t e from t h e s t r u c t u r e and in an area unaffected b y building c o n s t r u c t i o n . T h e physical characteristics of t h e soils o b t a i n e d from t h e adjacent and r e m o t e test holes can be compared. It should b e n o t e d t h a t for a building having cracking, t h e soils i m m e d i a t e l y b e l o w the f o u n d a t i o n level generally have b e e n w e t t e d excessively. L a b o r a t o r y testing will invariably show a low swell p o t e n t i a l . However, careful testing can reveal t h a t t h e material possesses a high swelling pressure. S o m e t i m e s , t h e actual swelling characteristics of t h e soil can only be revealed by air drying t h e soil sample, and t h e n subjecting it to w e t t i n g . In a n y event, samples o b t a i n e d from areas unaffected b y building c o n s t r u c t i o n should give i n f o r m a t i o n relative t o t h e soil behavior at t h e t i m e of c o n s t r u c t i o n . T h e m o s i t u r e c o n t e n t as well as t h e d r y d e n s i t y of all soil samples should b e d e t e r m i n e d . If possible, t h e m o i s t u r e c o n t e n t should be carefully c o m p a r e d w i t h t h e m o i s t u r e c o n t e n t of t h e soil prior t o building c o n s t r u c t i o n . Usually, in t h e course of nearly 1,200 cases investigated, the m o i s t u r e c o n t e n t b e n e a t h t h e building area had increased. T h e m a g n i t u d e of increase ranged from 2 t o 8 p e r c e n t . T h e c o m m o n l y assumed t h e o r y t h a t t h e soils b e n e a t h a s t r u c t u r e are subject t o w e t t i n g and d r y i n g , resulting in e x p a n s i o n and shrinkage, is n o t necessarily t r u e . In fact, drying and shrinkage of soils seldom or never cause cracking in a building. In o n e building, t h e soil i m m e d i a t e l y b e n e a t h the furnace in t h e b a s e m e n t was e x a m i n e d . T h e underslab m o i s t u r e c o n t e n t was high. N o shrinkage is likely to take place b e n e a t h t h e central p o r t i o n of a covered area. Survey T o m o s t s t r u c t u r a l engineers, t h e first o r d e r of investigation of a cracked building is a survey t o d e t e r m i n e w h i c h p a r t of t h e building has m o v e d and t h e m a g n i t u d e of m o v e m e n t . A d m i t t e d l y ,


190

a survey will assist in t h e d e t e r m i n a t i o n of t h e general trend of building m o v e m e n t ; however, it should never be used as a clue t o f o u n d a t i o n m o v e m e n t . A m o v e m e n t survey is of little value if n o t c o m p a r e d w i t h a previous survey r e c o r d . Also, t h e results will b e of d o u b t f u l value if a reliable b e n c h m a r k is n o t utilized. Bench m a r k s such as t h e t o p Of a fire h y d r a n t , t e l e p h o n e pole or m a n h o l e cover are subject t o m o v e m e n t in an expansive soil area, and readings w h i c h rely o n these b e n c h m a r k s can be entirely misleading. A reliable b e n c h m a r k should consist of a c o n c r e t e pier drilled d e e p i n t o b e d r o c k in a zone w h e r e m o i s t u r e change will n o t take place. T h e pier should b e well reinforced. By referring t o a reliable b e n c h m a r k and c o n d u c t i n g a survey at intervals of once a m o n t h , t h e m o v e m e n t of t h e building can t h u s b e m o n i t o r e d . T h e survey m u s t be c o n d u c t e d w i t h c o n t r o l p o i n t s carefully selected. Such u n d e r t a k i n g is costly and time c o n s u m i n g . O n l y in t h e course of an i m p o r t a n t s t r u c t u r e is such a survey w a r r a n t e d . T h e c o m m o n practice of referring t h e m o v e m e n t of t h e building t o a brick course or to t h e t o p of t h e grade b e a m , assuming t h a t these are level at t h e time of c o n s t r u c t i o n , can be totally misleading. A m o v e m e n t

survey of a cracked building is of little value unless c o n d u c t e d

professionally.

Test

pits T h e only positive m e t h o d of d e t e r m i n i n g t h e subsoil c o n d i t i o n and c o n s t r u c t i o n details in

t h e f o u n d a t i o n system is b y opening test pits. If t h e building has a crawl space, t h e investigation p r o c e d u r e can b e greatly simplified. Pits should be o p e n e d adjacent to the grade b e a m and n e x t t o t h e interior s u p p o r t s . By investigating t h e test pit, t h e following can be revealed: 1. T h e f o u n d a t i o n system, 2. C o n d i t i o n of t h e air space b e n e a t h t h e grade b e a m (drilled pier f o u n d a t i o n ) , 3 . C o n d i t i o n of t h e t o p of pier, t h e presence of m u s h r o o m s , 4 . T h e m o i s t u r e c o n t e n t of t h e soil (soil samples should b e t a k e n every 12 inches t o a d e p t h of at least 5 feet), 5. T h e presence of underslab gravel, gradation of gravel and thickness of t h e gravel layer, and 6. C o n d i t i o n of t h e c o n c r e t e slab, t y p e of r e i n f o r c e m e n t . Wherever possible, at least o n e pier should be uncovered for its entire d e p t h and examined for possible tension cracks o r voids at t h e b o t t o m of t h e pier.

CAUSE O F MOVEMENT When all t h e investigation and s t u d y outlined above have b e e n c o m p l e t e d , t h e cause of m o v e m e n t of t h e building can t h e n be d e t e r m i n e d . Obviously, n o m o v e m e n t will t a k e place in an expansive soil area unless t h e f o u n d a t i o n soil b e c o m e s w e t t e d excessively. of m o i s t u r e m u s t b e d e t e r m i n e d .

Therefore, t h e source


Foundation

191

design

F o u n d a t i o n designs m a d e before 1960 are based u p o n soil r e p o r t s w r i t t e n w i t h a limited k n o w l e d g e of swelling soil p r o b l e m s and their solutions. C o n s e q u e n t l y , it is n o t surprising t o find t h a t t h e criteria established in these early soil r e p o r t s are n o t sufficient t o cope w i t h t h e c o m p l e x i t y of t h e swelling p o t e n t i a l of t h e soil. T h e usual s h o r t c o m i n g s of t h e given criteria are insufficient d e a d load pressure exerted on a pier or pad f o u n d a t i o n system, insufficient pier length, and lack of r e i n f o r c e m e n t . A c o m m o n design defect is t h e use of m o r e s u p p o r t s t h a n necessary b e n e a t h t h e I-beam. Dead load pressure exerted o n t h e interior piers is, therefore, very low. These piers are subject t o uplift. A remedial m e a s u r e w o u l d b e t o use a heavier b e a m section w i t h fewer s u p p o r t s . T r a d i t i o n a l design is t o d o w e l t h e e x t e r i o r p a t i o slabs i n t o t h e grade b e a m s w i t h d o w e l bars. This p r o c e d u r e has caused a great deal of heaving p r o b l e m s as explained previously in c h a p t e r 6 u n d e r "Slabs o n Expansive Soils." Despite t h e possible deficiency of a drilled pier or footing system, a p r o p e r l y engineered f o u n d a t i o n system suffers only m i n o r distress even u n d e r t h e m o s t adverse c o n d i t i o n s . Most buildings t h a t suffer severe d a m a g e are designed and c o n s t r u c t e d b y c o n t r a c t o r s w i t h o u t t h e benefit of a soil or structural engineer. S o m e c o n t r a c t o r s t a k e the m a t t e r entirely i n t o their o w n h a n d s and p r e w e t t h e f o u n d a t i o n , p u d d l e t h e backfill, reinforce the footings instead of the f o u n d a t i o n walls, drill oversized piers and e x p e c t a stable building. U n f o r t u n a t e l y , o r f o r t u n a t e l y , these buildings m a y r e m a i n in good c o n d i t i o n for years and give an excuse to the builder to c o n t i n u e this undesirable practice. Construction Legally, if t h e c o n t r a c t o r follows every detail specified b y his consultants—the structural and soil engineers—then his liability in t h e event of future building damage will b e greatly reduced. Obviously, t h e c o n t r a c t o r is t h e first target of t h e o w n e r in a lawsuit filed

for

negligence. Damage caused b y swelling soils is n o t recognized b y t h e c o u r t as an act of G o d . In t h e c o u r t , every effort will be m a d e t o prove t h e c o n t r a c t o r ' s negligence. F o r i n s t a n c e , the absence of slab r e i n f o r c e m e n t should n o t be i m p o r t a n t w i t h respect t o slab cracking; however, in t h e c o u r t , this appears as a glaring m i s t a k e on t h e p a r t of t h e c o n t r a c t o r for n o t following the design. Most c o n t r a c t o r s a t t e m p t t o d o a good j o b in their building. Details such as t h e separation of slabs from bearing walls, t h e use of a d o w e l b a r t o c o n n e c t p a t i o slabs w i t h grade b e a m s , t h e absence of void spaces in t h e slab bearing p a r t i t i o n walls and o t h e r s are t h e results of i n c o m p l e t e specifications and t h e ignorance of t h e c o n t r a c t o r o n t h e t e c h n i q u e of building in expansive soil areas r a t h e r t h a n purposely o m i t t i n g t h e details. U n f o r t u n a t e l y , in the c o u r t of justice, n o differentiation can be m a d e b e t w e e n ignorance and i n t e n t i o n a l errors. T h e r e are e x c e p t i o n s w h e r e t h e c o n t r a c t o r abuses every rule of good c o n s t r u c t i o n practice in t h e f o u n d a t i o n c o n s t r u c t i o n and e x p e c t s t o go u n d e t e c t e d because t h e covered f o u n d a t i o n excavation will n o t be q u e s t i o n e d after t h e building is c o m p l e t e d . In o n e large a p a r t m e n t c o m p l e x , t h e piers were drilled off c e n t e r from t h e grade b e a m s , s o m e having missed t h e grade

192


b e a m s c o m p l e t e l y . Piers w e r e b o t t o m e d o n t h e u p p e r soils instead of t h e b e d r o c k , resulting in c h i m n e y tilting. Some of t h e c o n s t r u c t i o n defects are s h o w n on figure 120. Drainage Water entering f o u n d a t i o n soils can b e from o n e or m o r e of the following sources: 1. Rise of g r o u n d w a t e r or d e v e l o p m e n t of p e r c h e d water, 2. P o o r surface drainage causing surface w a t e r to e n t e r t h r o u g h backfill i n t o t h e f o u n d a t i o n soils, 3 . Breakage of utility lines. A perched w a t e r table c o n d i t i o n c a n n o t be foreseen at t h e t i m e of c o n s t r u c t i o n and is usually n o t m e n t i o n e d in t h e soil r e p o r t ; t h e r e f o r e , a s u b d r a i n system is generally n o t specified in t h e design. T h e responsibility of d a m a g e caused b y perched w a t e r is difficult to define. When

Figure 120. Timber supports used to correct the missed piers.

193


t h e r e is a possibility of p e r c h e d w a t e r , a subdrain system should b e provided. T o provide an effective subdrain system against perched w a t e r c o n d i t i o n involves considerable cost. Damage caused b y perched w a t e r should be in t h e category of " A n act of G o d " . Almost 9 0 p e r c e n t of t h e w a t e r t h a t enters f o u n d a t i o n soils is derived from surface w a t e r . As discussed in c h a p t e r 7, i n a d e q u a t e slope a r o u n d the building, loose backfill, i m p r o p e r location of t h e sprinkling system, and shrubs and flower b e d s planted adjacent t o the building are all p o t e n t i a l causes of w e t t i n g of f o u n d a t i o n soils. If civil action is instigated resulting from f o u n d a t i o n p r o b l e m s , the strongest defense t h e c o n t r a c t o r possesses against t h e o w n e r ' s suit is t h a t t h e o w n e r has n o t provided p r o p e r drainage a r o u n d t h e building. A n alert developer issues each h o m e o w n e r a m a n u a l on t h e care of drainage a r o u n d t h e h o u s e . In t h e event of a c o m p l a i n t , the b u i l d e r can t h e n have recourse back t o t h e owner. While it is t r u e t h a t p o o r drainage i n t r o d u c e s w a t e r i n t o t h e f o u n d a t i o n soils and causes heaving, t h e role of drainage in t h e cracked building has b e e n exaggerated. Moisture c o n t e n t in f o u n d a t i o n soils can increase substantially t o allow heaving even if t h e drainage a r o u n d t h e h o u s e is in excellent c o n d i t i o n . When investigating a cracked building, t h e m o s t obvious defect is t h a t of i m p r o p e r drainage a r o u n d t h e building. C o n s e q u e n t l y , m a n y investigators c o n t e n d t h a t b y correcting t h e surface drainage, t h e p r o b l e m is solved. Usually, this is far from t h e real cause of f o u n d a t i o n m o v e m e n t . When a soil engineer provides t h e f o u n d a t i o n design criteria, h e should design for s a t u r a t e d soil c o n d i t i o n s . In t h e field, he selects the soil w i t h the highest swell p o t e n t i a l for testing. In the l a b o r a t o r y , h e saturates t h e soil sample to d e t e r m i n e t h e m a x i m u m swell p o t e n t i a l . His design criteria are actually based o n t h e worst possible c o n d i t i o n s . P r e c a u t i o n s given o n drainage are only an added factor of safety. U n f o r t u n a t e l y , the art of coping w i t h present-day expansive soil p r o b l e m s is far from c o m p l e t e and t h e soil engineer can only h o p e that his r e c o m m e n d a t i o n s are carried o u t in full, t h u s minimizing any possible future d a m a g e .

REMEDIAL MEASURES It is relatively easy t o r e c o m m e n d the necessary remedial c o n s t r u c t i o n for a cracked building w h e n t h e cause of f o u n d a t i o n m o v e m e n t has b e e n d e t e r m i n e d . However, it is seldom possible t o r e t u r n t h e building to its original c o n d i t i o n . After t h e remedial c o n s t r u c t i o n has b e e n c o m p l e t e d , it takes a long t i m e for t h e s t r u c t u r e to adjust t o t h e improved state. In t h e m e a n t i m e m i n o r cracks will c o n t i n u e t o o p e n . It m a y t a k e as long as a year before t h e s t r u c t u r e is finally stabilized and cosmetic w o r k can be started. Remedial c o n s t r u c t i o n differs in each case, just as in the case of t h e prescription s u b m i t t e d t o t h e p a t i e n t . It is u n u s u a l w h e n t w o p a t i e n t s receive t h e same prescription. In the last 15 years, m o r e t h a n o n e t h o u s a n d distressed s t r u c t u r e s have b e e n investigated. Of this n u m b e r m o r e t h e n 50

p e r c e n t involved

residential houses w h e r e t h e investigation m e r e l y

consisted

of visual

inspection. Generally, for t h e private dwelling t h e r e c o m m e n d e d remedial c o n s t r u c t i o n is only partially c o m p l e t e d and t h e p r o b l e m c o n t i n u e s .

194


O t h e r cases involve school buildings, w a r e h o u s e s , religious buildings, i n s t i t u t i o n s , swimming p o o l s , and o t h e r lightly loaded s t r u c t u r e s . T h e remedial measures r e c o m m e n d e d for such s t r u c t u r e s are generally followed. In a b o u t 75 p e r c e n t of these cases, t h e o w n e r s are h a p p y w i t h t h e results o b t a i n e d from remedial c o n s t r u c t i o n and the case is closed. T h e r e m a i n d e r of t h e cases r e p o r t c o n t i n u e d m o v e m e n t , t h o u g h varying greatly in degree. Again, referring to t h e case of p a t i e n t s and d o c t o r s , s o m e p a t i e n t s are in terminal c o n d i t i o n and t r e a t m e n t can only prolong t h e life span. O t h e r p a t i e n t s visit t h e d o c t o r at an early d a t e , have the disease accurately diagnosed, and t h e p r o b l e m is solved. In a broad sense, the c o m m o n l y used remedial measures are as follows: F o r drilled pier f o u n d a t i o n : 1. Loosen soils a r o u n d t h e pier to reduce uplift pressure. 2. R e c o n s t r u c t void space b e n e a t h t h e grade beams. 3 . Eliminate t h e m u s h r o o m at t h e t o p of piers. 4 . Cut t h e t o p of pier and adjust t h e elevation of t h e piers b y shims. F o r c o n t i n u o u s footing f o u n d a t i o n : 1. Provide voids b e n e a t h t h e footings at calculated intervals t o increase t h e dead load pressure. 2. Post-tension t h e f o u n d a t i o n t o provide structural stability. 3 . Reinforce

existing f o u n d a t i o n

walls w i t h new reinforced

grade b e a m to tie the

s t r u c t u r e as in b o x c o n s t r u c t i o n . 4 . U n d e r p i n t h e s t r u c t u r e w i t h piers drilled i n t o b e d r o c k . F o r individual pad f o u n d a t i o n : 1. Decrease pad size t o increase dead load pressure. 2. U n d e r p i n t h e pad w i t h piers drilled i n t o b e d r o c k . For basement: 1. Saw cut t h e slab along t h e f o u n d a t i o n wall to allow free slab m o v e m e n t . 2. Adjust screw j a c k o n t o p of pipe c o l u m n t o relevel interior I-beam. 3 . Provide slip j o i n t s to all interior slab-bearing p a r t i t i o n walls, including d o o r frames and staircase walls. F o r exterior: 1. R e m o v e and r e c o m p a c t backfill. 2. Provide positive drainage a r o u n d the building. 3 . Provide a d e q u a t e o u t l e t for all d o w n s p o u t s . 4 . Provide c o n c r e t e a p r o n a r o u n d t h e h o u s e . 5. Relocate all lawn sprinkling h e a d s t o a distance of at least 10 feet from t h e building. 6. Remove all shrubs and flower beds which are planted adjacent to the h o u s e . F o r subdrain: 1. Provide a subdrain a r o u n d t h e building in the interior or e x t e r i o r at least 2 4 inches b e l o w t h e lower floor slab. 2. Provide a positive o u t l e t for t h e existing subdrain system. 3 . Provide a u t o m a t i c s u m p p u m p s in the b a s e m e n t .

Case I

DISTRESS CAUSED BY PIER UPLIFT

GENERAL T h e case s t u d y is t h a t of a school building (fig. 121) and is typical of pier uplift. T h e pier load is heavy, c o n s t r u c t i o n is in general u p t o s t a n d a r d , design is a d e q u a t e , and crawl space c o n s t r u c t i o n allows t h e s t r u c t u r e t o b e free of possible damaging effects of slab heaving. Yet damage t o t h e building caused b y uplift, before positive remedial measures were t a k e n , was so severe t h a t evacuation of t h e building was considered for reasons of safety.

HISTORY T h e school was c o m p l e t e d in 1962. It is founded w i t h piers drilled i n t o b e d r o c k . T h e b e d r o c k consists of essentially claystone and s a n d s t o n e shale located at d e p t h s 4 to 23 feet below t h e g r o u n d surface. T h e piers were designed for an end bearing pressure of 2 0 , 0 0 0 psf and a skin friction value of 2 , 0 0 0 psf. T h e piers were also designed for a m i n i m u m dead load pressure of 15,000 psf. T h e piers were t o p e n e t r a t e t h e shale b e d r o c k b y at least 4 feet and only t h e skin friction in t h e b e d r o c k was t o be assumed. T h e pier design system was considered t o be s o u n d in view of t h e limited k n o w l e d g e of pier design in 1 9 6 0 . Shortly after c o m p l e t i o n , distress of t h e building was n o t i c e d . In J u n e , 1 9 6 4 , t h e c o n t r a c t o r was advised t o repair t h e existing d a m a g e . In N o v e m b e r , 1964, t h e piers were found t o be in good c o n d i t i o n b u t surface drainage had n o t b e e n p r o p e r l y provided for and w a t e r had p e n e t r a t e d b e n e a t h t h e s t r u c t u r e causing soil swelling. In J u l y 1966 various c o l u m n s were jacked u p t o level t h e building and steel c o l u m n s were inserted for s u p p o r t . In F e b r u a r y 1 9 7 0 , as a safety p r e c a u t i o n a precast panel over a d o o r w a y had t o be r e m o v e d . In July and D e c e m b e r 1970 repairs were m a d e o n several piers. In J u n e 1971 further repairs were m a d e to a n u m b e r of interior c o l u m n s . A n inspection in July 1971 indicated t h a t m o v e m e n t was still c o n t i n u i n g . In March 1972 t h e a u t h o r was engaged t o m a k e a c o m p l e t e i n d e p e n d e n t investigation i n t o t h e cause of cracking and t o d e t e r m i n e t h e necessary remedial measures. T h e school building consists of t h r e e levels. T h e lower level is designated as Wing C. This level has a lower floor and o n e s t o r y above t h e lower floor. T h e m i d d l e level, designated as Wing Β N o r t h , and t h e u p p e r level, designated as Wing Β S o u t h , are b o t h o n e story high w i t h n o lower floor. T h e g y m n a s i u m , cafeteria, and m u s i c hall are all one s t o r y w i t h a high ceiling and designated as Wing D. T h e n o r t h e r n p o r t i o n of Wing D has a b a s e m e n t locker r o o m . This is t h e only p o r t i o n of t h e entire building w h e r e slab-on-ground c o n s t r u c t i o n is used. T h e r e m a i n d e r of the building is crawl space t y p e c o n s t r u c t i o n . Wing A is located at t h e west side of t h e building

196


WING Β

WING C

Figure 121. Exterior view of school building under study.


197

and is occupied b y a library and a d m i n i s t r a t i o n building. T h e remedial c o n s t r u c t i o n o n Wing C began in 1973 and was c o m p l e t e d in 1 9 7 4 . T h e various wings are s h o w n on figure 122. Most of t h e remedial measures u n d e r t a k e n in t h e past 10 years were c e n t e r e d at Wing C. In t h e crawl space u n d e r Wing C, m a n y c o n c r e t e pedestals w e r e removed and replaced w i t h steel H c o l u m n s . Figure 123 shows t h a t t h e c o n c r e t e pedestals were crushed b y pier uplifting force in t h e same m a n n e r as c o n c r e t e cylinders are crushed in t h e c o m p r e s s i o n testing m a c h i n e . Along t h e n o r t h wall in t h e crawl space, w a t e r seeped freely i n t o t h e crawl space t h r o u g h t h e backfill. Water has b e e n entering b e l o w t h e grade b e a m for a lengthy period of t i m e . A t t h e n o r t h w e s t corner, evidence was found t h a t w a t e r has flowed in freely and has washed t h e soil in t h e crawl space forming channels. In Wing B, b o t h n o r t h and s o u t h , t h e g r o u n d surface was relatively d r y , b u t t h e r e was evidence t h a t t h e soil has been w e t t e d in t h e past d u e t o infiltration of surface w a t e r . T h e lower level c o n s t r u c t i o n is confined in Wing C. T h e east wall of t h e w o r k s h o p revealed severe m o v e m e n t . T h e arts and crafts r o o m s also s h o w e d m a n y areas of extensive d a m a g e .

Figure 122. Plan of school building under study.

198


Figure 123. Compression failure of pedestal placed above pier and beneath grade beam. Uplifting pressure 30,000 psf.


199

Surprisingly, t h e slab-on-ground p o r t i o n of t h e locker and boiler r o o m s showed practically n o foundation movement. In t h e u p p e r level, n u m e r o u s cracks were found in t h e i n t e r i o r walls of Wings Β and C. Almost every interior p a r t i t i o n in these areas was cracked. T h e ceiling had pulled away from t h e structural walls b y as m u c h as 3 inches at t h e n o r t h end of t h e n o r t h - s o u t h corridor b e l o w Wing D and t h e rest of t h e building as s h o w n on figure 124. E x t e r i o r cracks were found in t h e grade b e a m s and brick courses, particularly o n t h e n o r t h wall of Wing C. Hairline cracks were found in t h e reinforced c o n c r e t e b e a m s in the crawl spaces, particularly in Wing C.

INVESTIGATION Swelling

potential

Six test holes and t w o test pits were excavated in 1972 at t h e locations s h o w n on figure 122 and u n d i s t u r b e d samples t a k e n from t h e test holes. Swell tests c o n d u c t e d o n samples t a k e n from test holes located at t h e s o u t h side and west side of t h e building indicate t h a t t h e e x p a n s i o n is a b o u t 1 t o 2 p e r c e n t and t h e swelling pressure is a b o u t 3 , 0 0 0 t o 4 , 0 0 0 psf. Since t h e test h o l e s were drilled adjacent to the building, it is likely t h a t d u e t o excessive w e t t i n g c o n d i t i o n , the soil

Figure 124. Cracks and separation of brick from ceiling.

200


has already swelled to its maximum limit. Consequently, the swell tests cannot reveal the initial soil condition. Swell tests performed on undisturbed samples taken from test holes drilled outside of the building area present a different condition. The upper clays swell about 6 percent with a swelling pressure as high as 25,000 psf. The typical dry clays found near the crawl space, after remolding and upon subsequent wetting, exhibit high swell potential as shown on figure 125. The swell characteristic represents the actual condition of the subsoil at the time the building was constructed. Assuming the swelling pressure of the upper clays is 15,000 psf, the following calculation will indicate the stress condition around the piers: Data:

Pier diameter

= 30 in.

Pier circumference

= 7.8 ft.

Pier end area

= 4.9 sq. ft.

Portion of pier in upper clay

= 8.0 ft.

Portion of pier in bedrock

= 4.0 ft.

Swelling pressure of upper clays

Figure 125.

= 15,000 psf

Typical swell test performed on remolded samples.


201

P o r t i o n of swelling pressure responsible t o uplift

• 15,000 x 0.15 = 2 , 2 5 0 psf

Skin friction

= 2 , 0 0 0 psf

then Total uplifting

:

force

(Total area of pier exposed t o w e t t i n g ) χ (Unit uplift)

= 7.8 χ 8 χ 2 , 2 5 0 :

Total withholding

force

140.4 kips

= (Total area of pier in b e d r o c k ) χ (Unit skin friction) = 7.8x4x2,000 = 6 2 . 4 kips

Net uplift

Moisture

= 1 4 0 . 4 - 6 2 . 4 = 7 8 . 0 kips

force

analysis

T h r e e piers, S-40, T-20, and W-38 were excavated t o t h e i r full d e p t h and samples t a k e n t o show t h e variation in soil p r o p e r t i e s w i t h respect t o b o t h d e p t h and radial distance from t h e piers. T h e m o i s t u r e c o n t e n t d e t e r m i n e d from samples t a k e n in t h e test pits and test holes was c o m p a r e d w i t h t h e values o b t a i n e d o n samples t a k e n in 1 9 6 1 . Samples t a k e n in 1972 b o t h adjacent t o t h e building and well away from t h e building were c o m p a r e d to d e t e r m i n e t h e n a t u r e of m o i s t u r e m o v e m e n t s . T h e d a t a are summarized as follows:

N o r t h W a l l - U p p e r Clays Year

Location

Avg. m o i s t u r e c o n t e n t , p e r c e n t

1961

In test holes

17.1

1972

In test holes

21.1

1972

In test pits

28.1

1972

Along pier T-40

25.8

1972

Along pier S-40

28.9

1972

R e m o t e from t h e building,

15.2

Test hole 6

T h e d a t a are n o t definitive, b u t suggest t h a t a significant increase in m o i s t u r e c o n t e n t has occurred in t h e soil n e x t t o t h e building and a r o u n d t h e piers.


202 North Wall-Bedrock Location

Year

Avg. m o i s t u r e c o n t e n t , p e r c e n t

1961

In test holes

21.0

1972

In test holes

20.3

1972

Along pier T-40

23.7

1972

Along pier S-40

23.4

1972

R e m o t e from t h e building,

19.3

Test hole 6 T h e above d a t a shows t h a t t h e m o i s t u r e c o n t e n t of t h e lower b e d r o c k has remained fairly uniform in t h e past 10 years, indicating t h a t it has n o t been substantially w e t t e d . T h e b e d r o c k i m m e d i a t e l y adjacent t o the pier appears to have slightly increased in m o i s t u r e c o n t e n t . East and West W a l l s - U p p e r Clay Avg. m o i s t u r e Year

Wall West

East

Source

c o n t e n t , percent

1961

F r o m test hole

17.7

1972

F r o m test hole

21.2

1961

F r o m test hole

18.3

1972

F r o m test hole

18.9

A significant increase in m o i s t u r e c o n t e n t has occurred for t h e west wall, b u t t h e change along t h e east wall is negligible. T h e b e d r o c k actually appeared t o be drier in 1972 than in 1 9 6 1 . T o d e t e r m i n e if t h e m o i s t u r e was p e n e t r a t i n g along t h e walls of t h e piers or soaking d o w n u n i f o r m l y from t h e surface, m o i s t u r e c o n t e n t samples were t a k e n adjacent t o t h e walls of piers S-40 and T-40 and also 3 feet away. T h e average m o i s t u r e c o n t e n t was as follows: Avg. m o i s t u r e c o n t e n t , p e r c e n t Pier

Bedrock

U p p e r clay At wall

T h r e e ft.

At wall

Three ft.

of pier

away

of pier

away

S-40

28.9

20.3

23.4

18.8

T-40

25.8

21.0

23.7

21.3

T h e d a t a strongly suggest t h a t t h e m a i n m o i s t u r e m o v e m e n t is i m m e d i a t e l y along the surface of the piers. Pier

uplift T h e increased m o i s t u r e c o n t e n t a r o u n d piers S-40 and T-40 suggest t h a t b o t h have been

subjected t o uplift. F o r pier T-40, it is possible t h a t surface w a t e r has entered along t h e face of


203

this pier and m a y have even reached near t h e b o t t o m of t h e pier. C o n s e q u e n t l y , t h e entire pier has lifted. F o r pier S-40, a 3/8-inch-wide h o r i z o n t a l crack was found just above b e d r o c k . Since excavation of t h e pit a r o u n d t h e pier relieved all t h e uplift forces on t h e side of t h e pier in the clay, t h e pier should have gradually settled as t h e pit was excavated and it was theorized t h a t the crack m u s t have been o p e n b y m o r e than 3 / 8 inch prior t o excavation. Tension cracks developed in t h e pier clearly indicate t h a t t h e u p p e r soils have e x e r t e d uplifting pressure on t h e u p p e r p o r t i o n of t h e pier, and t h e p o r t i o n of the pier in b e d r o c k is w i t h h o l d i n g t h e pier. T h e uplifting

pressure exerted

on the pier d e p e n d s on t h e swelling pressure of the

s u r r o u n d i n g soils. T h e uplifting force exerted on each pier m a y reach as high as 2 0 0 kips. This force is sufficient t o crush t h e c o n c r e t e pedestal formed o n t o p of t h e pier. Also, w h e n all the piers in Wing C were exposed during t h e remedial c o n s t r u c t i o n , it was found t h a t at least five piers had a distinct shear failure p a t t e r n as s h o w n on figures 126 and 127.

Figure 126. Failure of pier by shear resulting from uplift.


204

Figure 127. Failure of pier by shear resulting from uplift.

CAUSE O F MOVEMENT In general, t h e cause of m o v e m e n t of t h e building is d u e t o t h e uplifting of the piers. T h e m o v e m e n t is m o r e severe at t h e n o r t h side u n d e r Wing C. T o t h e west of Wing D , the entire school building is c o n n e c t e d w i t h grade b e a m s . Wing D is separated from t h e r e m a i n d e r of the school building w i t h e x p a n s i o n j o i n t s . C o n s e q u e n t l y , at t h e eastern p o r t i o n of t h e school building f o u n d a t i o n m o v e m e n t is distributed t h r o u g h o u t t h e system and is n o t conspicuous, while at the n o r t h - s o u t h c o r r i d o r t h e entire system is separated. This explains w h y severe m o v e m e n t along t h e n o r t h - s o u t h corridor is n o t i c e d . In a d d i t i o n t o t h e uplifting of t h e piers, several o t h e r c o n s t r u c t i o n defects were found. T w o piers in Wing C are b o t t o m e d o n t h e u p p e r clay instead of drilled i n t o b e d r o c k as s h o w n on figure


205

128. T h e piers were 3 6 inches in d i a m e t e r and 3 t o 4 feet in length, r a t h e r t h a n 12 inches in d i a m e t e r and drilled i n t o b e d r o c k as had b e e n designed. Since t h e u p p e r clays have a m a x i m u m soil bearing value of a b o u t 3 , 0 0 0 psf, it is possible t h a t s e t t l e m e n t of these piers has t a k e n place. T h e entire length of air space b e n e a t h t h e n o r t h wall in Wing C was carefully i n s p e c t e d . T h e r e was a m i n i m u m of air space. R e m n a n t s of c a r d b o a r d used for forming t h e air space were found, b u t it appears t h a t t h e air space was n o t p r o p e r l y c o n s t r u c t e d , as s h o w n o n figure 129. Either t h e air space was n o t formed t o t h e specified thickness or t h e uplifting of t h e soil has closed t h e air space. In any event, along the n o r t h wall in Wing C, t h e soil has exerted uplifting pressure on t h e grade b e a m t h a t can reach as high as 2 5 , 0 0 0 psf. N o t all t h e distress manifest in t h e building was caused by f o u n d a t i o n m o v e m e n t . All t h e p a r t i t i o n walls in t h e classrooms show cracks. T h e p a t t e r n of t h e cracks indicates t h a t t h e b e a m s s u p p o r t i n g t h e slabs were deflected. T h e cracks in t h e p a r t i t i o n wall are typical distress d u e t o t h e deflection and plastic flow of t h e long-span c o n c r e t e floor b e a m s .

Figure 128. Improperly placed pier. Pier length should be 20 feet and bearing on bedrock. Actual length only 4 feet and bearing on clay.

206


Figure 129. Four-inch void, which has completely closed, beneath the grade beam.

It is i m p o r t a n t t o isolate structural defects from f o u n d a t i o n m o v e m e n t w h e n investigating a cracked building so t h a t t h e cause m a y be d e t e r m i n e d . In m a n y cases, structural defects and f o u n d a t i o n defects take place in t h e same s t r u c t u r e .

REMEDIAL CONSTRUCTION Since t h e cause of f o u n d a t i o n m o v e m e n t and t h e source of m o i s t u r e t h a t entered i n t o t h e f o u n d a t i o n soils have b e e n defined, t h e remedial measures should consist essentially of relieving t h e uplifting pressure e x e r t e d o n t h e piers and preventing additional w a t e r from entering t h e f o u n d a t i o n soils. T h e remedial measures consist of t h e following: 1. R e m o v e all backfill a r o u n d t h e building and replace c o m p a c t e d t o at least 9 0 p e r c e n t standard P r o c t o r d e n s i t y at o p t i m u m m o i s t u r e c o n t e n t . Backfill along t h e n o r t h wall of Wing C should consist of n o n e x p a n s i v e soils instead of t h e original soil. T h e a d e q u a t e c o m p a c t i o n of t h e backfill soil is very i m p o r t a n t to insure t h a t any surface w a t e r will n o t p e n e t r a t e t h r o u g h t h e backfill and i n t o t h e f o u n d a t i o n soils. 2. All void space b e n e a t h the grade b e a m should be re-formed t o insure t h a t there will be at least 4 inches of space b e t w e e n t h e soil and t h e grade b e a m . At t h e same t i m e , care should be t a k e n t o insure t h a t there will b e n o large m u s h r o o m s present on t o p of t h e


207

piers. T h e air space should b e formed adjacent t o t h e sides of t h e piers. With t h e air space p r o p e r l y f o r m e d , t h e load of t h e building will t h e n b e e x e r t e d on the piers. 3 . In Wing C, it will b e necessary t o loosen or remove t h e soils above b e d r o c k from a r o u n d all piers. Such u n d e r t a k i n g will have t o b e p e r f o r m e d b y h a n d inside t h e crawl space. T h e d e p t h of loosening or removing of soil should be at least 8 feet. 4 . Drainage a r o u n d t h e building m u s t be i m p r o v e d , and should consist of t h e following: a. I m p r o v e t h e drainage in t h e c o u r t y a r d

area. R e m o v e t h e asphalt paving in t h e

c o u r t y a r d and replace w i t h c o n c r e t e . b . R e c o n s t r u c t the c o n c r e t e side Walk a r o u n d t h e building t o provide an a d e q u a t e slope. Also provide an a d e q u a t e e x p a n s i o n j o i n t b e t w e e n t h e sidewalk and the grade b e a m . c. Slope t h e g r o u n d surface a r o u n d t h e building away from t h e s t r u c t u r e t o allow p r o p e r drainage.

T h e above remedial measures will p r e v e n t further d a m a g e t o t h e school s t r u c t u r e d u e t o expansive soils. After t h e above remedial measures are m a d e , the d e a d load pressure will b e fully exerted o n t h e piers and t h e u p p e r soils will n o t exert uplifting pressure o n t h e piers. By preventing w a t e r from entering t h e crawl space area, m o v e m e n t of t h e piers should be arrested. T h e releveling p r o c e d u r e can b e started, as follows:

1. Carefully establish t h e elevation of all piers b e n e a t h t h e school building b y referring t o t h e established b e n c h m a r k at t h e n o r t h side of t h e building. 2. A s t r u c t u r a l engineer should be c o n s u l t e d t o d e t e r m i n e t h e a p p r o p r i a t e n e w elevation of t h e school building c o m m e n s u r a t e w i t h t h e initial c o n s t r u c t i o n . 3 . T h e piers a r o u n d t h e e x t e r i o r of t h e school building have lifted; h o w e v e r , at t h e central p o r t i o n of t h e building t h e piers have m a i n t a i n e d t h e i r original position. It is reasonable t o lower t h e e x t e r i o r piers and allow t h e interior piers t o m a i n t a i n their original position.

In 1 9 7 2 , remedial m e a s u r e s as r e c o m m e n d e d above were started. T o facilitate c o n s t r u c t i o n , t h e entire crawl space area b e n e a t h Wing C was lowered. This n o t only allowed w o r k m e n t o m o v e freely in t h e w o r k area b u t also remove at least 4 feet of soil a r o u n d the piers (fig. 130). T h e entire crawl space was lighted and c o n v e y o r belts installed for e a r t h removal. Each pier was carefully e x a m i n e d for defects after t h e s u r r o u n d i n g soil was r e m o v e d . Steel rings were installed a r o u n d t h e t o p of t h o s e piers t h a t suffered shear failure. All grade b e a m s were e x a m i n e d for s t r u c t u r a l s t r e n g t h . Heavy steel girders w e r e i n t r o d u c e d t o s t r e n g t h e n t h e defected b e a m s (fig. 131). O t h e r remedial m e a s u r e s such as providing a d e q u a t e air space b e n e a t h t h e grade b e a m s , removing and r e c o m p a c t i n g backfill, installing s u m p p u m p s t o eliminate perched w a t e r , and relocating t h o s e piers having insufficient length were p e r f o r m e d u n d e r close supervision. T h e n t h e o p e r a t i o n of releveling started. T h e grade b e a m s w e r e raised w i t h high-capacity jacks (fig. 132), and t h e t o p of pier cut-and-shimmed w i t h steel plates. T h r e e o r four piers were releveled in o n e o p e r a t i o n . A t o t a l of 56 piers w e r e releveled in Wing C over a period of 4 m o n t h s . During t h e leveling o p e r a t i o n , careful surveys w e r e c o n d u c t e d t o d e t e r m i n e t h e vertical m o v e m e n t . Typical records are s h o w n o n figure 1 3 3 .

208


Figure 130. Loosening of soil around the pier to eliminate uplifting pressure.

F o u r sets of major readings w e r e t a k e n as follows: 1. Pier elevation before remedial c o n s t r u c t i o n , 2. Pier elevation after air space b e n e a t h t h e grade b e a m s was cleared and load of building c o n c e n t r a t e d o n t h e piers, 3 . Pier elevation after t h e removal of soils s u r r o u n d i n g t h e piers, t h u s partially eliminating t h e uplifting pressure exerted on t h e face of the piers, and 4 . Pier elevation after t h e releveling o f t h e piers. F r o m figure 1 3 3 , t h e effect of t h e various stages of remedial c o n s t r u c t i o n can be reflected b y t h e s e t t l e m e n t of t h e piers. T h e case study of this school is a typical e x a m p l e of failure d u e t o pier uplift. In a d d i t i o n t o t h e c o n s t r u c t i o n defects, present k n o w l e d g e of a drilled pier system in expansive soils calls for


209

Figure 131. Steel ring placed around the defective pier and steel girder installed t o strengthen the grade beam.

Figure 132.

Jacking the grade beam in the releveling operation.

210

Figure 133.

THE

PIER S

Pier settlement after various stages of remedial construction.


PIER ELEVATIO N AFTE R LOOSENIN G SOIL S AROUND N AFTE R RELEVELIN G OPERATIO N PIER ELEVATIO


211

b o t h a d e q u a t e r e i n f o r c e m e n t of t h e pier t o resist tension and d e e p p e n e t r a t i o n i n t o b e d r o c k t o provide for anchorage. Such p r e c a u t i o n s could have resisted t h e uplift pressures. R e m e d i a l c o n s t r u c t i o n for this school building has b e e n confined t o Wing C. After a period of 6 m o n t h s , t h e building is still undergoing s t r u c t u r a l a d j u s t m e n t . Minor cracks appeared in t h e b l o c k wall as t h e result of releveling adjustment. It is e x p e c t e d t h a t a stabilized c o n d i t i o n can be achieved in t h e building w i t h i n a year.

Case II

DISTRESS CAUSED BY IMPROPER PIER DESIGN AND CONSTRUCTION

GENERAL This is a typical case of i m p r o p e r design and c o n s t r u c t i o n of a drilled pier f o u n d a t i o n system. T h e building is a residential h o u s e located in west Denver, C o l o r a d o .

EXISTING CONDITION Design T h e residence is a split-level s t r u c t u r e facing east, w i t h t h e finished b a s e m e n t at t h e s o u t h end, crawl space u n d e r t h e living p o r t i o n and garage at t h e n o r t h end. It is a brick veneer and w o o d frame s t r u c t u r e w i t h a trussed r o o f system and s u p p o r t e d o n piers (fig. 134). A subsoil investigation was m a d e before c o n s t r u c t i o n . T h e subsoils consist of a b o u t 4 feet of stiff clays overlying claystone b e d r o c k . T h e w a t e r table was found at a d e p t h of 7 feet b e l o w t h e original g r o u n d surface. A pier f o u n d a t i o n was r e c o m m e n d e d . T h e piers w e r e designed for a m a x i m u m end pressure of 1 5 , 0 0 0 psf, a skin friction of 1,500 psf and a m i n i m u m dead load pressure of 1 5 , 0 0 0 psf. It was also r e c o m m e n d e d

t h a t t h e piers should be drilled at least 4 feet i n t o c l a y s t o n e . (As

claystone b e d r o c k was practically exposed in t h e excavation, t h e length of t h e piers d o e s n o t exceed 4 feet). Distress T h e h o u s e was built in 1 9 6 1 . Cracks a p p e a r e d in t h e h o u s e 6 m o n t h s after o c c u p a n c y . T h e e x t e n t of m o v e m e n t began increasing steadily. A subdrainage system leading t o a s u m p p u m p was later installed

in t h e crawl space area of t h e h o u s e . T h e m o s t severe m o v e m e n t t o o k place

b e t w e e n t h e crawl space area and t h e living r o o m area. T h e s e p a r a t i o n of t h e crawl space from t h e t w o - s t o r y p o r t i o n of t h e h o u s e is s h o w n o n figure 135. T h e w i d t h of t h e separation measures as m u c h as 1-1/2 inches. T h e p i c t u r e w i n d o w above t h e crawl space area is also separated from t h e wall b y as m u c h as 1 inch. Cracks also appeared at t h e rear of t h e h o u s e b e t w e e n t h e o n e and t w o - s t o r y p o r t i o n s . E x t e r i o r d o o r s w e r e j a m m e d and t h e p a t i o slab was a p p r o x i m a t e l y 1 inch l o w e r t h a n its original position.


214

Figure 134. Location of exterior cracks.

In t h e interior of t h e h o u s e , severe cracks were found near t h e staircase leading to the b a s e m e n t (fig. 136). Cracks were also found above m o s t d o o r s and w i n d o w s , indicating severe m o v e m e n t . Most of t h e d o o r s in the h o u s e were j a m m e d . T h e I-beam w h i c h s u p p o r t s t h e u p p e r floor appears t o have m o v e d . O n e of t h e posts in t h e crawl space area was loose, indicating the uplifting of t h e s u p p o r t u n d e r t h e I-beam. Slabs were raised. In general, t h e e x t e n t of cracking in this h o u s e is considered to be very severe, and from the p a t t e r n of t h e cracks and t h e n a t u r e of the swelling of t h e f o u n d a t i o n soils, severe uplifting m o v e m e n t of t h e soil b e n e a t h t h e f o u n d a t i o n has t a k e n place.

CAUSE O F M O V E M E N T T h e cause of f o u n d a t i o n m o v e m e n t for this h o u s e can b e summarized as follows: 1. U n d i s t u r b e d h a n d drive samples were t a k e n in the crawl space area b e n e a t h t h e grade b e a m s . Tests indicated

t h a t t h e w e a t h e r e d claystone possessed high swell p o t e n t i a l .

Typical test results are s h o w n o n figure

137. Figure 137 indicates t h a t t h e swelling

pressure is a b o u t 1 6 , 0 0 0 psf. 2. N o air space was found b e n e a t h t h e grade b e a m near t h e m a i n e n t r a n c e in t h e crawl space. T h e t o t a l length of t h e p o r t i o n of grade b e a m w i t h o u t void-forming cardboard is a p p r o x i m a t e l y 8 feet. T h e lower w e a t h e r e d claystone exerted direct uplifting pressure o n t h e grade b e a m in this p o r t i o n of t h e h o u s e . With 8-foot-long grade b e a m s , 9 inches w i d e ,

DISTRESS CAUSED BY IMPROPER PIER DESIGN

215

Figure 135. Separation of living room from the two-story portion. (See previous figure)

w i t h o u t air space, t h e total uplifting pressure exerted o n t h e grade b e a m can reach as high as 9 6 , 0 0 0 lbs. This pressure is sufficient t o cause t h e severe m o v e m e n t b e t w e e n t h e one-story p o r t i o n and t h e t w o - s t o r y p o r t i o n of t h e h o u s e . 3 . Since t h e piers are o n l y 4 feet in length, t h e soils exerted n o t o n l y uplift pressure a r o u n d t h e p e r i m e t e r of t h e pier b u t also acted directly o n t h e b o t t o m of t h e piers. T h e m a x i m u m possible uplift in this case is as follows: Data:


= 1 2 in.

Pier circumference

= 3 . 1 4 ft.

Pier end area

= 0 . 7 8 5 sq. ft.

216


Figure 136. Separation of house due to differential expansion ·

P o r t i o n of pier in bedrock

= 3.0 ft.

Swelling pressure in bedrock

= 16,000 psf

P o r t i o n of swelling pressure responsible t o uplift

= 16,000 X 0.15 = 2 , 4 0 0 psf

then Total uplift

force F r o m pier end

= 16,000 X 0 . 7 8 5 = 12.5 kips

217


Ο

\

2

Noturol

Dry

Natural

Moisturt

Unit

\

\

=

107.5

pcf

=

22.9

p«rctnt

Jll (1er

Εxp( ns ioi cue t<

Weight Contint

Mi

con$1ant ι >res sur e

t ig.

(

\

>

\

\

\ Λ s

\

\

\ Swelling Pies re inr Of i irnil

0.1

:

1.0 APPLIED

\

16,000

10 PRESSURE

-

100

kit

Figure 137. Typical sample of weathered claystone obtained from beneath grade beam.

F r o m pier wall

= 2 , 4 0 0 X 3 X 3.14 = 2 2 . 6 kips

Total

= 3 5 . 1 kips

and Total withholding

force

(Dead load pressure)

= 15,000 X 0.785 = 11.8 kips

It is obvious t h a t t h e dead load pressure exerted o n t h e pier is n o t sufficient t o prevent uplift. T h e interior piers have even less dead load t h a n t h e e x t e r i o r piers. C o n s e q u e n t l y ,


218

t h e piers b e n e a t h the I-beam have lifted, causing t h e I-beam t o m o v e , t h u s disturbing t h e entire u p p e r s t r u c t u r e . 4 . T h e p a r t i t i o n walls in t h e t w o - s t o r y p o r t i o n of t h e h o u s e are slab-bearing, and w h e n t h e slabs heave, t h e walls i m p a r t direct uplifting pressure t o t h e I-beam w h i c h disturbs t h e upper structure. 5. T h e cause of w e t t i n g of t h e soils b e n e a t h t h e f o u n d a t i o n is from a high w a t e r table and p o o r drainage a r o u n d t h e h o u s e . Initial f o u n d a t i o n investigation indicates t h a t t h e w a t e r table is n e a r t h e b a s e m e n t floor level. Initially, consideration should have b e e n given t o t h e effect of w e t t i n g o n t h e structural stability. T h e cause of m o v e m e n t of this h o u s e is d u e t o t h e swelling of the soils beneach t h e grade beam

and

t h e uplifting

of t h e piers. T h e design and

construction

of t h e h o u s e

cannot

a c c o m m o d a t e t h e severe uplifting of t h e soils.

REMEDIAL MEASURES T h e following remedial m e a s u r e s w e r e r e c o m m e n d e d : 1. Excavate a r o u n d t h e b a s e m e n t p o r t i o n of t h e h o u s e t o e x p o s e t h e grade b e a m . R e m o v e all m u s h r o o m s above t h e piers and r e c o n s t r u c t t h e air space in t h e same m a n n e r as in t h e crawl space p o r t i o n of t h e h o u s e . 2. F r e e t h e piers from t h e grade b e a m . 3 . Precise leveling should be m a d e using t h e central grade b e a m as a reference p o i n t t o relevel t h e entire h o u s e . It is possible t o definitely establish t h e a m o u n t of a d j u s t m e n t w h i c h is required for each individual pier t o r e t u r n the h o u s e t o a level position.

It is

e x p e c t e d t h a t after this has b e e n d o n e , t h e existing cracks will be partially closed. 4 . T h e piers, after a d j u s t m e n t , should b e shimmed w i t h steel plates. 5. T h e backfill in t h e b a s e m e n t p o r t i o n of t h e h o u s e should b e provided w i t h d e e p wells a p p r o x i m a t e l y 3 feet in d i a m e t e r a r o u n d t h e five exposed piers so t h a t in t h e future, a d j u s t m e n t of t h e piers will b e possible w i t h o u t again removing all backfill. 6. T h e t o p of the wells should be covered w i t h suitable material so t h a t surface w a t e r will n o t seep i n t o t h e wells. 7. Readjust t h e I-beam t o level the u p p e r s t r u c t u r e . 8. R e m o v e all slab-bearing s t r u c t u r e s , such as t h e stairway, interior c u p b o a r d s , b o o k c a s e s , furnace, and so forth, and provide slip j o i n t s so t h a t further slab m o v e m e n t will n o t affect t h e u p p e r s t r u c t u r e . 9. Check t h e grade b e a m b e n e a t h t h e walk-out d o o r at t h e rear p o r t i o n of t h e h o u s e t o insure t h a t t h e grade b e a m is tied in as a unit. If necessary, n e w grade b e a m s should be c o n s t r u c t e d to span above the walk-out d o o r . 10. R e m o v e t h e rear p a t i o slab for t h e entire length so t h a t t h e slab will be free from the grade b e a m . 11. F r e e t h e b a s e m e n t floor slab a r o u n d t h e p e r i m e t e r of t h e grade b e a m s . 12. E x t e n d t h e e x t e r i o r subdrainage system from t h e rear side of t h e h o u s e t o t h e s o u t h of the garage t o i n t e r c e p t all possible sources of free w a t e r from entering the h o u s e .


219

13. R e c o m p a c t t h e backfill in thin, m o i s t e n e d layers w i t h a m e c h a n i c a l t a m p e r . 14. Regrade t h e backfill around t h e h o u s e so t h a t surface w a t e r will drain away from the house. 15. R e m o v e all shrubs and flower b e d s from a r o u n d t h e h o u s e and extend t h e d o w n s p o u t s . This specific case w e n t to t h e court and t h e c o n t r a c t o r was ordered to p a y t h e $ 1 1 , 0 0 0 cost of remedial c o n s t r u c t i o n w h i c h a m o u n t e d to a b o u t 50 p e r c e n t of t h e cost of t h e h o u s e . T h e remedial measures w e r e c o m p l e t e l y carried o u t . Shortly after t h e h o u s e was releveled, s o m e of t h e m o r e severe cracks started t o close as s h o w n o n figure 138. It t o o k m o r e t h a n 6 m o n t h s before t h e s t r u c t u r e was stablized. S o m e 6

Figure 138. Movement of the house before and after remedial correction.

220


years after t h e remedial c o n s t r u c t i o n , n o serious f o u n d a t i o n m o v e m e n t had t a k e n place in this h o u s e (fig. 1 3 9 ) ; t h e r e f o r e , r e a d j u s t m e n t of t h e piers was n o t necessary.

Figure 139. Condition of front of house in 1974.

Case III

DISTRESS CAUSED BY HEAVING OF FOOTING PAD AND FLOOR SLAB

GENERAL This case s t u d y is typical of t h a t of u n d e r e s t i m a t i n g t h e swelling p o t e n t i a l of t h e soil. Individual f o u n d a t i o n p a d s have heaved and severe floor heave has t a k e n place. Because of cost, remedial measures were only partially carried o u t . More t h a n 2 years have elapsed since t h e remedial w o r k was c o m p l e t e d and the buildings remain in perfect c o n d i t i o n .

HISTORY T h e buildings u n d e r investigation are in a State-operated ward for h o u s i n g t h e severely retarded and consist of 6 cottages, 3 t o t h e east and 3 t o t h e west. T h e 3 cottages located at t h e western p o r t i o n of t h e site are identified as C h e r u b , Aspen, and Birch. T h e eastern c o t t a g e s are t h e Starlight, Crescent, and B u t t e r c u p . Each g r o u p is c o n n e c t e d t o t h e service buildings as indicated o n figures 140 t h r o u g h 1 4 3 . T h e cottages were c o m p l e t e d in 1962. T h e original soil r e p o r t r e c o m m e n d e d

that the

buildings be founded w i t h spread footings o n a c o m b i n a t i o n of c o m p a c t e d fill and t h e in-place natural sandy clays, designed for a m a x i m u m soil pressure of 3 , 0 0 0 psf and a m i n i m u m dead load pressure of 1,500 psf. T h e s t r u c t u r a l design of t h e buildings indicates t h a t t h e y were f o u n d e d o n individual pads designed for t h e pressures r e c o m m e n d e d . T h e footings and slabs are f o u n d e d partly on c o m p a c t e d fill and partly o n n a t u r a l soils. T h e fill was c o m p a c t e d t o 100 p e r c e n t standard P r o c t o r density u n d e r the footings and t o 95 p e r c e n t s t a n d a r d P r o c t o r d e n s i t y u n d e r t h e slabs. Cracks first

appeared

in t h e building in 1963 and have c o n t i n u e d steadily since. A n

investigation i n t o t h e cause of cracking was m a d e b y a soils engineer in O c t o b e r 1 9 6 6 . A t t h a t t i m e , it was r e c o m m e n d e d t h a t drainage a r o u n d t h e e x t e r i o r of t h e buildings be i m p r o v e d .

DISTRESS

In general, t h e e x t e n t of cracking is m o r e severe at t h e eastern g r o u p of cottages t h a n at t h e western g r o u p . Typical k i n d s of cracking w h i c h t o o k place at t h e various buildings are as follows: 1. T e n s i o n cracks near t h e t o p of t h e c o n c r e t e c o l u m n s ,

222


Figure 140. View of cottages

2. Corridors w h i c h c o n n e c t

t h e service buildings to the cottages were separated

both

h o r i z o n t a l l y and vertically. Diagonal cracks are general in t h e brick at t h e j u n c t i o n of t h e corridors w i t h t h e service buildings, 3. Slab bearing p a r t i t i o n walls were cracked and slightly b u c k l e d , 4. T h e e n t r a n c e t o t h e service buildings had m o v e d and diagonal cracks were found in t h e brick veneer, and 5. A p o r t i o n of t h e floor slabs had moved w i t h respect to the grade b e a m s . T h e e x t e n t of cracking in t h e Aspen and C h e r u b cottages was less severe t h a n in the o t h e r four cottages. Typical distresses are s h o w n o n figures 144 t h r o u g h 146. T h e following slab m o v e m e n t d a t a was o b t a i n e d : Buttercup

2.3 in.

B u t t e r c u p t o Service

2.1 in.

Crescent

4.4 in.

Crescent t o Service

0.3 in. 0.7 in.

Starlight

4.1 in.

Starlight t o Service

Cherub

2.3 in.

C h e r u b t o Service

1.2 in.

Birch

3.1 in.

Birch t o Service

0.9 in.

Aspen

2.4 in.

Aspen t o Service

2.0 in.

DISTRESS CAUSED BY HEAVING OF SLABS

223

Ο

Figure 141. Location of exploratory holes for the cottages.

T h e above d a t a indicates t h a t t h e differential slab m o v e m e n t is t h e greatest at Crescent, while t h e B u t t e r c u p building shows only a 2.3 inch differential slab m o v e m e n t . Considering t h e a m o u n t of w e t t i n g of t h e slab in this building ( B u t t e r c u p ) , it is likely t h a t t h e slab raised m o r e u n i f o r m l y t h a n in t h e o t h e r buildings. T h e corridors c o n n e c t i n g t h e service buildings t o t h e various cottages indicate definite m o v e m e n t . This m o v e m e n t can b e associated w i t h t h e different loading c o n d i t i o n s in t h e cottages w i t h respect to t h e corridors.

224


Figure 142. Plan of test hole and test pit location for west cottages.

INVESTIGATION In August

1968 investigation i n t o t h e cause of cracking of t h e buildings, t h e source of

m o i s t u r e t h a t entered the subsoils, and t h e possibility t h a t a neighboring w a t e r t a n k had an u n d e c t e c t e d leak t h a t provided t h e m o i s t u r e was initiated, as well as t h e possible remedial measures n e e d e d . In t h e first phase of t h e investigation, 2 4 e x p l o r a t o r y holes were drilled at the site, 15 of w h i c h were drilled adjacent t o t h e cracked buildings. T h e r e m a i n d e r of t h e test holes were drilled away from t h e cracked buildings at locations s h o w n o n figure 1 4 1 . A c o m p l e t e r e p o r t , including r e c o m m e n d e d remedial measures, was s u b m i t t e d in S e p t e m b e r 1968. F o r m o r e t h a n 1-1/2 years, n o corrective action was t a k e n . In t h e m e a n t i m e , t h e c o n d i t i o n of all buildings c o n t i n u e d t o d e t e r i o r a t e . Aspen and C h e r u b , w h i c h were in relatively good c o n d i t i o n in 1 9 6 8 , n o w showed severe m o v e m e n t . T h e m o v e m e n t of t h e various buildings b e c a m e so severe t h a t it was necessary t o evacuate t h e p a t i e n t s from all six buildings. In April 1970 a decision was m a d e t o initiate a second phase of investigation. With t h e t w o phases of investigation, all possible factors t h a t could influence t h e effectiveness of t h e remedial c o n s t r u c t i o n w o u l d b e covered.


Figure 143.

225

Plan of test hole and test pit location for east cottages.

In the second phase of the investigation, the concrete slab was core drilled in 12 locations in each building and hand augered in the core hole to a depth of 6 feet. Undisturbed samples were obtained in each auger hole. A total of 82 holes was drilled inside the building. The location of all test holes and test pits is shown on figures 141 through 143. The investigation was directed mainly toward the following items: 1. Determination of the variation of moisture content in the soil beneath the floor slab in each building to a depth of approximately 6 feet. 2. Determination of the swelling potential and the swelling pressure of the soils beneath the floor slabs at various depths. 3. Determination of the swelling potential and the swelling pressure of the soils directly beneath the exterior footings at each building. 4. Determination of the possible sources of moisture which entered the buildings. 5. Determination of the water table elevation in the area. 6. Prediction of the future behavior of the soils and of the effectiveness of the proposed remedial measures. The behavior of the soils involves many variables some of which cannot be determined with certainty. This investigation is based solely on the statistical average behavior of the soils rather


226

Figure 144. Upper rounds - West complex - Cherub ward - Wall braced t o prevent falling in.

t h a n t h e result of a single observation or test. T h e evaluation of the behavior of t h e soils is m u c h m o r e complicated and difficult t h a n for o t h e r elastic engineering materials. In this investigation, m o r e t h a n 150 tests o n swelling characteristics, over 3 0 0 tests on m o i s t u r e c o n t e n t , and m a n y o t h e r tests were m a d e so t h a t p r o p e r conclusions could b e d r a w n along w i t h r e c o m m e n d a t i o n s for remedial measures. Subsoil

conditions

Subsoil c o n d i t i o n s at t h e site consist essentially of 0 t o 8 feet of fill overlying soft t o stiff clays. Bedrock was found at d e p t h s ranging from 8 t o 29 feet. T h e characteristics of t h e various subsoil strata are described as follows:


227

Figure 145. Corner of Cherub ward. Interior wall pulling away from exterior wall.

F i l l - T h e fill consists of t h e on-site soils and it was often difficult t o distinguish b e t w e e n t h e fill and t h e natural soil. T h e fill was placed u n d e r controlled c o n d i t i o n and t h e o p t i m u m m o i s t u r e c o n t e n t ranged from 16.3 t o 18.6 p e r c e n t . T h e actual m o i s t u r e c o n t e n t of the in-place fill ranged from 10.7 t o 16.1 p e r c e n t . C l a y - T h e clays at t h e site had fairly uniform characteristics. T h e soil could be classified as on t h e borderline b e t w e e n C L and CH w i t h t h e liquid limit ranging from 4 2 . 0 t o 5 3 . 6 p e r c e n t and t h e plasticity i n d e x ranging from 2 6 . 8 t o 3 2 . 5 p e r c e n t . T h e stiffness of the clay varied w i t h the m o i s t u r e c o n t e n t . In general, t h e u p p e r soils were soft and their stiffness increased w i t h d e p t h . An X-ray diffraction analysis indicated t h a t t h e total clay mineral ( n o t including clay-size q u a r t z or calcite, etc.) was p r o b a b l y n o t m o r e t h a n 5 p e r c e n t b y v o l u m e of t h e t o t a l sample. Major minerals were q u a r t z and calcite, especially in t h e d e c a n t e d fractions. Minor minerals


228

Figure 146. Interior Aspen ward — Floor crack.

were m o n t m o r i l l o n i t e (unusually b r o a d 14-angstrom lines) w i t h possibly a trace of kaolinite and mica. H y d r o m e t e r analysis indicated t h a t the clay fraction ( p e r c e n t m i n u s 0 . 0 0 2 m m ) of t h e typical sample was less t h a n 35 p e r c e n t , and colloid c o n t e n t (percent m i n u s 0.001 m m ) less t h a n 22 p e r c e n t . T h e shrinkage limit of typical clays ranged from 9.7 t o 14.0 p e r c e n t . T h e above physical analysis of t h e clay soils indicated t h a t in accordance with the established m e t h o d s of classifying expansive soils, t h e f o u n d a t i o n soil u n d e r t h e various buildings falls i n t o t h e category of "highly expansive soils." Bedrock—Bedrock consisted of c l a y s t o n e and s a n d s t o n e . T h e u p p e r p o r t i o n of claystone was highly w e a t h e r e d . T h e claystone b e d r o c k had a resemblance t o stiff clay and it was difficult t o distinguish b e t w e e n t h e claystone and the u p p e r clay. T h e claystone b e d r o c k had essentially the same physical characteristics as t h e u p p e r clay.


229

Stabilized free w a t e r in the area was found at d e p t h s of 5 t o 19 feet b e l o w t h e t o p of t h e floor level. However, at t h e site of t h e buildings, t h e w a t e r table was at least 10 feet b e l o w t h e floor level. Method

of

approach

After all t h e test d a t a had b e e n a c c u m u l a t e d , t h e m e t h o d of a p p r o a c h used in solving t h e p r o b l e m was as follows: Swelling P o t e n t i a l - S w e l l i n g p o t e n t i a l is an index t h a t indicates t h e degree of v o l u m e change of t h e soil after s a t u r a t i o n . F r o m t h e swell p o t e n t i a l , it is possible t o e s t i m a t e the m a g n i t u d e of floor and f o u n d a t i o n heaving. T h e swelling p o t e n t i a l of soils u n d e r each building was o b t a i n e d and a curve w h i c h graphically s u m m a r i z e d t h e results was p r o p o s e d b y p l o t t i n g t h e m o i s t u r e c o n t e n t versus swelling p o t e n t i a l . A curve was p r e p a r e d for each building (fig. 147). Swelling P r e s s u r e - S w e l l i n g pressure can b e defined as t h e pressure required t o k e e p t h e v o l u m e of t h e sample c o n s t a n t . F o r c o n s t a n t d e n s i t y , swelling pressure should have a c o n s t a n t value. With variable m o i s t u r e c o n t e n t and d e n s i t y , t h e swelling pressure varies as s h o w n on figure 148. Figure 148 is a typical g r a p h of swelling pressure versus m o i s t u r e c o n t e n t for each building. Average M o i s t u r e - T h e m o i s t u r e c o n t e n t of all soil samples b e n e a t h b o t h t h e slab and t h e footings was o b t a i n e d . A n average m o i s t u r e c o n t e n t was d e t e r m i n e d , w h i c h provided i n f o r m a t i o n o n t h e following: 1. Average m o i s t u r e c o n t e n t for t h e entire building at various d e p t h s , 2. Average m o i s t u r e

consent

in t h e p e r i m e t e r of t h e building at various d e p t h s , and

3. Average m o i s t u r e c o n t e n t at t h e central p o r t i o n of each building at various d e p t h s . T h e r e c o m m e n d e d remedial measures for each building are essentially based o n the swelling pressure,

swelling p o t e n t i a l , and m o i s t u r e d i s t r i b u t i o n . T h e conclusions are based o n

the

statistical average of soil behavior. Source of

moisture

N o v o l u m e change will take place in t h e expansive soils unless t h e r e is a change in t h e a m o u n t of m o i s t u r e in t h e soil. T h e increase of m o i s t u r e c o n t e n t can b e caused b y various factors as follows: 1. Surface runoff including rain, m e l t i n g s n o w , and lawn sprinkler water, 2. Leaks in t h e u n d e r slab h e a t i n g system, 3 . Leaks in t h e sewer s y s t e m , 4 . Leaks in t h e d o m e s t i c w a t e r s y s t e m , and 5. Possible rising w a t e r table c o n d i t i o n d u e t o increase in subsurface w a t e r v o l u m e . T h e m o s t difficult aspect in investigating t h e source of w a t e r was in d e t e r m i n i n g w h e t h e r t h e underslab soils were w e t t e d b y the i n t r o d u c t i o n of surface w a t e r d u e t o p o o r e x t e r i o r drainage or b y leakage of t h e underslab utility system. Since each building is s u r r o u n d e d b y grade

230


Figure 147.

Moisture and swelling potential relationship at Aspen.

beams 3 feet deep, surface water can enter the subsoils only at a depth of at least 3 feet below the top of the floor slab. However, it is most likely that exterior surface water has entered the underslab soils through the void space beneath the grade beams; therefore, the following conclusions can be established: 1. If the moisture content directly beneath the concrete slabs (within 24 inches below the top of the floor slab) is high, then a leak in the underslab utility lines is suggested. 2. If the moisture content around the perimeter of the building at a depth of more than 3 feet below the floor slab is high, then the migration of exterior surface runoff into the underslab soils is suggested.


231

20,000 ASPEN

10,000

£

5,000

• • •

1,000

12

14

16 MOISTURE

20

18 CONTENT

22

24

(%)

Figure 148. Moisture and swelling pressure relationship at Aspen.

3 . If t h e m o i s t u r e c o n t e n t a r o u n d t h e p e r i m e t e r of t h e building and t h e m o i s t u r e c o n t e n t of t h e building's i n t e r i o r are b o t h low, t h e n n o i n t r o d u c t i o n of surface runoff or leakage in t h e utility lines is suggested. 4 . If t h e m o i s t u r e c o n t e n t a r o u n d the p e r i m e t e r of t h e building and t h e m o i s t u r e c o n t e n t of t h e building's i n t e r i o r are b o t h high, t h e n b o t h t h e i n t r o d u c t i o n of surface runoff and leakage in the utility lines are suggested. T h e average m o i s t u r e c o n t e n t at various d e p t h s for each building will give a clear i n d i c a t i o n as t o t h e source of m o i s t u r e t h a t has entered i n t o t h e buildings.

TREATMENT Based on t h e above reasoning, the p r o b l e m t h a t existed in each building can be established and remedial measures prescribed.


232

Treatment

at Birch

Birch is the east building of the west c o m p l e x . F o u n d a t i o n soils at this building site consist essentially of controlled c o m p a c t e d fill. A s t u d y of t h e original soil r e p o r t indicates t h a t at t h e n o r t h side of this building t h e r e is a p p r o x i m a t e l y 1/2 foot of cut and at t h e s o u t h side there is a p p r o x i m a t e l y 7 feet of fill. T h r e e test pits were o p e n e d at the e x t e r i o r of the building, adjacent t o the grade b e a m . In Test Pits 101 and 102, w a t e r was flowing from u n d e r n e a t h t h e slab. F u r t h e r testing indicated that t h e underslab heating system had leaked and t h e sewer line had b r o k e n . This resulted in t h e flooding of t h e underslab soils. Most of t h e c o m p a c t e d fill soil b e n e a t h t h e footings possessed only low swell p o t e n t i a l . T h e possibility of f o u n d a t i o n m o v e m e n t is relatively slim. Moisture c o n t e n t s for t h e entire building at various d e p t h s are relatively u n i f o r m w i t h the lowest m o i s t u r e c o n t e n t 15.8 p e r c e n t and the highest m o i s t u r e c o n t e n t 22.6 p e r c e n t . F u r t h e r slab m o v e m e n t should n o t exceed 1/2 inch. T h e following remedial measures are r e c o m m e n d e d for this building: 1. T h e sewer line which r u n s u n d e r t h e building and branches i n t o the various b a t h r o o m s should be exposed and carefully checked for leakage. 2. U n d e r p i n n i n g of t h e e x t e r i o r o r interior footings will n o t be necessary. Remedial measures t o t h e footings are n o t r e c o m m e n d e d . 3. T h e p e r i m e t e r of t h e floor slab should be saw cut t o insure t h a t the slab is separated from t h e grade b e a m s and t h a t there will be free m o v e m e n t of t h e slab w i t h respect t o the grade b e a m s . 4. T h e slab-bearing p a r t i t i o n walls in this building should be r e c o n s t r u c t e d in such a m a n n e r t h a t slab m o v e m e n t will n o t affect t h e stability of the s t r u c t u r e . A vertical slip j o i n t should also be provided w h e r e the p a r t i t i o n walls c o n n e c t with the exterior walls or columns. Treatment

at

Aspen

Aspen is t h e n o r t h building of t h e west c o m p l e x . T h e f o u n d a t i o n soils at this building consist entirely of t h e natural soils. T h e a m o u n t of cut in t h e site grading ranged from 5 t o 20 feet. The swelling p o t e n t i a l of t h e soils b e n e a t h t h e e x t e r i o r footings is high w i t h a p e r c e n t of swell of 5.5 p e r c e n t and swelling pressure of 1 0 , 0 0 0 psf. The average m o i s t u r e c o n t e n t b e n e a t h the interior footings was 14.7 p e r c e n t . This c o r r e s p o n d s t o an average swelling p o t e n t i a l of 3.2 percent, (fig. 147) and an average swelling pressure of 6,200 psf (fig. 148). At a d e p t h of 6 feet below t h e t o p of t h e slab, t h e average m o i s t u r e c o n t e n t decreased t o 11.6 p e r c e n t . This corresp o n d s t o an average swelling p o t e n t i a l of 6.5 p e r c e n t and average swelling pressure of 14,000 psf. T h e m o i s t u r e d i s t r i b u t i o n indicates t h a t the lower soils are in a very d r y s t a t e , and if the soils b e c o m e excessively w e t t e d , swelling will take place. In 1 9 6 8 , a mechanical engineer found t h a t there was only slight leakage in the underslab h o t w a t e r heating system b y c o n d u c t i n g pressure tests. A n o t h e r pressure test was c o n d u c t e d in April

233


1970, and t h e pressure d r o p p e d from 130 to 27 psi in 20 m i n u t e s . It was obvious t h a t in the preceeding m o n t h s m o r e leakage had developed in t h e u n d e r s l a b heating system which a c c o u n t e d for t h e severe m o v e m e n t in this building. T h e soils directly b e n e a t h t h e floor slab h a d an average m o i s t u r e c o n t e n t of 13.8 p e r c e n t . This is low c o m p a r e d w i t h u n d e r s l a b m o i s t u r e c o n t e n t of t h e o t h e r buildings. T h e m o v e m e n t of the floor slab in this building h a d only begun and future severe floor m o v e m e n t will take place even t h o u g h the u n d e r s l a b h e a t i n g system is entirely d i s c o n n e c t e d . T h e existing p o c k e t s of high m o i s t u r e c o n t e n t soils caused b y leakage of the h e a t i n g system will migrate t o t h e drier phase of soils and cause d a m a g e . This c a n n o t be prevented unless all p r o b l e m soils b e n e a t h the slab are removed. T h e following remedial measures were r e c o m m e n d e d for this building: 1. U n d e r p i n t h e e x t e r i o r footings w i t h piers drilled i n t o b e d r o c k . T h e piers should be designed for a m a x i m u m end pressure of 3 0 , 0 0 0 psf and a skin friction of 3 , 0 0 0 psf for t h a t p o r t i o n of the pier in b e d r o c k . T h e piers should also be designed for a m i n i m u m dead load pressure of 2 0 , 0 0 0 psf. T h e piers can be drilled in a slanted position, or t w o piers can be drilled u n d e r each c o l u m n w i t h a grade b e a m spanning over t h e t w o piers. 2. T h e interior footings should also be u n d e r p i n n e d ; however, it is almost impossible t o u n d e r p i n t h e interior footings w i t h o u t demolishing t h e entire building. T h e r e f o r e , it was r e c o m m e n d e d t h a t t h e interior footings be decreased in area b y c u t t i n g off the c o n c r e t e pad and t h u s increasing t h e unit dead load pressure. It was e s t i m a t e d t h a t t h e dead load pressure on each pad could be increased t o over 6 , 0 0 0 psf b y reducing t h e area of the concrete pad. 3. T h e floor slabs in this building should be entirely removed and the soils b e n e a t h the slab removed

for a d e p t h of 3 feet. These soils should b e discarded and replaced w i t h

n o n e x p a n s i v e , impervious, granular soils c o m p a c t e d t o at least 9 0 p e r c e n t

standard

P r o c t o r density at o p t i m u m m o i s t u r e c o n t e n t . If for some reason t h e u n d e r s l a b soils c a n n o t be r e m o v e d , t h e r e is every possibility t h a t the floor slab will raise as m u c h as 3 inches above t h e present level. Slip j o i n t s in the p a r t i t i o n walls will prevent t h e d i s t u r b a n c e of the u p p e r s t r u c t u r e , b u t unsightly cracks in the p a r t i t i o n walls and the floor slab will t a k e place.

Treatment

at

Cherub

C h e r u b is t h e west building of t h e west c o m p l e x . T h e f o u n d a t i o n soils b e n e a t h this building are m o s t l y fill. At t h e n o r t h side, t h e r e is 1 foot of cut and at t h e s o u t h side there is a b o u t 7 feet of fill. Bedrock is shallow at t h e west side of the building. T h e average m o i s t u r e c o n t e n t of the soils b e n e a t h t h e e x t e r i o r footings was a b o u t 2 2 . 8 p e r c e n t which c o r r e s p o n d s t o a swelling p o t e n t i a l of less t h a n 1 p e r c e n t . T h e possibility of f o u n d a t i o n m o v e m e n t was r a t h e r r e m o t e . Most of t h e interior footings are placed o n n a t u r a l soils. T h e average m o i s t u r e c o n t e n t of the soils b e n e a t h t h e interior footings was a b o u t 14.5 p e r c e n t . This c o r r e s p o n d s t o a swelling p o t e n t i a l of 5.0 p e r c e n t . Since the m o i s t u r e c o n t e n t was

234


low, t h e chance of increase in m o i s t u r e c o n t e n t b e n e a t h t h e interior footings is high and there is a strong possibility t h a t t h e footing f o u n d a t i o n will have future m o v e m e n t . In 1 9 6 8 , a mechanical engineer again c o n d u c t e d pressure tests o n the underslab h o t w a t e r heating s y s t e m . T h e tests indicated t h a t there was n o leakage in t h e system. O n J a n u a r y 2 8 , 1970, similar tests were m a d e in this building which indicated t h a t t h e pressure d r o p p e d from 9 0 t o 7 6 psi in 15 m i n u t e s and from 100 t o 52 psi in 75 m i n u t e s . This indicates t h a t t h e r e is leakage in t h e underslab heating system. T h e m o i s t u r e c o n t e n t d i s t r i b u t i o n analysis indicates t h a t t h e m o i s t u r e c o n t e n t near footing level was higher t h a n for the soils directly b e n e a t h t h e floor; also, the e x t e r i o r m o i s t u r e c o n t e n t was generally high, a b o u t 4 p e r c e n t higher t h a n t h e interior m o i s t u r e c o n t e n t . This definitely indicated t h a t m o s t of t h e w e t t i n g of this building had b e e n caused b y the migration of surface w a t e r i n t o t h e f o u n d a t i o n soils. T h e leakage of the underslab h e a t i n g system had t a k e n place only r e c e n t l y and t h e effect of t h e leakage had n o t been reflected in the m o i s t u r e c o n t e n t of the soils. T h e soils directly b e n e a t h t h e floor slabs had an average m o i s t u r e c o n t e n t of 12.1 p e r c e n t which c o r r e s p o n d s t o a swelling p o t e n t i a l of m o r e t h a n 8 p e r c e n t . T h e m o v e m e n t of t h e floor slab of this building has o n l y begun and severe floor m o v e m e n t will o c c u r even t h o u g h the underslab heating system is entirely d i s c o n n e c t e d . T h e existing local high m o i s t u r e c o n t e n t in t h e soil will migrate t o t h e drier soil and cause floor d a m a g e . T h e following remedial measures are r e c o m m e n d e d for this building: 1. It is n o t necessary t o u n d e r p i n t h e e x t e r i o r footings. T h e m o i s t u r e c o n t e n t a r o u n d t h e p e r i m e t e r of t h e building is high and further swelling of t h e soils b e n e a t h the footings is unlikely. 2. T h e r e is a strong possibility t h a t the i n t e r i o r footings will have m o v e m e n t . T h e interior footings should be decreased in area b y cutting off t h e c o n c r e t e pad and t h u s increasing the u n i t dead load pressure, and 3 . T h e floor slabs in this building should be entirely r e m o v e d and t h e soils b e n e a t h the slabs removed t o a d e p t h of 3 feet. These removed soils should b e replaced w i t h n o n e x p a n s i v e , granular soils c o m p a c t e d t o at least 9 0 p e r c e n t standard P r o c t o r density at o p t i m u m m o i s t u r e c o n t e n t . If this is d o n e , t h e n the possibility of further floor m o v e m e n t will be remote. Treatment

at

Buttercup

B u t t e r c u p is t h e east building of t h e east c o m p l e x . T h e f o u n d a t i o n soils at this building site consist of 2 t o 5 feet of controlled c o m p a c t e d fill. It was suspected t h a t all footings, b o t h interior and exterior, were f o u n d e d o n s t r u c t u r a l fill. T h e average m o i s t u r e c o n t e n t of t h e soils directly b e n e a t h the footings was 2 5 . 6 p e r c e n t . This c o r r e s p o n d s t o a swelling pressure of 2 , 0 0 0 psf and a swelling p o t e n t i a l of less t h a n 1 p e r c e n t . At a lower d e p t h , t h e natural soils were generally d r y w i t h t h e m o i s t u r e c o n t e n t ranging from 18 t o 20 p e r c e n t and the swelling pressure reaching as high as 9 , 0 0 0 psf. Judging from t h e m o i s t u r e c o n d i t i o n of the soils b e n e a t h t h e e x t e r i o r footings, it was n o t necessary t o u n d e r p i n t h e e x t e r i o r footings. T h e possibility of footing f o u n d a t i o n m o v e m e n t was relatively r e m o t e .

235


T h e average m o i s t u r e c o n t e n t b e n e a t h t h e interior footings

was 2 2 . 1 p e r c e n t .

This

c o r r e s p o n d s t o an average swelling p o t e n t i a l of less t h a n 1 p e r c e n t . T h e soils b e n e a t h t h e exterior footings had relatively u n i f o r m m o i s t u r e c o n t e n t w i t h a m i n i m u m m o i s t u r e c o n t e n t of 20.1 percent

and

maximum

moisture

content

of

26.9 p e r c e n t . T h e possibility of

foundation

m o v e m e n t for t h e interior footings was also r e m o t e . It was, t h e r e f o r e , n o t necessary t o u n d e r p i n o r m a k e remedial c o n s t r u c t i o n o n the i n t e r i o r footings. In 1 9 6 8 , a pressure test was m a d e on the underslab h e a t i n g s y s t e m , t h e results of which showed t h a t t h e pressure d r o p p e d from 3 2 t o 0 psi in 6 0 seconds. This definitely indicates that t h e r e is a large leak in the u n d e r s l a b h e a t i n g s y s t e m . Free w a t e r was found n o t only in the e x t e r i o r test pits b u t also in t h r e e test holes inside t h e building. T h e a m o u n t of w a t e r t r a p p e d in the underslab soils m u s t b e near s a t u r a t i o n which a c c o u n t s for the steady flow of w a t e r from the underslab soils i n t o Test Pit 110. Also, sewer tests t h a t were c o n d u c t e d indicate t h a t e x t e r i o r test pits had filled during the tests. It was c o n c l u d e d t h a t there are very definitely leaks in t h e sewer lines of the east complex. T h e m o i s t u r e c o n t e n t of t h e soils at t h e p e r i m e t e r of t h e building was only slightly higher t h a n t h e m o i s t u r e c o n t e n t at t h e central p o r t i o n of t h e building. This indicates t h a t t h e a m o u n t of w a t e r t h a t seeped i n t o t h e soils from t h e e x t e r i o r of t h e building was relatively l o w . Most of t h e w a t e r present in t h e underslab soils is derived from t h e leakage of t h e u n d e r s l a b heating system and possibly from leaking sewer lines. T h e possible behavior of the floor slabs at this building can be evaluated b y studying the m o i s t u r e d i s t r i b u t i o n at various d e p t h s . T h e following facts were n o t i c e d :

1. T h e soils directly b e n e a t h t h e slabs had an average m o i s t u r e c o n t e n t of 2 1 . 0 p e r c e n t . This c o r r e s p o n d s t o an average swelling p o t e n t i a l of 1.2 p e r c e n t which is considered t o be low. 2. T h e r e was n o large difference b e t w e e n t h e m o i s t u r e c o n t e n t in t h e c e n t e r p o r t i o n and t h e m o i s t u r e c o n t e n t a r o u n d t h e p e r i m e t e r of t h e building

(19.9

versus 21.2

percent);

t h e r e f o r e , t h e m i g r a t i o n of m o i s t u r e from interior t o e x t e r i o r is unlikely t o t a k e place. 3 . D u e t o t h e excessive leakage of t h e underslab h e a t i n g s y s t e m , a large p o r t i o n of t h e soils had reached a state of s a t u r a t i o n . It is n o t necessary t o replace t h e underslab soils and the chance and m a g n i t u d e of future slab m o v e m e n t is l o w . All backfill a r o u n d this building should b e removed t o e x p o s e t h e f o u n d a t i o n s y s t e m . After t h e backfill has been r e m o v e d , the excavation should remain o p e n for a period of at least 2 weeks t o insure t h a t all w a t e r t r a p p e d u n d e r the floor slabs can b e effectively drained.

Treatment

at

Starlight

Starlight is t h e east building of t h e east c o m p l e x . T h e f o u n d a t i o n soils at this site consist of b o t h cut and fill. At t h e n o r t h side, t h e r e is 2 feet of c u t and at t h e s o u t h side there is 4 feet of fill. It is likely t h a t all footings, b o t h e x t e r i o r and interior, are founded o n t h e natural soils.

236


T h e swelling p o t e n t i a l b e n e a t h t h e footings was erratic, ranging from 0.3 t o 3.4 p e r c e n t , w i t h swelling pressure ranging from 2 , 5 0 0 t o 1 2 , 0 0 0 psf. Since t h e footings are only lightly l o a d e d , further m o v e m e n t of t h e footings is possible. A s t u d y of t h e average m o i s t u r e c o n t e n t at t h e p e r i m e t e r of t h e slab indicated t h a t t h e average m o i s t u r e c o n t e n t was 2 1 . 1 p e r c e n t , which c o r r e s p o n d s t o a swelling p o t e n t i a l of 1.5 p e r c e n t . T h e p o t e n t i a l m o v e m e n t of t h e e x t e r i o r footings is at least 50 p e r c e n t . T h e average m o i s t u r e c o n t e n t b e n e a t h t h e interior footings was

15.0

p e r c e n t . This

c o r r e s p o n d s t o an average swelling p o t e n t i a l of 4.5 p e r c e n t and an average swelling pressure of 6 , 0 0 0 psf. At t h e lower d e p t h , a p p r o x i m a t e l y 6 feet b e l o w t h e t o p of t h e slab, t h e average m o i s t u r e c o n t e n t decreased t o 14.1 p e r c e n t . This c o r r e s p o n d s t o an average swelling p o t e n t i a l of 6.0 p e r c e n t and an average swelling pressure of 15,000 psf. It was obvious t h a t t h e p o t e n t i a l m o v e m e n t of t h e interior footings is high and in fact, at a d e p t h of 6 feet b e l o w the t o p of the floor slab, some of t h e soils had a swelling p o t e n t i a l as high as 9 p e r c e n t and a swelling pressure as high as 3 0 , 0 0 0 psf. Again in 1 9 6 8 , t h e underslab h e a t i n g system was checked and only slight leakage was found. During this test, t h e pressure only d r o p p e d from 3 4 t o 3 3 . 5 psi. N o n e of t h e test pits n o r test holes, either inside or outside of the building, showed signs of t h e presence of free water. This indicated t h a t leakage of the utility lines was n o t t h e main p r o b l e m at this building. T h e p e r i m e t e r m o i s t u r e c o n t e n t at a d e p t h of 3 feet was 21.1

percent

and t h e m o i s t u r e c o n t e n t at t h e central p o r t i o n of t h e building at t h e same d e p t h was

15.0

p e r c e n t . This definitely indicated t h a t t h e source of m o i s t u r e t h a t entered this building was from surface runoff. P o o r drainage a r o u n d t h e building was directly responsible for the w e t t i n g of the f o u n d a t i o n soils. T h e soils directly b e n e a t h t h e floor slab had an average m o i s t u r e c o n t e n t of 16.9 p e r c e n t , c o r r e s p o n d i n g t o an average swelling p o t e n t i a l of 3.5 p e r c e n t . Moisture d i s t r i b u t i o n in t h e underslab soils was e x t r e m e l y erratic, w i t h t h e highest m o i s t u r e c o n t e n t being 22.7 percent and t h e lowest

10.6 p e r c e n t . T h e p o t e n t i a l for further floor m o v e m e n t at this building is great.

Moisture d i s t r i b u t i o n was erratic, and even t h o u g h t h e source of m o i s t u r e t h a t entered t h e building had b e e n cut off b y improving t h e drainage, t h e r e is still the c h a n c e of m o i s t u r e migration from t h e wet area t o t h e dry area, and this can cause d a m a g e . T h e remedial m e a s u r e s given u n d e r " T r e a t m e n t at A s p e n " can be applied in their entirety h e r e . Since t h e m a i n source of w a t e r w h i c h entered t h e building is from t h e surface runoff, special a t t e n t i o n should be directed t o improving t h e surface drainage c o n d i t i o n .

Treatment

at

Crescent

Crescent is t h e n o r t h building of t h e east c o m p l e x . T h e f o u n d a t i o n soils at this site consist of p a r t fill and p a r t cut. At t h e west side t h e r e is 3 feet of cut and at t h e east side t h e r e is 1-1/2 feet of fill. It is likely t h a t all footings at this building are placed o n n a t u r a l soils. T h e c o n d i t i o n of soils b e n e a t h t h e e x t e r i o r footings is represented b y Test Pits 112 and 1 1 3 . Free w a t e r was found in Test Pit 112. T h e w a t e r seeped i n t o t h e pit from t h e underslab gravel and drained away in several d a y s . Soils at footing level in Test Pit 112 possessed only low swelling potential.


237

In Test Pit 1 1 3 , n o free w a t e r was found. T h e swelling p o t e n t i a l at footing level was low b u t at lower d e p t h s , t h e swelling p o t e n t i a l reached as high as 2.4 p e r c e n t w i t h a swelling pressure of 6 , 0 0 0 psf. The

average

perimeter

moisture

content

was

on

the

order

of

21.2

percent.

This

c o r r e s p o n d s t o an average swelling p o t e n t i a l of 1.8 p e r c e n t and a swelling pressure of 3 , 0 0 0 psf. F r o m t h e s t u d y of t h e soil b e h a v i o r b e n e a t h t h e e x t e r i o r footings, t h e possibility of f o u n d a t i o n m o v e m e n t is r e m o t e . It is n o t necessary t o u n d e r p i n t h e footings. Most of t h e interior footings are placed o n n a t u r a l soils. T h e average m o i s t u r e c o n t e n t of t h e soils b e n e a t h t h e interior footings was 19.2 p e r c e n t .

This c o r r e s p o n d s t o an average swelling

p o t e n t i a l of 2.3 p e r c e n t and an average swelling pressure of 3 , 5 0 0 psf. T h e m o i s t u r e c o n t e n t decreased w i t h d e p t h . A t a d e p t h of 6 feet b e l o w t h e t o p of t h e floor slab, some of t h e n a t u r a l soils possessed

high swelling p o t e n t i a l ,

as m u c h as 8.3 p e r c e n t .

T h e r e is some chance of

m o v e m e n t of t h e interior footings. Again in 1 9 6 8 , definite leakage was found in t h e underslab heating system. Tests indicated t h a t t h e pressure d r o p p e d from 35 t o 2 2 psi in 5 m i n u t e s . A s t u d y of t h e m o i s t u r e c o n t e n t of t h e underslab soils indicated t h a t t h e m o i s t u r e c o n t e n t directly b e n e a t h t h e floor slab was high and t h e p e r i m e t e r m o i s t u r e c o n t e n t was even higher t h a n t h e central m o i s t u r e c o n t e n t . This indicated t h a t t h e underslab soils were w e t t e d b y leakage of t h e utility lines as well as b y surface runoff. T h e soils directly b e n e a t h t h e floor slab had an average m o i s t u r e c o n t e n t of 20.5

percent.

This c o r r e s p o n d s t o an average swelling p o t e n t i a l of 1.9 p e r c e n t and an average swelling pressure of 3 , 0 0 0 psf. T h e m o i s t u r e c o n t e n t was relatively u n i f o r m w i t h i n 4 feet below t h e surface of t h e slab. With p r o p e r drainage, t h e c h a n c e of further floor m o v e m e n t is n o t great. T h e remedial m e a s u r e s for this building are essentially t h e same as t h o s e r e c o m m e n d e d for Birch. F o r t h e floor slabs, t h e r e c o m m e n d e d p r o c e d u r e is a choice b e t w e e n removal of t h e soils b e n e a t h t h e slab as r e c o m m e n d e d for Aspen or saw-cutting t h e floor slabs and improving t h e p a r t i t i o n walls as r e c o m m e n d e d for Birch. Drainage

improvement

In a d d i t i o n t o t h e remedial measures given for each building, t h e following

general

t r e a t m e n t for improving t h e e x t e r i o r drainage was r e c o m m e n d e d : 1. R e m o v e all backfill t o e x p o s e t h e f o u n d a t i o n system. After removal, a careful check should b e m a d e of t h e void space b e n e a t h t h e grade b e a m s t o assure t h a t it is p r o p e r l y formed. T h e excavation should remain o p e n for a period of at least 1 week so t h a t all w a t e r w h i c h is t r a p p e d u n d e r t h e floor slab can be drained o u t . 2. A c o n c r e t e walk should be provided a r o u n d t h e e x t e r i o r of each cottage. T h e walk should b e at least 8 feet w i d e w i t h sufficient slope t o allow free drainage of w a t e r away from t h e cottages. T h e walks should n o t be tied i n t o t h e grade b e a m s . An e x p a n s i o n j o i n t should b e provided b e t w e e n t h e walk and t h e grade b e a m . 3 . T h e g r o u n d surface s u r r o u n d i n g t h e cottages should be sloped t o drain away from t h e cottages. A slope of 10 inches for t h e first 10 feet is r e c o m m e n d e d . If this slope in u n a t t a i n a b l e , it m a y be necessary t o install swales at various l o c a t i o n s o u t s i d e of t h e cottages t o lower t h e g r o u n d surface and allow free drainage.

238


4. T h e lawn sprinkler heads should be located at least 10 feet from t h e f o u n d a t i o n walls of t h e buildings. Spray from

t h e sprinkler heads should

n o t be directed t o w a r d s t h e

buildings. 5. T h e r o o f d o w n s p o u t s are n o t efficient. It is entirely possible t h a t during heavy s t o r m s , w a t e r could collect, overflow, and n o t drain away t h r o u g h t h e d o w n s p o u t s . I m p r o v e m e n t is necessary in this area. 6. Drainage is n o t a b l y p o o r in front of t h e service buildings. These areas should be paved w i t h c o n c r e t e if positive surface drainage is n o t possible.

REMEDIAL CONSTRUCTION R e m e d i a l c o n s t r u c t i o n started in S e p t e m b e r 1 9 7 1 . All r e c o m m e n d a t i o n s were carried o u t e x c e p t t h e u n d e r p i n n i n g of t h e e x t e r i o r footings. Budget limitations did n o t allow t h e full remedial c o n s t r u c t i o n . T h e remedial c o n s t r u c t i o n , at a cost of a half million dollars, was c o m p l e t e d in J u l y , 1 9 7 2 . Figures 149 t h r o u g h 154 show certain c o n s t r u c t i o n p r o c e d u r e s . Considering t h e e x t e n t of original d a m a g e , t h e remedial c o n s t r u c t i o n has proven successful. It is u n f o r t u n a t e t h a t t h e severe swelling p o t e n t i a l of t h e subsoil had n o t b e e n recognized in t h e design stage. Had t h e buildings b e e n founded o n piers r a t h e r t h a n o n individual pads and had t h e underslab heating system b e e n eliminated, m o s t of t h e damage could have b e e n avoided.

Figure 149. Installation of subdrains around the perimeter of the building.


Figure 150. Checking the void forming material beneath the grade beams. 239

240


Figure 151. Repair of leaking plumbing.


241

Figure 152. Drainage around exterior of the building has been improved. Note catch basins in the lawn area.

242

Figure 154. Concrete apron placed around the building w i t h properly constructed mastic joints.


Figure 153. Severely cracked brick wall, patched and repaired. Note: Further cracking has not occurred.

Case IV

DISTRESS CAUSED BY HEAVING OF CONTINUOUS FOOTINGS

GENERAL This case s t u d y is typical of w h a t h a p p e n s w h e n c o n t i n u o u s footings are placed o n expansive soil w i t h o u t considering uplift forces. In this instance t h e s t u d y involves a h o u s e t h a t was c o n s t r u c t e d w i t h o u t benefit of a d e q u a t e design. S t r u c t u r a l s t r e n g t h in t h e f o u n d a t i o n walls was lacking; t h e r e f o r e , it was n o t possible t o u n d e r p i n t h e building. Use of post-tensioned steel o r pouring of a n e w f o u n d a t i o n wall appears t o b e t h e o n l y possible remedial c o n s t r u c t i o n . By so doing, t h e h o u s e will be tied t o g e t h e r as a b o x and will be able t o w i t h s t a n d further differential movement.

HISTORY T h e h o u s e is located in Broomfield, C o l o r a d o , a small c o m m u n i t y n o r t h of Denver, Color a d o . This area is well k n o w n for its swelling soil p r o b l e m . T h e h o u s e was c o n s t r u c t e d in 1 9 6 0 . N e i t h e r a soil investigation r e c o r d n o r s t r u c t u r a l design drawings were available. T h e h o u s e has a full b a s e m e n t and a t t a c h e d garage. Test pits were excavated t o e x a m i n e t h e f o u n d a t i o n system of t h e h o u s e . This revealed t h a t t h e h o u s e is f o u n d e d w i t h c o n t i n u o u s spread footings o n t h e n a t u r a l soils. T h e footings are 2 0 inches wide and 8 inches in d e p t h . T o c o n f o r m w i t h t h e n a t u r a l g r o u n d c o n t o u r , t h e b a s e m e n t f o u n d a t i o n wall is s t e p p e d d o w n from full b a s e m e n t height at t h e s o u t h end t o o n l y 2 4 inches at t h e n o r t h e n d . T h e space b e t w e e n t h e c o n c r e t e wall and brick course is filled w i t h cinder b l o c k . N o r e i n f o r c e m e n t was found in t h e c o n c r e t e f o u n d a t i o n wall o r in t h e footings. Such f o u n d a t i o n design creates a structural weakness in t h e m i d d l e p o r t i o n of t h e b a s e m e n t wall. W i t h o u t r e i n f o r c e m e n t in t h e c o n c r e t e , slight f o u n d a t i o n m o v e m e n t will result in severe cracking. Also, at t h e n o r t h end of* t h e b a s e m e n t , t h e r e is a s t r u c t u r a l d i s c o n t i n u i t y at t h e w a l k - o u t e n t r a n c e , w h e r e t h e grade b e a m is d i s r u p t e d . N o n e of t h e slab-bearing p a r t i t i o n walls have slip j o i n t s e i t h e r at t h e t o p or b o t t o m . A n y slab m o v e m e n t will cause t h e p a r t i t i o n wall t o p u s h against t h e u p p e r floor joists and cause m o v e m e n t in t h e u p p e r levels. Exterior A t t h e e x t e r i o r of t h e h o u s e , severe cracks were found, b o t h o n t h e west and east sides in t h e brick walls above t h e b a s e m e n t . Most of t h e cracks appeared directly below and above

244


w i n d o w s in t h e m i d d l e p o r t i o n of t h e b a s e m e n t section of t h e h o u s e . Cracks o p e n e d (fig. 155), as m u c h as three-quarters of an inch, and e x t e n d e d t o t h e c o n c r e t e b e n e a t h t h e cinder b l o c k . At the n o r t h side of t h e building, severe cracks were also found below t h e w i n d o w s . N o severe cracks were found on t h e s o u t h side of t h e building. Basement

interior

T h e b a s e m e n t floor slab had b e e n r e m o v e d , exposing a p o r t i o n of t h e footings. Prior t o removal, t h e b a s e m e n t floor slab was b a d l y cracked. Cracks were found in t h e footings as well as in t h e f o u n d a t i o n walls. A t r e n c h was opened a r o u n d t h e b a s e m e n t p o r t i o n of t h e h o u s e in an a t t e m p t t o install drain tile a r o u n d t h e b a s e m e n t wall, leading t o a s u m p . A 4-inch gravel layer was placed b e n e a t h t h e b a s e m e n t slab. Upper

floor

T h e a m o u n t of cracking in t h e u p p e r floor is also severe as s h o w n o n figure 156. Cracks were evident above w i n d o w s , at t h e east side of t h e h o u s e , and a few d o o r s were j a m m e d in t h e u p p e r level. Exterior

drainage

E x t e r i o r drainage c o n d i t i o n s of t h e h o u s e are p o o r , particularly at t h e n o r t h side of t h e h o u s e w h e r e t h e walk-out b a s e m e n t d o o r is located. N a t u r a l drainage is from west t o east, and there is a strong t e n d e n c y for surface w a t e r t o e n t e r t h e f o u n d a t i o n soils from t h e west side.

1 -_

_ ™

I

"

]]·]ΐ|Μμ|

_,1

zzrr

I

• ι

I

1

T

1

cri

Figure 155. Cracks below window.


245

Figure 156. Severe cracking above closet.

SUBSOIL CONDITION T h e subsoil c o n d i t i o n s at t h e site consist essentially of slightly p o r o u s s a n d y clays at t h e s o u t h end, and highly w e a t h e r e d , m o i s t c l a y s t o n e at t h e n o r t h end. U n d i s t u r b e d h a n d drive samples were t a k e n from t h e test pits. Swell-consolidation tests, p e r f o r m e d o n t h e u n d i s t u r b e d h a n d drive samples, indicated t h a t t h e u p p e r s a n d y clays at t h e s o u t h side of t h e building possessed o n l y low swelling p o t e n t i a l ; while t h e w e a t h e r e d c l a y s t o n e at t h e n o r t h side of t h e building possessed m o d e r a t e t o high swelling p o t e n t i a l . T h e soils, at present, are in a very m o i s t c o n d i t i o n . It is a p p a r e n t t h a t t h e soils were in a m u c h drier c o n d i t i o n w h e n t h e h o u s e was c o n s t r u c t e d , and t h e swelling p o t e n t i a l of t h e claystone should b e m u c h higher t h a n t h e tests indicate. N o free w a t e r was found in t h e test pits w h i c h had a d e p t h of 5 feet b e n e a t h t h e b o t t o m of t h e footings.

CAUSE O F MOVEMENT T h e cause of m o v e m e n t of t h e h o u s e is d u e t o a c o m b i n a t i o n of uplifting m o v e m e n t of t h e f o u n d a t i o n soils and p o o r s t r u c t u r a l design as s u m m a r i z e d b e l o w : 1. T h e r e is a difference in d e p t h of t h e c o n c r e t e f o u n d a t i o n walls, and also t h e f o u n d a t i o n walls are n o t reinforced.

T h e s t r u c t u r e of t h e h o u s e c a n n o t w i t h s t a n d even slight


246

differential m o v e m e n t . This is s u b s t a n t i a t e d b y t h e fact t h a t t h e f o u n d a t i o n wall, as well as t h e f o u n d a t i o n , have b o t h cracked severely. 2. T h e a m o u n t of swelling of t h e claystone at t h e n o r t h end of t h e building is a b o u t 5 times as m u c h as t h e a m o u n t of swell of t h e sandy clays at t h e s o u t h end of t h e building u n d e r t h e same m o i s t u r e c o n d i t i o n s . C o n s e q u e n t l y , t h e f o u n d a t i o n at t h e n o r t h end of t h e building has lifted. M e a s u r e m e n t confirms t h a t t h e r e is a difference in elevation b e t w e e n t h e n o r t h and s o u t h e n d s of t h e building b y as m u c h as 5 inches. 3. A t o n e t i m e , w a t e r seeped i n t o t h e b a s e m e n t at t h e west side t h r o u g h t h e seams b e t w e e n t h e c o n c r e t e and cinder b l o c k wall. Such w e t t i n g c o n d i t i o n s have caused t h e f o u n d a t i o n soils t o swell. N o free w a t e r was found b e n e a t h t h e footings at t h e t i m e of inspection. Judging from t h e high m o i s t u r e c o n t e n t of t h e f o u n d a t i o n soils, it is evident t h a t t h e entire area has b e e n u n d e r severe w e t t i n g c o n d i t i o n s . T h e swelling of t h e w e a t h e r e d claystone requires o n l y slight m o i s t u r e increase. T h e presence of free w a t e r is n o t necessarily t h e cause of these swelling c o n d i t i o n s .

REMEDIAL MEASURES Since t h e cause of m o v e m e n t is d u e t o b o t h t h e structural weakness of t h e building and t h e swelling

of

the

claystone

beneath

the

footings,

the

following

remedial

measures

are

recommended: 1. A n e w f o u n d a t i o n wall should b e p o u r e d a r o u n d t h e interior of t h e b a s e m e n t . T h e new grade b e a m s should b e reinforced w i t h t w o 5/8-inch bars, t o p and b o t t o m , and should be tied in w i t h t h e existing f o u n d a t i o n wall in t h e m a n n e r s h o w n in figure 157. T h e new f o u n d a t i o n wall should have a d e p t h of a p p r o x i m a t e l y t h e full height of t h e b a s e m e n t . This wall will tie t h e b a s e m e n t t o g e t h e r structurally and eliminate t h e existing structural weakness. 2. T h e soils should b e removed intervals, as s h o w n in figure

from

b e n e a t h t h e footings at a p p r o x i m a t e l y

10-foot

158. In so doing, t h e weight of t h e building will be

c o n c e n t r a t e d at isolated locations and dead load pressure increased. It is estimated t h a t t h e a m o u n t of d e a d load pressure exerted o n t h e e x t e r i o r footings is a b o u t 6 0 0 psf. Such dead load pressure is insufficient t o prevent t h e uplifting of t h e c l a y s t o n e b e n e a t h t h e footings. With voids formed b e n e a t h t h e footings, t h e dead load pressure will b e increased t o a b o u t 3 , 0 0 0 psf, eliminating t h e uplifting p r o b l e m . Air space b e n e a t h t h e footings should b e formed b y t h e use of void forming material. 3 . A n alternative remedial c o n s t r u c t i o n m e t h o d is installation of post-tensioned steel cables in b o t h t h e o u t s i d e and inside of t h e f o u n d a t i o n walls in t h e m a n n e r indicated in figure 159. T h e p u r p o s e of t h e post-tensioned cables is t o tie t h e entire s t r u c t u r e t o g e t h e r t o prevent u n e q u a l m o v e m e n t . 4 . T h e drainage a r o u n d t h e building should b e improved so t h a t w a t e r will drain away from t h e building. All shrubs and flower b e d s adjacent t o t h e building should b e removed and all r o o f d o w n s p o u t s should e x t e n d well b e y o n d t h e limit of all backfill.

247


SECTION

AA

SECTION

Β Β

Figure 157. Sketch of new grade beam for basement.

5. T h e installation of a subsurface drainage system will n o t i m p r o v e t h e p r e s e n t situation. However, t h e use of a subsurface drainage system will k e e p t h e w a t e r table b e l o w b a s e m e n t level. T h e use of a gravel layer b e n e a t h t h e slab will n o t p r e v e n t cracking of t h e slab, b u t will b r e a k any capillary w a t e r rise. If t h e remedial m e a s u r e s presented above are p e r f o r m e d , it is believed t h a t m o v e m e n t of t h e h o u s e will s t o p , b u t it will t a k e a period of at least 6 m o n t h s b e f o r e equilibrium can b e established.

Interior

decorating

and

repairs

should

not

begin u n t i l equilibrium has b e e n

established. Elevation pins should b e established a r o u n d t h e h o u s e and r e c o r d s k e p t as t o t h e a m o u n t of m o v e m e n t . R e m e d i a l c o n s t r u c t i o n was started shortly after t h e h o u s e was investigated. T h e posttensioned cable system w a s used t o s t r e n g t h e n t h e f o u n d a t i o n (fig. 160). T h e residence is in good c o n d i t i o n after t h e remedial c o n s t r u c t i o n .

248


• Figure 158.

Pit I

Removal of soils beneath the footings.

249


PLAN

Post Tension Cables installed maximum Γ-θ" below window. Cable cased in grouted tubes. Applied tension on the order of 100 p.s.i.

Existing foundation wall (concrete in good condition) Floor Slab

7^

SECTION

A ·A

Figure 159. Remedial measures using post-tensioned steel cables.

250


Figure 160. Post-tensioning the foundation wall.

Case V

DISTRESS CAUSED BY RISE OF WATER TABLE

GENERAL This case s t u d y involves 3 9 t w o - s t o r y t o w n h o u s e s f o u n d e d o n a drilled pier system, t h a t had f o u n d a t i o n m o v e m e n t . T h e m o v e m e n t of t h e drilled pier system is essentially caused b y a rise of ground w a t e r . T h e following k n o w l e d g e was gained from this s t u d y : 1. In areas w h e r e t h e r e is a strong possibility of rise of g r o u n d w a t e r , t h e use of a drilled pier system should be carefully considered. 2. Heaving of t h e floor slab can t r a n s m i t high swelling pressure t o t h e grade b e a m s . 3. Chemical t r e a t m e n t of t h e u n d e r s l a b soil, in this case, is ineffective.

HISTORY T h i r t y - n i n e t o w n h o u s e s are u n d e r investigation for f o u n d a t i o n m o v e m e n t . T h e t o w n h o u s e s are located in s o u t h e a s t Denver, C o l o r a d o . T h e r e are four t o seven u n i t s in each building. A total of 2 5 1 u n i t s and a c l u b h o u s e were studied (fig. 161). Initial subsoil investigation for t h e site was m a d e in 1 9 6 5 . T h e r e p o r t resulting from t h a t investigation r e c o m m e n d e d

t h a t t h e buildings b e founded

w i t h piers drilled i n t o b e d r o c k

designed for a m a x i m u m end pressure of 2 0 , 0 0 0 psf and a m i n i m u m dead load pressure of 10,000 psf. C o n s t r u c t i o n of t h e building c o m p l e x was started in S e p t e m b e r , 1 9 6 5 . S h o r t l y after the c o m p l e t i o n of t h e various u n i t s , m o v e m e n t of t h e buildings b e g a n , and in s o m e buildings severe m o v e m e n t of b o t h f o u n d a t i o n walls and floor slabs was n o t i c e d . In March, 1 9 7 0 , a consulting soil engineer was engaged

t o m a k e a preliminary investigation i n t o t h e cause of cracking and

m o v e m e n t in t h e various u n i t s . A r e p o r t outlining remedial c o n s t r u c t i o n was prepared b u t no corrective a c t i o n was t a k e n . T h e following typical distress was observed in m o s t buildings: Foundation

walls

Cracks were found in t h e brick course in a diagonal p a t t e r n from t h e t o p of t h e w i n d o w or d o o r o n t h e g r o u n d level t o t h e b o t t o m of t h e w i n d o w in t h e u p p e r level. Most of t h e cracks had b e e n p a t c h e d and some had r e o p e n e d . T h e w i d t h of t h e cracks ranged from hairline thickness t o as m u c h as 1 inch as s h o w n o n figures 162 and 1 6 3 . T h e s e cracks definitely indicated pier


252

95

70

GROUNDWATER

CûNTO'Jf c

Figure 161. Building location plan and ground-water contour.

m o v e m e n t . In a d d i t i o n , b o t h vertical and diagonal cracks were found in t h e b a s e m e n t f o u n d a t i o n walls. Interior floor

slabs

Most of t h e b a s e m e n t areas were finished. C o n c r e t e floor slabs were covered with tile o r carpeting. T h e slabs had heaved in m a n y buildings and t h e cracks generally followed a p a t t e r n parallel t o t h e f o u n d a t i o n walls. This is a typical slab crack w h e r e separation b e t w e e n t h e slab and f o u n d a t i o n wall was n o t p r o p e r l y c o n s t r u c t e d . T h e slabs bind o n t h e f o u n d a t i o n walls and are n o t free t o accept vertical m o v e m e n t . C o n s e q u e n t l y , cracks appeared

parallel t o

the

f o u n d a t i o n walls. Such m o v e m e n t n o t o n l y caused t h e slab cracking, b u t also exerted uplift pressure o n t h e f o u n d a t i o n walls. Partition

walls

All buildings have u n i t s w i t h p a r t i t i o n s and in m o s t cases slip j o i n t s were n o t provided in t h e slab bearing p a r t i t i o n walls. C o n s e q u e n t l y , w h e n t h e slabs m o v e d u p w a r d , t h e y exerted uplifting pressure o n t h e u p p e r s t r u c t u r e . As a result, cracks developed in t h e u p p e r stories, j a m m i n g t h e d o o r s and d i s t o r t i n g t h e floor system.


Figure 162.

Typical cracking of the townhouse exterior walls.

253

Figure 163. Typical cracking of townhouse exterior walls indicating pier uplift.

254 FOUNDATIONS ON EXPANSIVE SOILS


255

In t h o s e t o w n h o u s e s w h i c h had b a s e m e n t areas t h a t w e r e n o t finished, slab m o v e m e n t d o e s n o t c o n t r i b u t e t o t h e distress of t h e u p p e r stories; however, t h e staircase walls in all cases rest directly o n t h e b a s e m e n t floor. T h e r e f o r e , slab m o v e m e n t can t r a n s m i t m o v e m e n t t o t h e u p p e r stories t h r o u g h t h e staircase p a r t i t i o n walls. Slab

treatment B e t w e e n F e b r u a r y and April of 1 9 6 6 , in several buildings mainly at t h e w e s t e r n p o r t i o n of

t h e site, t h e floor slab was removed and t h e soil b e n e a t h t h e slab injected w i t h stabilizing chemicals in an effort t o eliminate t h e swelling p o t e n t i a l of t h e lower soils. After t h e new slab had b e e n placed, m o v e m e n t of t h e floor slab was n o t checked and in m o s t cases, slabs again cracked. T h e results o f t h e swell tests indicated t h a t t h e swelling pressure of t h e u n d e r s l a b soils ranged from 5 0 0 t o 2 , 5 0 0 psf and t h e m o i s t u r e c o n t e n t s at various d e p t h s w e r e fairly u n i f o r m . Tests indicated t h a t t h e application of chemicals o n t h e u n d e r s l a b soil did n o t have a p r o n o u n c e d effect in reducing t h e swelling p o t e n t i a l . Aprons C o n c r e t e a p r o n s c o n s t r u c t e d at t h e rear of t h e buildings were in m o s t cases cracked. Gaps w e r e found b e t w e e n t h e slabs and t h e buildings, indicating separation. This can b e caused b y either t h e uplifting of t h e building relative t o t h e slab or t h e s e t t l e m e n t of t h e slab d u e t o i n a d e q u a t e backfill relative t o t h e building. T h e a p r o n s were m u d j a c k e d in several buildings in an a t t e m p t t o correct t h e c o n d i t i o n . In general, t h e cracks t h a t developed in this a p a r t m e n t c o m p l e x typify t h e cracks found in buildings f o u n d e d o n expansive soils. T h e r e is n o d o u b t t h a t t h e m o v e m e n t of t h e buildings can be a t t r i b u t e d t o a swelling soil p r o b l e m .

SUBSOIL CONDITIONS Subsoil c o n d i t i o n s at t h e site consist of 7 t o 2 2 feet of stiff t o m e d i u m stiff clays overlying claystone b e d r o c k . T h e clays have an average m o i s t u r e c o n t e n t of 2 0 . 3 p e r c e n t w i t h t h e highest m o i s t u r e c o n t e n t of 2 3 . 8 p e r c e n t and t h e lowest of o n l y 11.6 p e r c e n t . This m o i s t u r e c o n t e n t is considerably higher t h a n t h e m o i s t u r e c o n t e n t r e p o r t e d b y t h e testing laboratories in August of 1 9 6 5 . A t t h a t t i m e , t h e average m o i s t u r e c o n t e n t of t h e u p p e r soil w a s o n l y 14.6 p e r c e n t . T h e swelling p o t e n t i a l of t h e u p p e r clays ranges from 0.5 t o 3 p e r c e n t w i t h t h e swelling pressure ranging from 0 t o 5 , 0 0 0 psf. T h e lower b e d r o c k consists basically of claystone and w e a t h e r e d claystone. S o m e s a n d s t o n e lenses were found in t h e claystone. A s t u d y of m o i s t u r e c o n t e n t of t h e c l a y s t o n e b e d r o c k also indicates an increase in t h e last few years. Since t h e m o i s t u r e c o n t e n t of t h e c l a y s t o n e b e d r o c k is affected b y t h e p a t t e r n of seams and fissures of b e d r o c k , t h e increase of m o i s t u r e c o n t e n t is n o t as obvious as t h e increase of m o i s t u r e c o n t e n t o f t h e u p p e r clays. T h e swelling pressure of t h e lower claystone ranges from 5 , 0 0 0 t o 2 5 , 0 0 0 psf. T h e average swelling pressure is a b o u t 1 0 , 0 0 0 psf.


256

It is c o n c l u d e d from t h e l a b o r a t o r y testing t h a t t h e u p p e r clays possess o n l y low swelling p o t e n t i a l and t h e lower c l a y s t o n e possess high swelling p o t e n t i a l . Most of t h e f o u n d a t i o n wall m o v e m e n t is t h e d i r e c t result of t h e swelling of t h e lower c l a y s t o n e ; however, t h e swelling of t h e u p p e r clays is sufficient t o cause t h e floor slabs t o heave and crack.

WATER TABLE When t h e soil and f o u n d a t i o n investigation was m a d e in A u g u s t , 1 9 6 5 , n o free w a t e r was found in a n y of t h e 2 1 e x p l o r a t o r y holes. S o m e of t h e test holes were m o r e t h a n 2 0 feet d e e p . In J u l y , 1970 w h e n this investigation was m a d e , free w a t e r was found in almost every h o l e . Figure 161 indicates an a p p r o x i m a t e c o n t o u r of equal elevation t o w a t e r t a b l e . T h e following c o n d i t i o n s w e r e observed: 1. T h e water-table elevation is high at t h e w e s t e r n p o r t i o n of t h e site and low at t h e eastern p o r t i o n . A difference in t h e water-table elevation of 3 0 feet was observed b e t w e e n t h e n o r t h w e s t c o r n e r and t h e s o u t h e a s t c o r n e r of t h e p r o p e r t y . 2. T h e water-table c b n t o u r follows fairly well w i t h t h e b e d r o c k c o n t o u r . Water was found at t h e t o p o r i m m e d i a t e l y b e l o w t h e surface of b e d r o c k . 3 . T h e w a t e r table follows fairly well w i t h t h e g r o u n d surface c o n t o u r . T h e g r o u n d surface was high in t h e west and low in t h e east. After carefully s t u d y i n g t h e c o n t o u r of equal elevation t o t h e w a t e r table and t h e general water-table c o n d i t i o n s in t h e area, it is c o n c l u d e d t h a t in t h e last 5 years, after area d e v e l o p m e n t , t h e r e was a definite change of water-table c o n d i t i o n s . T h e general rise of t h e water-table is essentially caused b y a p e r c h e d w a t e r table in t h e developed residential area. B e d r o c k in this area is shallow and c o m p o s e d essentially of claystone w h i c h is relatively impervious. Free w a t e r flows m o s t l y o n t o p of b e d r o c k and also flows in t h e fissures and seams of t h e b e d r o c k . Surface w a t e r d o e s n o t necessarily p e n e t r a t e directly from t h e g r o u n d surface i n t o t h e u n d e r l y i n g b e d r o c k b u t will flow from t h e high p o i n t t o t h e low p o i n t along t h e surface of bedrock. T h e rise of w a t e r table definitely has a bearing o n t h e f o u n d a t i o n m o v e m e n t of this a p a r t m e n t c o m p l e x . As seen o n figure 1 6 1 , t h e buildings suffering t h e m o s t severe d a m a g e are located in high water-table elevation areas. Where t h e water-table elevation is low, relatively m i n o r d a m a g e t o t h e buildings was e x p e r i e n c e d .

CAUSE O F MOVEMENT In reviewing t h e f o u n d a t i o n design, t h e grade b e a m s and pier system and t h e results of this investigation, t h e following w e r e derived:


Foundation

257

design

T h e f o u n d a t i o n design criteria calls for an end-bearing pressure for piers of 2 0 , 0 0 0 psf and a m i n i m u m dead load pressure of 1 0 , 0 0 0 psf. T h e piers should p e n e t r a t e t h e shale a m i n i m u m d e p t h of 5 feet. T h e side of t h e pier excavated i n t o t h e shale should b e r o u g h e n e d t o provide resistance t o uplift.

T h e drill logs provided b y t h e driller indicated t h a t t h e required

pier

p e n e t r a t i o n had b e e n fulfilled in m o s t cases. T h e load carrying capacity of t h e piers was reviewed, including t h e d e a d load pressure r e q u i r e m e n t , and w e r e found t o b e in a c c o r d a n c e w i t h t h e r e c o m m e n d a t i o n s , e x c e p t for t h e piers s u p p o r t i n g i n t e r i o r c o l u m n s and b e a m s . In m o s t cases, t h e swelling pressure of t h e l o w e r b e d r o c k is a b o u t 1 0 , 0 0 0 psf w i t h a few cases reaching as high as 2 5 , 0 0 0 psf. T h e r e c o m m e n d e d d e a d load pressure of 1 0 , 0 0 0 psf is low, b u t considering t h e subsoils and t h e w a t e r table c o n d i t i o n s at t h e t i m e t h e subsoil investigation was m a d e , t h e design r e c o m m e n d a t i o n s are in a range o f s o u n d engineering p r a c t i c e .

Foundation

construction

F r o m t h e i n f o r m a t i o n o b t a i n e d b y excavating from t h e inside and from t h e o u t s i d e of t h e building exposing t h e grade b e a m s y s t e m , t h e c o n s t r u c t i o n was in a c c o r d a n c e w i t h t h e s t r u c t u r a l f o u n d a t i o n design. In several places, m u s h r o o m s w e r e found o n t o p of t h e piers and t h e void forming m a t e r i a l b e n e a t h t h e grade b e a m s i m m e d i a t e l y adjacent t o t h e piers was a b s e n t . S u c h defects decrease t h e d e a d load pressure e x e r t e d o n t h e piers; h o w e v e r , t h e effect is n o t sufficient t o cause t h e p r e s e n t distress of t h e various buildings. Slab

construction T h e m a j o r p r o b l e m is in t h e area of slab c o n s t r u c t i o n . In principle, free floating slabs should

b e provided. T h e r e should b e positive e x p a n s i o n j o i n t s b e t w e e n t h e slabs and t h e grade b e a m s t o allow free slab m o v e m e n t . A t p r e s e n t , t h e j o i n t b e t w e e n t h e slabs and t h e grade b e a m s is n o t effective in m o s t cases. Slab m o v e m e n t d u e t o expansive soils has t r a n s m i t t e d uplifting pressure from t h e slabs t o t h e f o u n d a t i o n walls. T h e swelling pressure of t h e u n d e r s l a b soil is a b o u t 2 , 0 0 0 psf. In e x t r e m e cases, t h e pressure t r a n s m i t t e d from t h e slab t o t h e f o u n d a t i o n wall can reach 2 , 0 0 0 p o u n d s per linear foot. With t h e piers spaced o n 11-foot centers, each pier can b e subject t o a b o u t 22 kips of uplifting pressure.

Pier

uplifting T h e m o s t i m p o r t a n t reason for t h e m o v e m e n t of t h e f o u n d a t i o n walls is t h e uplifting of t h e

piers. In t h e original f o u n d a t i o n design, it was assumed t h a t t h e l o w e r b e d r o c k w o u l d n o t b e c o m e w e t t e d and t h e skin friction b e t w e e n t h e b e d r o c k and t h e piers w o u l d provide an additional factor of safety against pier uplifting. With t h e rise of t h e w a t e r t a b l e , t h e entire lower b e d r o c k b e c a m e w e t t e d ; c o n s e q u e n t l y , t h e skin friction value dissipated and r a t h e r t h a n holding t h e piers,

258


t h e swelling of t h e b e d r o c k actually lifted t h e piers. As an e x a m p l e , pier N o . 8 in Building 8 was checked b y calculation as follows: A c t u a l dead load pressure o n t h e pier

2 0 . 6 kips


10 inches

Average swelling pressure

10,000 psf

Pier p e n e t r a t i o n i n t o b e d r o c k

7 feet

Uplifting pressure d u e t o s a t u r a t i o n of b e d r o c k - 7X2.62X 1 0 , 0 0 0 X 0 . 1 5

27.5 kips

U n b a l a n c e d pressure 27.5 - 2 0 . 6

7.1 kips

F o r interior piers w h e r e t h e actual dead load pressure exerted o n t h e pier ranges from 2.6 t o 20.9 kips, t h e u n b a l a n c e d pressure is in t h e range of 10 t o 3 0 kips. It is concluded t h a t t h e w e t t i n g of t h e b e d r o c k plus t h e pressure t r a n s m i t t e d from slab heaving t o t h e grade b e a m had lifted t h e piers. In o n e test pit, t h e entire length of t h e pier was e x p o s e d . A gap of a p p r o x i m a t e l y 3 inches b e t w e e n t h e soil and t h e b o t t o m of t h e pier was found. This verified t h a t t h e pier actually pulled o u t of t h e g r o u n d and w o u l d have cracked h a d it n o t b e e n reinforced.

SOURCE O F MOISTURE T h e rise of g r o u n d w a t e r in this d e v e l o p m e n t is m a i n l y derived from surface w a t e r . T h e source of surface w a t e r is from the following: Precipitation Precipitation, either from rain or from melting snow, can c o n t r i b u t e t o t h e rise of w a t e r table. Before t h e d e v e l o p m e n t was c o n s t r u c t e d , m o s t of t h e precipitation drained from t h e site as surface runoff. O n l y a p o r t i o n o f t h e p r e c i p i t a t i o n p e n e t r a t e d t h e g r o u n d . With building c o n s t r u c t i o n , in isolated areas, precipitation will p e n e t r a t e t h e soils t h r o u g h the loose backfill a r o u n d each building and d u e t o p o o r drainage c o n d i t i o n s , t e n d s t o a c c u m u l a t e instead of r u n n i n g off t h e site. This c o n s t i t u t e s an i m p o r t a n t factor t o w a r d t h e rise of g r o u n d water. Lawn

irrigation After a d e v e l o p m e n t is c o m p l e t e d , lawn irrigation in t h e area will generally create perched

water-table c o n d i t i o n s . It is conceivable t h a t a large a m o u n t of lawn irrigation w a t e r will travel t h r o u g h t h e u p p e r soils and b e c o m e t r a p p e d at t h e surface of t h e b e d r o c k . This practice is n o t necessarily limited t o t h e p r o p e r t y

investigated b u t pertains t o t h e entire general area. High

water-table c o n d i t i o n s prevailed in several areas in t h e general vicinity after t h e d e v e l o p m e n t was completed.


Pipe

259

leakage Leakage of t h e w a t e r lines, resulting from c o r r o d e d service saddles, was found in front of

Buildings 8 0 , 8 3 , and 8 4 . It is n o t k n o w n h o w long t h e leakage had t a k e n place, b u t m o n t h l y w a t e r c o n s u m p t i o n from J a n u a r y t o O c t o b e r of 1969 was 2 4 . 2 million gallons. During this same period in 1 9 7 0 , t h e c o n s u m p t i o n was 2 8 . 3 million gallons, w h i c h shows an increase of 4 million gallons over a period of 10 m o n t h s . This increase in c o n s u m p t i o n can be partially explained b y pipe leakage. These 4.1 million gallons of w a t e r will eventually flow o n t h e surface of t h e b e d r o c k and cause a general rise of t h e w a t e r t a b l e .

EVALUATION O F BUILDING CONDITIONS F o r discussion p u r p o s e s , t h e t o w n h o u s e c o m p l e x was divided i n t o t h r e e areas (fig. 161), and t h e evaluations are as follows: Area I In this area, severe f o u n d a t i o n m o v e m e n t has t a k e n place. T h e f o u n d a t i o n walls as well as t h e floor slabs have cracked and heaved. Most of t h e floor slabs in this area have b e e n replaced and t h e soils b e n e a t h t h e floor slabs have b e e n t r e a t e d w i t h special chemicals. T h e water-table elevation is high in this area and t h e b e d r o c k has b e e n w e t t e d excessively. F o u n d a t i o n m o v e m e n t in this area is caused b y t h e uplifting of t h e piers.

Area II In this area, relatively m i n o r f o u n d a t i o n m o v e m e n t has t a k e n place. S o m e of t h e floor slabs have b e e n replaced and t r e a t m e n t of t h e underslab soils was m a d e in several buildings. T h e buildings in this area have n o t suffered severe f o u n d a t i o n m o v e m e n t w h i c h is p r o b a b l y d u e t o t h e following:

1. T h e water-table elevation is relatively d e e p and t h e b o t t o m s of t h e piers are above t h e present w a t e r table. T h e piers still m a i n t a i n an anchorage effect and expansive soils have n o t acted u p o n t h e surface of t h e piers. 2. T h e l o w e r b e d r o c k consists of a c o m b i n a t i o n of s a n d s t o n e and c l a y s t o n e w h i c h d o e s n o t possess a high swell p o t e n t i a l . T h e r e f o r e , t h e piers are relatively stable. It is possible t h a t these buildings will n o t suffer severe f o u n d a t i o n m o v e m e n t in t h e f u t u r e ; however, changing w a t e r table and o t h e r local c o n d i t i o n s m a y affect t h e stability of t h e s t r u c t u r e . F o r instance, in Buildings 8 8 , 9 0 and 9 2 , it is a n t i c i p a t e d t h a t severe m o v e m e n t will t a k e place in t h e near future. This is because of t h e high swell p o t e n t i a l of t h e l o w e r soils and t h e close p r o x i m i t y of t h e w a t e r table.

260


Area III This area is located at t h e s o u t h w e s t p o r t i o n of t h e site w h e r e g r o u n d surface is high and t h e w a t e r table elevation is d e e p . At t h e n o r t h e a s t p o r t i o n of t h e site, t h e g r o u n d w a t e r is well below t h e b o t t o m of t h e piers. T h e c o n d i t i o n of these buildings is relatively g o o d . T h e floor slabs have n o t b e e n replaced, n o r have t h e r e b e e n a n y remedial m e a s u r e s t a k e n .

REMEDIAL MEASURES R e m e d i a l measures d e p e n d o n t h e e x t e n t of t h e present d a m a g e and can be best described u n d e r t h e t h r e e areas m e n t i o n e d b e f o r e : Area I In this area, t h e d a m a g e is of such e x t e n t t h a t drastic m e a s u r e s should b e t a k e n as recommended: 1. All piers should be cut free from t h e f o u n d a t i o n system so t h a t t h e entire building will n o t b e associated w i t h t h e lower b e d r o c k . This is necessary because t h e source of t h e p r o b l e m , for t h e buildings in this area, is caused b y t h e e x p a n s i o n of t h e lower b e d r o c k . Since t h e water-table c o n d i t i o n s c a n n o t b e changed, it is necessary t o prevent direct c o n t a c t of t h e f o u n d a t i o n system w i t h t h e b e d r o c k . 2. Individual p a d s should b e provided b e n e a t h t h e f o u n d a t i o n walls t o s u p p o r t t h e building. T h e pads should b e designed for a m a x i m u m soil pressure of 2 , 5 0 0 psf and as m u c h dead load pressure as possible. Since t h e swelling pressure o f t h e u p p e r soils in this area is a b o u t 2 , 0 0 0 psf, t h e r e should b e little danger of f o u n d a t i o n m o v e m e n t d u e t o t h e swelling of t h e u p p e r soils. 3 . Shims should b e provided o n t o p of each pad so t h a t t h e elevation of t h e building can b e adjusted. T h e shims should b e adjusted w i t h an engineering level i m m e d i a t e l y

after

c o m p l e t i o n o f t h e p a d s and should b e readjusted after a period of 6 m o n t h s . T h e above described u n d e r p i n n i n g o p e r a t i o n can be e x e c u t e d either from inside t h e b a s e m e n t area o r from o u t s i d e . Since it is necessary in m o s t cases t o r e m o v e t h e floor slabs, it will probably

be more

economical

to

perform

the

underpinning

o p e r a t i o n inside

the

basement. 4. All floor slabs should b e separated from t h e bearing walls w i t h a positive expansion j o i n t . If effective e x p a n s i o n j o i n t s c a n n o t b e o b t a i n e d , a gap of a p p r o x i m a t e l y o n e half inch should b e left all a r o u n d t h e slab t o insure t h a t t h e slabs will n o t bind against t h e bearing walls. When t h e b a s e m e n t is finished, this gap can b e tiled t o prevent dirt entering t h e gap. 5. T h e use of interior slab-bearing p a r t i t i o n walls should b e discouraged. If such is necessary, t h e n slip j o i n t s should b e provided t o insure free p a r t i t i o n wall m o v e m e n t . T h e slip j o i n t should apply t o all d o o r frames and staircase walls. It should b e emphasized t h a t sheetr o c k o n b o t h sides of t h e p a r t i t i o n wall should also b e provided w i t h slip j o i n t s .


261

6. In s o m e cases, it m a y be necessary t o provide a subsurface drainage system a r o u n d t h e p e r i m e t e r of t h e b a s e m e n t area. 7. It will be necessary t o carefully check all sewage and w a t e r pipes t o insure t h a t n o leakage has t a k e n place. T h e above remedial m e a s u r e s for Area I are expensive and difficult t o carry o u t , b u t t o insure t h a t n o further f o u n d a t i o n m o v e m e n t will t a k e place and t o eliminate s o m e of t h e existing d a m a g e , such remedial m e a s u r e s are necessary and unavoidable. Area II R e m e d i a l m e a s u r e s for t h e buildings in Area II d e p e n d greatly u p o n individual building c o n d i t i o n s . In general, t h e following are r e c o m m e n d e d : 1. Increase t h e d e a d load pressure exerted o n t h e piers b y eliminating a n u m b e r of piers and increasing t h e span b e t w e e n t h e piers. This should b e carefully designed and p l a n n e d b y a s t r u c t u r a l engineer. T h e dead load pressure exerted o n t h e piers should b e n o t less t h a n 2 0 , 0 0 0 psf. T h e end bearing pressure of t h e piers should n o t greatly exceed 4 0 , 0 0 0 psf, w i t h a skin friction of 4 , 0 0 0 psf for t h e p o r t i o n of pier in b e d r o c k . 2. Where existing d a m a g e is relatively severe, t h e entire building should b e releveled using shims o n t o p of each pier. 3 . Air space b e n e a t h t h e grade b e a m s should b e carefully checked for effectiveness. All m u s h r o o m s above t h e piers should b e r e m o v e d . 4 . Careful i n s p e c t i o n of t h e c o n d i t i o n of t h e floor slabs should be m a d e . If t h e slabs are binding against t h e grade b e a m s , t h e n t h e slabs should b e r e m o v e d and replaced w i t h an effective j o i n t system. Every effort should b e m a d e t o avoid t h e transmission of pressure from t h e slabs t o t h e f o u n d a t i o n walls. 5. Interior slab bearing p a r t i t i o n walls and drainage s y s t e m s should b e t r e a t e d as described u n d e r Area I Area III N o remedial m e a s u r e s are necessary in this area. T h e buildings are in relatively g o o d c o n d i t i o n and unnecessary a l t e r a t i o n s in this area should b e avoided. However, close observation of f o u n d a t i o n m o v e m e n t should b e m a i n t a i n e d . If it is found at a later d a t e t h a t t h e r e are signs of f o u n d a t i o n m o v e m e n t , b o t h t h e s t r u c t u r a l engineer and t h e soils engineer should be informed so t h a t t h e y m a y d e t e r m i n e if remedial m e a s u r e s are necessary. Since

the

investigation

covers

as m a n y

as 2 5 1

u n i t s , it is difficult

t o specify

the

r e c o m m e n d e d remedial p r o c e d u r e for each building. T h e soils engineer should be at t h e site during t h e e x e c u t i o n of t h e remedial measures r e c o m m e n d e d b y t h e s t r u c t u r a l engineer and t h e soils engineer.

APPENDIX A SUGGESTED METHOD OF TEST FOR ONE-DIMENSIONAL EXPANSION A N D UPLIFT PRESSURE OF CLAY SOILS1 S U B M I T T E D B Y W.

1. Scope 1.1 This method explains how to make expansion tests on undisturbed or compacted clay soil samples that have no particle sizes greater than τ$ in. (passing the No. 4 standard ASTM sieve 3). The test is made to determine (1) magnitude of volume change under load or no-load conditions, (2) rate of volume change, (3) influence of wetting on volume change, and (4) axial permeability of laterally confined soil under axial load or no-load during expansion. Saturation (no drainage) takes place axially. Permeant water is applied axially for determining the effect of saturation and permeability. The specimens prepared for this test may also be used to determine the vertical or volume shrinkage as the water content decreases. Total volume change for expansive soils is determined from expansion plus shrinkage values for different ranges of water content. 1.2 Expansion test data may be used to estimate the extent and rate of uplift in subgrades beneath structures or in structures formed from soils, and shrinkage tests may be used to estimate the volume changes which will occur in soils 1

T h i s s u g g e s t e d m e t h o d h a s n o official s t a t u s i n t h e S o c i e t y b u t is p u b l i s h e d a s i n f o r m a t i o n o n l y . T h e m e t h o d is b a s e d o n t h e experience of the submitter. C o m m e n t s are solicited. 2 Consulting Civil Engineer, Denver, Colo. 3 S e e A S T M S p e c i f i c a t i o n E l l , for W i r e C l o t h S i e v e s for T e s t i n g P u r p o s e s , Annual Book of ASTM Standards, P a r t 3 0 .

G.

HOLTZ2

upon drying, provided that natural conditions and operating conditions are duplicated. 2. Significance 2.1 The expansion characteristics of a soil mass are influenced by a number of factors. Some of these are size and shape of the soil particles, water content, density, applied loadings, load history and mineralogical and chemical properties. Because of the difficulty in evaluating these individual factors, the volume-change properties cannot be predicted to any degree of accuracy unless laboratory tests are performed. When uplift problems are critical, it is important to test samples from the sites being considered. 2.2 The laboratory tests described herein are primarily intended for the study of soils having no particles larger than the No. 4 standard sieve size (^ξ in.). If the test is made on the minus No. 4 fraction of soils containing gravel material (plus No. 4), some adjustment is required in any analysis. Gravel reduces volume change because it replaces the more active soil fraction. 3. Apparatus 3.1 Consolidometer—Conventional laboratory consolidometers are used for the expansion test. Consolidometers most used in the United States are of the fixed-ring and floating-ring types. Figure 1 illustrates the fixed-ring type. Either


264

of these is suitable. Both types are available commercially. In the fixed-ring container, all specimen movement relative to the container is upward during expansion. In the floating-ring container, movement of the soil sample is from the top and bottom away from the center during expansion. The specimen containers for the fixed-ring consolidometer and the floating-ring consolidometer consist of brass or plastic rings, and other

sion tests the larger diameter consolidation rings are preferred as they restrain fhe soil action to a lesser degree. In a test using the floating-ring apparatus, the friction between the soil specimen and container is smaller than with the fixed-ring apparatus. On the other hand,, the fixed-ring apparatus is more suitable for saturation purposes and when permeability data are required. Porous stones are required at the top and bottom of the

/Dial gage holder

Clamping ring and water container J-Connecting rod ^-Gasket |

'Base p l a t e - - ' ' F I G . 1—Fixed-Ring Consolidometer.

component parts. Sizes of container rings most commonly used vary between 4j-in. diameter by 1} in. deep and 2j-in. diameter by J in. deep, although other sizes are used. However, the diameter should be not less than 2 in. and the depth not greater than three tenths of the diameter, except that the depth must not be less than f in. for specimens of small diamter. Lesser depths introduce errors caused by the magnitude of surface disturbance, while large depths cause excessive side friction. For expan-

specimen to allow application of water. The apparatus must allow vertical movement of the top porous stone for fixed-ring consolidometers, or vertical movement for top and bottom porous stones for floating-ring consolidometers, as expansion takes place. A ring gage machined to the height of the ring container to an accuracy of 0.001 in. is required; thus, the ring gage for lj-in.high specimens will have a height of 1.250 in. Measure the diameter of the specimen container ring to 0.001 in.

265

APPENDIX A

3.2 Loading Device—A suitable device for applying vertical load to the specimen is required. The loading device may be platform scales of 1000 to 3000-lb capacity mounted on a stand and equipped with a screw jack attached underneath the frame. The jack operates a yoke which extends up through the scale platform and over the specimen container resting on the platform. The yoke is forced up or down by operating the jack, thus applying or releasing load to the soil specimen. The desired applied pressure, which is measured on the scale beam, becomes fully effective when the beam is balanced. 3.2.1 Another satisfactory loading device utilizes weights and a system of levers for handling several tests simultaneously. Hydraulic-piston or bellowstype loading apparatus are also very satisfactory if they have adequate capacity, accuracy, and sensitivity for the work being performed. Apparatus such as described in ASTM Method D 2435, Test for One-Dimensional Consolidation Properties of Soils, 4 is satisfactory and may be used. 3.3 Device

for

Cutting

Undisturbed

Specimens—This apparatus consists of a cutting bit of the same diameter as the ring container of the consolidometer, a cutting stand with bit guide, and knives for trimming the soil. Wire saws or trimming lathes may be used if a uniform tight fit of the specimen to the container is obtained. 3.4 Device for Preparation

of Remolded

Specimens—Compacted soil specimens are prepared in the consolidometer ring container. In addition to the container, the apparatus consists of an extension collar about 4 in. in depth and of the same diameter as the container. A compaction hammer of the same type required in Method A of ASTM Method D 698, Test for Moisture-Density Rela4

Annual Book of ASTM Standards,

Part 11.

tions of Soils, Using 5.5-lb Rammer and 12-in. Drop. 4 4. Procedure-Expansion Test 4.1 Preparation

of Undisturbed

Speci-

mens—Perform the tests on hand-cut cube samples or core samples o£ a size that will allow the cutting of approximately I in. of material from the sides of the consolidometer specimen. (Alternatively, obtain a core of a diameter exactly the same as the diameter of the consolidometer specimen container, and extrude the core directly into the container. This procedure is satisfactory provided that the sampling has been done without any sidewall disturbance and provided that the core specimen exactly fits the container. Place the undisturbed soil block or core on the cutting platform, fasten the cutting bit to the ring container, and place the assembly on the srmple in alignment with the guide arms. With the cutting stand guiding the bit, trim the excess material with a knife close to the cutting edge of the bit, leaving very little material for the bit to shave off as it is pressed gently downward. (Other suitable procedures to accommodate guides for wire saws, trimming lathes, or extrusion devices may be used in conformance with the use of alternative apparatus and samples.) In trimming the sample, be careful to minimize disturbance of the soil specimen and to assure an exact fit of the specimen to the consolidometer container. When sufficient specimen has been prepared so that it protrudes through the container ring, trim it flush with the surface of the container ring with a straightedge cutting tool. Place a glass plate on the smooth, flat cut surface of the specimen, and turn the container over. Remove the cutting bit, trim the specimen flush with the surface of the container ring, and cover it with a second


266

glass plate to control evaporation until it is placed in the loading device. 4.2 Preparation

of

Remolded

Speci-

mens—Use about 2 lb of representative soil that has been properly moistened to the degree desired and processed free from lumps and from which particles or aggregations of particles retained by a ï^-in. (No. 4) sieve have been excluded. Compact the specimen to the required wet bulk density after adding the required amount of water as follows : Place the extension collar on top of the container ring and fasten the bottom of the container ring to a baseplate. Weigh the exact quantity of the processed sample to give the desired wet density when compacted to a thickness \ in. greater than the thickness of the container ring. Compact the specimen to the desired thickness by the compaction hammer. Remove the extension collar and trim the excess material flush with the container ring surface with a straightedge cutting tool. Remove the ring and specimen from the baseplate and cover the specimen surfaces with glass plates until the specimen is placed in the loading device. If, after weighing and measuring the specimen and computing the wet density, as described below, the wet density is not within 1.0 lb/ft 3 of that required, repeat the preparation of the remolded specimen until the required accuracy is obtained. 4.3 Calibration

of Dial Gage for

Height

Measurements—Prior to filling the container ring with the soil specimen, place a ring gage in the specimen container with the same arrangement of porous plates and load plates to be used when testing the soil specimen. Place the assembly in the loading machine in the same position it will occupy during the test. After the apparatus has been assembled with the ring gage in place, apply a load equivalent to a pressure of 0.35 psi (or 0.025 kgf/cm 2) on the soil

specimen. The dial reading at this time will be that for the exact height of the ring gage. Mark the parts of the apparatus so that they can be matched in the same position for the test. 4.4 Initial

Height and Weight of Soil

Specimen—Clean and weigh the specimen container ring and glass plates and weigh them to ±0.01 g before the ring is filled. After filling and trimming is completed, weigh the soil specimen, ring, and glass plates to ± 0 . 0 1 g. Determine the weight of the soil specimen. Assemble the specimen container and place it in the loading device. If the specimen is not to be saturated at the beginning of the test, place a rubber sleeve around the protruding porous plates and load plates to prevent evaporation. Apply the small seating load of 0.35 psi (or 0.025 kgf/cm 2) to the specimen. By comparing the dial reading at this time with the dial reading obtained with the ring gage in place, determine the exact height of the specimen. Use this information to compute the initial volume of the specimen, the initial density, void ratio, water content, and degree of saturation. The true water content of the specimen will be determined when the total dry weight of the specimen is obtained at the end of the test. 4.5 Saturation

and Permeability

Data

—To saturate the specimen attach the percolation tube standpipe, fill it with water, and wet the specimen. Take care to remove any air that may be entrapped in the system by slowly wetting the lower porous stone and draining the stone through the lower drain cock. After the specimen is wetted, fill the pan in which the consolidometer stands with water. After saturation has been completed, permeability readings can be taken at any time during the test by filling the percolation tube standpipe to an initial reading and allow the water to percolate through the specimen. Measure the

267

APPENDIX A

amount of water flowing through the sample in a given time by the drop in head. 4.6 Expansion

Test:

4.6.1 General Comments—The expansion characteristics of an expansive-type soil vary with the loading history, so that it is necessary to perform a separate test or several specimens for each condition of loading at which exact expansion data are required. However, one procedure is to test only two specimens: (1) loadedand-expanded, and (2) expanded-and-

permeameter tube head should be sufficiently low so that the specimen is not lifted.) As the specimen begins to expand, increase the load as required to hold the specimen at its original height. Then reduce the load to \ , and \ of the maximum load and finally to the seating load of 0.35 psi (or 0.025 kgf/cm 2) and measure the height with each load. Use a greater number of loadings if greater detail in the test curve is required. Maintain all loads for 24 h, or longer if needed, to obtain constant values of height #

rSpecimt *n wetted

\

I

\

I" to

\

ted

^A^et

\

\ ·>

—

^ 0

20

10

- —

Specimen wetted,

Load-psi.

F i g . 2—Load-Expansion Curves.

loaded. From these data, an estimate of expansion can be made for any load condition as shown by Curve C, Fig. 2, in which Specimen No. 1 was loaded and expanded by saturation with water, (Curve B) and Specimen No. 2 was expanded by saturation with water and then loaded (Curve A). 4.6.2 Loaded and Expanded

Test—To

measure expansion characteristics where the soil specimen is saturated under full load and then allowed to expand, apply the seating load of 0.35 psi (or 0.025 kgf/cm 2) to Specimen No. 1, and secure initial dial readings. Then saturate the soil specimen as described in 4.5. (The

Remove the specimen from the ring container and weigh it immediately and again after drying to 105 C. From the water content, dry bulk density, and specific gravity of the specimen, calculate the volume of air and, assuming it to be the same as the volume of air following the determination of permeability, calculate the water content and degree of saturation. 4.6.3 Expanded

and Loaded

Test—To

measure expansion characteristics where the soil is allowed to expand before loading, apply the seating load of 0.35 psi (or 0.025 kgf/cm 2) to Specimen No. 2, and secure initial dial gage readings.

268


Then saturate the specimen as described in 4.5. Allow the specimen to expand under the seating load for 4$ h or until expansion is complete. Load the specimen successively to | , J, J and—t times the maximum load found in 4.6.2, to determine the reconsolidation characteristics of the soil. Use a greater number of loadings, if greater detail in the test curve is required. Follow the procedures specified in 4.6.2 for making loadings and all measurements and determinations. 4.6.4 Individual

Load-Expansion

Test

—When it is desired to perform separate expansion tests for other conditions of loading apply the seating load of 0.35 psi (or 0.025 kgf/cm 2) to the specimen and measure the initial height. Then load the specimen to the desired loading, saturate the specimen as described in 4.5, and allow the specimen to expand under the applied load for 48 h, or until expansion is complete. Measure the height of the expanded specimen. Reduce the load to that of the seating load. Allow the height to become constant and measure; then remove the specimen from the ring and make the determination specified in 4.6.2. 5. Procedure—Shrinkage Test 5.1 Specimen

Preparation—When

measurements of shrinkage on drying are needed, prepare an additional specimen as described in 4.1 or 4.2. Cut this specimen from the same undisturbed soil sample as the expansion specimens, or remolded to the same bulk density and water content conditions as the expansion specimens. Place the specimen in the container ring, and measure the initial volume and height as described in 4.4. Determine the water content of the soil specimen by weighing unused portions of the original sample of which the specimen is a part, drying the material in an oven to 105 C, and reweighing it. 5.2 Volume

and

Determinations—To

Height

measure

shrinkage, allow the specimen in the ring to dry in air completely or at least to the water content corresponding to the shrinkage limit (ASTM Method D 427, Test for Shrinkage Factors of Soils). 4 After the specimen has been air-dried, remove it from the ring container, and obtain its volume by the mercury-displacement method. 5.2.1 To perform the mercury displacement measurement, place a glass cup with a smoothly ground top in an evaporating dish. Fill the cup to overflowing with mercury, and then remove the excess mercury by sliding a special glass plate with three prongs for holding the specimen in the mercury over the rim. Pour the excess mercury into the original container and replace the glass cup in the evaporating dish. Then immerse the air-dried soil specimen in the glass cup filled with mercury using the special glass plate over the glass cup to duplicate the initial mercury volume determination condition. (See Method D 427 for general scheme of test and equipment.) Transfer the displaced mercury into a graduated cylinder, and measure the volume. If the shrinkage specimen is cracked into separate parts, measure the volume of each part, and add the individual volumes to obtain the total. (A paper strip wrapped around the specimen side and held by a rubber band is effective in holding the specimen intact during handling.) 5.2.2 If the height of the air-dried specimen is desired, place the specimen and ring container in the loading machine. Apply the seating load of 0.35 psi (or 0.025 kgf/cm 2), and then read the dial gage. 6. Calculations 6.1 Expansion

Test

Data—Calculate

the void ratio as follows:

Shrinkage

volume of voids

h — hQ

volume

volume of solids

h0

269

APPENDIX A

where: e = void ratio, h = height of the specimen, and ho = height of the solid material at zero void content Calculate the expansion, as a percentage of the original height, as follows : A, percent =

h2 — hi X 100 hi

where : Δ = expansion in percentage of initial volume, hi = initial height of the specimen, and hi = height of the specimen under a specific load condition. 6.2 Permeability

Test

Data—Calculate

the permeability rate by means of the following basic formula for the variable head permeameter: k =

Ap X U As X 12

1. 5i X - In — t H{

where: k — permeability rate, ft/year, A ρ = area of standpipe furnishing the percolation head, in. 2, Aa = area of the specimen, in. 2, Ls = length of the specimen, in., Hi = initial head, difference in head between headwater and tailwater, in., Hf = final head, difference in head between headwater and tailwater, in., and / = elasped time, years. 6.3 Shrinkage

Test

Data—Calculate

the volume shrinkage as a percentage of the initial volume as follows: Vi

where : Δ 8 = volume shrinkage in percentage of initial volume, i\ — initial volume of specimen (height of specimen times area of ring container), and

vd

= volume of air-dried specimen from mercury displacement method. Calculate the shrinkage in height as follows: Δ*. =

X 100

where: AhB — height of shrinkage in percentage of initial height, hi — initial height of specimen, and ha — height of air-dried specimen. 6.3.1 To calculate the total percentage change in volume from "air-dry to saturated conditions, ,, add the percentage shrinkage in volume on air drying Δ 8 to the percentage expansion in volume on saturation Δ 6 , as described in 6.1. This value is used as an indicator of total expansion but is based on initial conditions of density and water content. Since expansion volume data are determined for several conditions of loading, the total volume change can also be determined for several conditions of loading. 6.3.2 To calculate the total percentage change in height from saturated to airdry conditions, add the percentage shrinkage in height Ahs to the percentage expansion Δ wThen the specimen is saturated under specific load conditions. 7. Plotting Test Data 7.1 Expansion Test—The test data may be plotted as shown on Fig. 2. 8. Reports 8.1 Expansion Test—Include the following information on the soil specimens tested in the report : 8.1.1 Identification of the sample (hole number, depth, location). 8.1.2 Description of the soil tested and size fraction of the total sample tested. 8.1.3 Type of sample tested (remolded or undisturbed; if undisturbed, describe the size and type, as extruded core, hand-cut, or other).

270

8.1.4 Initial moisture and density conditions and degree of saturation (if remolded, give the comparison to maximum density and optimum water content (see Methods D 698)). 8.1.5 Type of consolidometer (fixed or floating ring, specimen size), and type of loading equipment. 8.1.6 A plot load versus volume change curves as in Fig. 1. A plot of void ratio versus log of pressure curve may be plotted if desired. 8.1.7 A plot log of time versus deformation if desired. 8.1.8 Load and time versus volume-


change data in other forms if specifically requested. 8.1.9 Final water content, bulk dry density, and saturation degree data. 8.1.10 Permeability data and any other data specifically requested. 8.2 Shrinkage Test—For the report on shrinkage, include data on the decrease in volume from the initial to air-dried condition and, if desired, other information such as the total change in volume and total change in height. Report the load conditions under which the volume change measurements were obtained. Include also Items 8.1.1 through 8.1.5 and 8.1.9.

APPENDIX Β T h e following are s o m e q u e s t i o n s c o m m o n l y raised b y o w n e r s , builders, and architects concerning t h e solutions a n d p r e c a u t i o n s for s t r u c t u r e f o u n d e d on expansive soils. T h e answers given are based m o s t l y o n e x p e r i e n c e r a t h e r t h a n o n the t h e o r e t i c a l a p p r o a c h .

TRUE AND FALSE 1.

It is t r u e t h a t a subsoil investigation should b e c o n d u c t e d for each s t r u c t u r e t o b e b u i l t in an expansive soil area. Using subsoil i n f o r m a t i o n t h a t was o b t a i n e d in t h e vicinity r a t h e r t h a n at t h e specific s t r u c t u r e l o c a t i o n can be b o t h misleading and d a n g e r o u s .

2.

It is t r u e t h a t f o u n d a t i o n design based u p o n subsoil investigation should be o b t a i n e d before construction. P r o p e r f o u n d a t i o n systems, such as size of footing, length of pier, thickness of slab, e t c . , m u s t be carefully designed.

3.

It is false t h a t w h i t e streaks in clay are b e n t o n i t e . White streaks in t h e clay are calcium deposits and n o t b e n t o n i t e . When w e t t e d , t h e heavy calcium c o n t e n t in t h e soil can cause excessive s e t t l e m e n t b u t n o t swelling.

4.

It is false t h a t heaving a c t i o n will stabilize after a n u m b e r of years. O n c e t h e f o u n d a t i o n soils have b e c o m e w e t t e d , t h e r e is n o w a y t o r e m o v e t h e m o i s t u r e . M o v e m e n t of t h e s t r u c t u r e will c o n t i n u e , b u t t h e m a g n i t u d e m a y vary.

5.

It is false t h a t expansive soil b e n e a t h t h e f o u n d a t i o n s will settle b a c k u p o n removal of source of w a t e r o r o n p r o l o n g e d drying. Moisture c o n t e n t b e n e a t h a n y s t r u c t u r e seldom decreases, w i t h possible e x c e p t i o n of areas i m m e d i a t e l y b e n e a t h h e a t d u c t s or furnaces. U p o n w e t t i n g , t h e soil heaves. After t h e soil has reached its m a x i m u m swell p o t e n t i a l , m o i s t u r e migrates t o t h e drier p o r t i o n of t h e soil and heaving action c o n t i n u e s .

6.

It is false t h a t r e i n f o r c e m e n t should b e placed in c o n t i n u o u s footings. T h e d e p t h of c o n t i n u o u s footings is usually o n l y 8 t o 12 inches. Such d e p t h is n o t sufficient t o r e n d e r an effective r e i n f o r c e m e n t . R e i n f o r c e m e n t should be placed in t h e f o u n d a t i o n walls.

7.

It is t r u e t h a t heavy r e i n f o r c e m e n t in t h e f o u n d a t i o n wall will m i n i m i z e cracking. R e i n f o r c e m e n t will span across u n e q u a l heaving and r e d u c e cracking.

272

8.


It is false t h a t chemical stabilization can provide an answer t o all expansive soil p r o b l e m s . Present day k n o w l e d g e of chemical stabilization is in its infantile stage. Extensive research will be required, especially in t h e area of field application, before chemical stabilization can be widely a d o p t e d .

9.

It is false t o assume t h a t if a building is situated o n high g r o u n d , there will be n o swelling problem. G r o u n d w a t e r does n o t follow g r o u n d c o n t o u r , and a p e r c h e d w a t e r table usually follows t h e b e d r o c k c o n t o u r . Therefore, it is possible t h a t a high w a t e r c o n d i t i o n can trigger swelling even o n high g r o u n d .

1.0.

It is false t h a t free water is necessary t o cause swelling. A m o i s t u r e increase of as little as 1 p e r c e n t b y weight is sufficient t o cause heaving action. Installation of a subdrainage system a r o u n d a building m a y i n t e r c e p t free w a t e r b u t will n o t prevent t h e increase of m o i s t u r e c o n t e n t in t h e f o u n d a t i o n soils.

11.

It is false t h a t b y p o n d i n g t h e soil before c o n s t r u c t i o n , t h e heaving p r o b l e m can be eliminated. Ponding will affect o n l y soils b e l o w ground surface t o a shallow d e p t h , while t h e r e is great danger of triggering t h e expansion of d e e p seated expansive soils after t h e building is completed.

12.

It is t r u e t h a t good surface drainage will reduce t h e risk of f o u n d a t i o n m o v e m e n t in an expansive soil area. G o o d surface drainage is a necessary r e q u i r e m e n t b u t b y itself is n o t sufficient t o prevent heaving

and

cracking.

Other

factors

such

as a d e q u a t e

structural design and

proper

c o n s t r u c t i o n t e c h n i q u e s are equally i m p o r t a n t . 13.

It is t r u e t h a t a subdrainage system is always a desirable e l e m e n t in t h e f o u n d a t i o n system. Subdrains will prevent free w a t e r from entering t h e f o u n d a t i o n soils. However, t o be effective, t h e drains should be placed at t h e p r o p e r d e p t h w i t h p r o p e r o u t l e t s .

14.

It is true t h a t if all the g r o u n d surface s u r r o u n d i n g a building is paved, swelling p r o b l e m s can be controlled. Extensive paving a r o u n d a building can effectively c o n t r o l t h e migration of surface w a t e r i n t o t h e f o u n d a t i o n soils. This a c c o u n t s for t h e surprisingly few cases r e p o r t e d o n t h e cracks of gasoline service s t a t i o n structures in expansive soils areas.

15.

It is t r u e t h a t d o w n s p o u t s should be long e n o u g h t o drain w a t e r away from a building. D o w n s p o u t s are preferred t o t h e h i d d e n r o o f drain s y s t e m . Defective d o w n s p o u t s can b e i m m e d i a t e l y d e t e c t e d while t h e built-in r o o f drain system m a y develop leakage t h a t goes u n d e t e c t e d for m a n y years.

APPENDIX Β

16.

273

It is false t h a t b y providing plastic m e m b r a n e s a r o u n d t h e h o u s e w h i c h are t o p p e d w i t h gravel b e d d i n g , water will n o t be able t o seep i n t o t h e f o u n d a t i o n soils. Plastic m e m b r a n e s will allow surface w a t e r t o leak t h r o u g h t h e seams and holes b u t will n o t allow e v a p o r a t i o n , c o n s e q u e n t l y , m o i s t u r e c o n t e n t in t h e soil b e n e a t h t h e plastic will increase steadily. After m a n y years, t h e a c c u m u l a t i o n of m o i s t u r e will eventually cause problems.

17.

It is t r u e t h a t lawn sprinkling systems should n o t be installed adjacent t o t h e building. Lawn sprinkling systems should be installed at least 10 feet away from t h e building, w i t h nozzles directed away from t h e s t r u c t u r e . T h u s t h e chance of saturating t h e backfill can b e reduced.

18.

It is t r u e t h a t a structural floor system or a s u s p e n d e d floor system is t h e only solution t o slabs-on-ground c o n s t r u c t i o n in expansive soil areas. Whenever possible a suspended floor system should b e a d o p t e d , b u t o f t e n t i m e s this is n o t economically feasible.

19.

It is false t h a t sand and gravel placed b e n e a t h the floor slab will r e d u c e the uplift pressure of e x p a n d i n g clay b y providing void space i n t o w h i c h t h e clay can flow as it e x p a n d s . Swelling clay can exert uplifting pressure on t h e interlocking gravel particles causing the floor t o heave. It is d o u b t f u l t h a t any clay will flow i n t o t h e voids.

20.

It is t r u e t h a t b u r i e d utility lines should be avoided, w h e n e v e r possible, in an expansive soil area. Swelling soil can shear w a t e r and sewer lines and cause leakage. Initial small leakage will trigger m o r e e x p a n s i o n causing greater leakage, resulting in severe d a m a g e .

21.

It is false t h a t p u d d l i n g of backfill can achieve t h e desired c o m p a c t i o n . Simulated l a b o r a t o r y tests can easily d e m o n s t r a t e t h a t p u d d l i n g will n o t increase t h e soil d e n s i t y . Excessive p u d d l i n g can i n t r o d u c e a large a m o u n t of w a t e r i n t o the f o u n d a t i o n soils.

22.

It is t r u e t h a t t h e use of expansive soil as backfill can exert swelling pressure o n the wall and cause cracking. H o r i z o n t a l swelling pressure is a p p r o x i m a t e l y equal in m a g n i t u d e t o t h e vertical swelling pressure. However, since m o s t backfill is n o t tightly c o m p a c t e d , such h o r i z o n t a l pressure seldom fully develops.

23.

It is false t h a t lateral expansion is t h e cause of separation of w i n d o w s and d o o r s from frames. Most lateral m o v e m e n t is caused b y differential heaving w h i c h creates an impression of pushing away or pulling a p a r t .

24.

It is false t h a t a drilled pier f o u n d a t i o n system provides t h e best answer t o s t r u c t u r e s f o u n d e d on expansive soils.

274


A drilled pier s y s t e m , if intelligently designed and c o n s t r u c t e d , can solve m u c h of t h e swelling soil p r o b l e m . However, in areas w h e r e there is a possibility of rising g r o u n d water, a drilled pier f o u n d a t i o n m a y n o t be effective. Statistically, t h e r e are p r o b a b l y m o r e cracked buildings f o u n d e d on piers t h a n w i t h spread footings. 25.

It is t r u e t h a t b y placing t h e building o n a single pier, there w o u l d never be a swelling problem. Who can afford t h a t ?

INDEX A Absorption, 9 Activity Method, 16,21 Adsorbed cation, 11 Adsorbed water, 13,176 Airport, 8, 173 Air space, 90, 96, 190, 205, 207, 246 Apron,37,121, 136,146,194, 255 Artesian, 33 Asphalt mat, 147 Asphalt membranes, 147, 148 Atmospheric pressure, 34 Atterberg limits, 10,11, 18,20, 112, 176 Attractive force, 1 3 , 1 4 , 4 3 Australia, 1 , 3 , 3 3 , 3 5 Β

Backfill, 100,101, 110, 122, 127, 146,149,170, 193,194, 206,219,235 Base exchange, 11, 171 Base exchange capacity, 17, 171 Basement, 5 , 9 , 6 2 , 121, 124, 125, 149, 184, 246 deep, 65, 66, 170 slab, 136, 244 wall, 90,99, 100,104,106, 134 Bearing capacity, 72, 73, 106, 165 Bearpaw shale, 2 Belled pier advantage of, 83 cleaning of, 84 construction of, 84 disadvantage of, 83 isolated-shaft, 86 shaft of, 84 system, 83, 84 Bench mark, 6 1 , 190, 207 Bentonite, 19,21,271 Black cotton, 5, 146 Blow count, 65, 72 Bond,86, 87 Box construction, 104,194, 243 Brick wall, 90,187,251 Building Research Advisory Board, 111, 122 C Caisson, 64, 71 Canada, 1, 3 Canal, 151,163 Capillary force, 33, 34

Capillary (cont.) fringe, 33 moisture movement, 4 1 , 151,152,154 potential, 24 rise, 149, 159,247 tube, 33 Catalytically-blown asphalt, 147 Cation, 10, 13, 176 Cation exchange, 10, 11, 16 Cation exchange capacity, 16 Cement, 175 Cement stabilization, 175,176,177, 272 Chemical analysis, 16, 17 Chemical injection, 159 Chemical stabilization, 11,175, 255 Cinder block wall, 90, 104, 131, 246 Clay fissured, 77, 149, 151 impervious, 150 mineral, 9, 10, 11, 16, 175 partially saturated, 162 shale, 26, 39, 77 size, 19, 24 stiff, 3 3 , 1 1 3 , 1 6 5 , 172,255 structure, 10,11 type of, 20 Claystone shale, 6, 44, 63, 93, 98, 195, 228, 255 Climate condition, 3 5 , 4 0 , 1 1 1 , 160 Climate rating, 111,112,113,120 Cohesion, 73, 86 Colorado, 6, 62, 64,111,183 Colorado School of Mines, 1 Colorado State Highway Department, 24, 168 Colloid content, 18,19, 20, 2 1 , 22, 228 Colloidal clay, 17 Compaction, 149, 206 control, 159, 163, 165,169,232 degree of, 3 9 , 5 6 , 1 6 9 effort, 149 method of, 27,149,163 Compactor, 150 Cone penetration, 88 Consolidation test, 67, 72 Consolidometer, 26, 27, 29, 4 3 , 4 5 , 67, 68 cantilever, 26,68 conbel, 26 fixed-ring, 38 one-dimensional, 26 platform, 2 6 , 6 7 , 6 8 ring, 45 Control joint, 122

276

Court yard, 156,207 Covered area, 33, 34, 35, 37,145, 148, 154, 159 Crack pattern, 185, 186 Cracked building, 9 1 , 109,190 Crawl space, 121, 125, 185, 190, 199,207,213,214 214 Curing, 8, 124 Curing time, 27, 39,40 Curling, 8, 124

D Darcy's law, 35 Dead load, 27, 28, 29, 45,47, 52, 81, 83, 92, 93, 9 5 , 1 0 3 , 1 0 6 , 1 0 9 , 184,194, 207, 233 Deep plow, 173 Degree of expansion, 19, 21, 28 Degree of saturation, 47, 4 8 , 4 9 , 56, 165 Denver, 28,44, 73,84, 113, 126, 168,176 Denver Blue Shale, 7 1 , 73 Denver formation, 1 Depth of desiccation, 35, 36,40 Depth of penetration, 74 Desiccate, 28, 29, 33, 173 Design criteria, 66 Differential drying, 155 heaving, 103, 104 movement, 104 settlement, 4 1 , 42, 124 thermal analysis, 16,17 Direct measurement, 16, 20, 26, 27, 35 Discontinuity of structure, 104 Distress study, 185 Doorframe, 131,135 Double layer structure, 13, 14 Double oedometer method, 41 Dowel bar, 95,129,191 Drainage, 4 1 , 192,193,207,229 Drain tile, 49 Drilled pier, 71, 74, 75, 86, 273 Drilling, 63 auger, 63, 66 percussion, 64 rotary, 64, 66 Dry density, 13, 21, 28, 160, 162, 189 initial, 39, 40, 55, 165 maximum, 47, 78 Drying and shrinkage, 160, 189 Drying and wetting, 29 Dye adsorption, 16, 17 Ε

Earth pressure, 8, 101, 104

INDEX

Ecca shale, 1 Effective stress, 42 Electron microscope resolution, 16, 17, 18 Electrostatic force, 13 End bearing capacity, 72, 74, 78, 195 Environmental condition, 33, 35,40, 4 1 , 44, 60 Evaporation, 34 Evapotranspiration, 3 Excavation, 33, 169 Existing structure, 63, 72 Expansion joint, 8, 89, 127, 207, 237, 257, 260 Expansive soils, 1, 3, 8, 11, 17, 27, 71 damage caused by, 8 distribution of, 3 nature of, 1 origin of, 1 physical properties of, 27 recognition of, 16 structure of, 33 F

Factor of safety, 73, 74, 83, 193, 25η Fatigue of swelling, 29 Federal Housing Administration, 22, 24, 101, 113,122 Fill, 110, 165, 167, 169,227 Fissures, 151, 152, 162, 255, 256 Floating slab, 127, 170 Flood plain, 62 Flooding, 110,159, 160, 161, 185 Floor level, 61, 66 Floor load, 124 Floor slab, 27, 89,124, 129, 159, 222, 252 Flower bed, 155, 182, 193, 194, 219, 246 Fly ash, 175 Footings, 27,45, 50, 52, 158, 183, 221 continuous, 103, 243 foundation, 103, 173 individual, 29,221 interrupted, 109, 248 pad, 29, 85, 86, 104, 221 wall, 103, 109 Foundation deep, 64 information, 183 mat, 111 movement, 46, 104, 166, 183, 187,190, 259 plan, 184 shallow, 64, 66 system, 63, 64, 93, 190, 271 type, 29, 61, 183 wall, 29, 100, 105, 245, 246, 252 Foundation soil, 135, 145, 150, 157, 173, 271 Free draining gravel, 124 Free swell, 18,19,44 Free water, 34, 151, 246, 256, 272 Furnace duct, 135, 142,271

INDEX

G

277

Garage slab, 135,141 Grade beam and pier system, 75, 76, 89, 218, 256 Gaade Grade beams, 89, 94, 96, 99,107, 125,127, 129, 136, 190, 192,251 Granite, 108 Granular soils, 110, 165, 167, 170 Gravitational migration, 37 Gravitational potential, 24 Gravity, 33 Gravity flow, 151, 152,154 Ground water, 40, 63, 84, 89, 101 rise of, 101, 122,124, 151, 170, 192, 258

Landslide, 62, 63, 151 Laramie formation, 1 Lateral pressure, 101, 127, 128 Lateral resistance, 85 Lateral wall movement, 101 Lawn watering, 35, 122, 148, 184, 194, 258, 272 Lightly loaded structure, 28,44, 89, 93, 192 Lime, 170, 171, 174 Lime stabilization, 171, 173, 174 Lime-treated subgrade, 171 Lime slurry, 173 Linear shrinkage, 18, 19 Liquid limit, 10, 28, 44, 77, 161, 168,175, 227 Load bearing walls, 135 Load test, 72, 73

H

M

Heaving differential, 163, 175, 187 movement, 106 potential, 26, 169 Highway, 8, 37, 171, 175 Honeycomb floor system, 125, 126, 127 Horizontal swelling pressure, 9 Humidity, 27, 35 Hydrometer analysis, 228 I I-beam, 187, 188,218 Illite, 9, 10, 13,21,27 Impervious, 110, 150, 152,256 Index property, 16,19, 28 India, 1,3,5 Indirect measurement, 22 Initial moisture content, 27, 28, 29, 38, 39,40, 49, 50, 56 Intercepting ditch, 151, 152 Intercepting drains, 151 Ion, 9, 13,26, 171 Irrigation ditch, 62, 151 Isolation of pier uplift, 85 Isomorphism, 17 Israel, 1 , 3 , 5 , 104 J

J-void, 125, 126 Κ Kaolinite, 9, 10, 1 1 , 2 1 , 2 7 , 4 4 L Laboratory testing, 66, 189

Masonry construction, 90,104 Masonry wall, 135, 136 Mat foundation, 111 behavior, 113 design of, 111, 113, 115, 116, 117 Matrix suction, 24, 26 Membrane, 145 asphalt, 147, 148 plastic, 146, 273 polyethylene, 145, 148, 149 Menard pressurementer, 72, 73, 74, 88 Meteorological, 61 Method of compaction, 40 Mexico, 3, 5 Mexico City, 5 Mineralogical composition, 9, 17, 18, 33, 39 Mineralogical identification, 16 Model pier test, 76, 77, 82 Moisture content, 21, 27, 34, 35, 111, 202 deficient, 34, 149, 154, 158, 163 distribution, 35, 146, 160 equilibria, 34 fluctuation, 35, 37, 146 migration, 3 3 , 3 4 , 4 1 , 4 9 , 122, 145, 148, 152, 162,236 movement, profile, 35, 160 transfer, 33, 37, 40 Moisture barrier, 174 horizontal, 145, 149 vertical, 145, 148, 149, 150 Montana, 2, 8 Montmorillonite, 1 , 3 , 6 , 9 , 13, 17, 1 8 , 2 7 , 4 4 , 171,172,227 Mud jack, 255 Mushroom, 92, 95, 96, 190, 194, 206, 218, 257

278

Ν

Negative moment, 111 Negative pore pressure, 25, 26 Normal stress, 42 Ο Optimum moisture content, 19, 38,44, 50, 77,150, 164 ,225 Organic compound, 176 Osmosis pressure, 13, 14, 16 Osmotic consolidometer, 26 Osmotic potential, 24 Overburden pressure, 3 4 , 4 1 , 1 1 2 Overburden soil, 53, 62, 73 Ρ

Pad, 29, 85, 93, 98,104, 106, 260 deep, 108 foundation system, 104,107,189 Partition wall, 129,132, 136,127, 252 Patio, 129,135, 187, 190,218 Pavement, 2 9 , 3 4 , 3 5 , 37,147,162 Penetration resistance, 2 8 , 6 5 , 72, 74, 88 Penetration test, 64, 65, 66 Perched water, 83, 122, 124,151,152, 192, 193,256,258,272 Peripheral drain, 152, 155 Permeability, 35, 38, 63, 176 Physiography, 61 Pierre formation, 1, 2, 63 Piers, 29, 3 7 , 4 4 , 4 7 , 1 9 5 anchorage, 29,47, 84, 93,210, 259 bearing capacity of, 71,74, 85, 85, 89, 98 belled, 71, 83 design capacity of, 74 diameter of, 94 drilled, 7 1 , 89,104,184 failure of, 89 foundation, 45, 125 friction, 66, 7 1 , 73, 77, 86, 88 length of, 92, 93,184, 191,205 reinforcement, 92, 95, 184, 210 settlement, 7 2 , 9 2 , 9 8 , 205, 208 shaft, 47, 73, 77, 85,99 size, 92,182 spacing, 92 straight-shaft, 71, 74, 84, 85 system, 7 3 , 1 0 1 , 189,208 uplift, 62, 75, 8 3 , 9 3 , 202, 254, 257 Placement condition, 3 9 , 4 4 , 1 6 4 Plastic limit, 34, 77,146 Plastic membranes, 34 Plasticity Index, 1 0 , 1 8 , 2 1 , 2 2 , 2 3 , 4 4 , 78, 111, 112,159,171,175,227

INDEX

Plumbing, 157 Poisson's equation, 13 Poisson-Boltzmann equation, 13 Ponding, 155,156,159, 160,161,163, 272, 273 Pore pressure, 34, 35,42 Portland cement, 175 Portland Cement Association, 121,122, 125 Post-tension, 118,194, 246, 247, 249, 250 Potential volume change, 22,25 Precipitation, 3, 10, 33,34, 35, 37, 111, 152, 185,258 Pressure injection, 173 Pressure release, 33 Prewetting, 5 2 , 1 5 9 , 1 6 2 , 1 6 3 , 165,190 Proctor density, 44, 56, 6 0 , 1 1 0 , 1 5 0 , 1 6 9 , 206, 221 Proprietary fluid, 176 Puddling, 149,150,192 PVC meter method, 1 6 , 2 1 , 2 4 , 1 1 2 PVC rating, 21 Q

Quartz, 44 R Raised floor system, 125,126 Rational design, 47, 85 Rational pier formula, 82, 88 Reinforced brickwork, 104 Remedial measures, 183, 193 Remolded sample, 21, 28, 38, 39, 44, 55, 56 Repulsive force, 13,43 Residential houses, 4 4 , 1 1 1 , 121,136,193 Resistivity, 63 Rocky Mountain Area, 28, 38, 43, 7 1 , 73, 74, 92, 111 Roof downspout, 122, 156, 194, 238, 246, 272 Roof drain, 35, 156 S

Sampler California, 66 Shelby tube, 66 split spoon, 65 Samples core, 66, 72 disturbe,d, 68 representative, 68 undisturbed (see Undisturbed) Sample thickness, 40 Sampling, 63, 66 Sampling method, 27 Sandstone, 64, 108,255

279

INDEX

Saturation, 27 complete, 47, 52 partial, 34,43 Seasonal cycle, 86,189 Seasonal moisture change, 148, 149 Seepage, 3 3 , 6 2 , 1 5 1 , 1 8 4 , 1 8 5 Seismic survey, 61,63 Selected fill, 109,110,169 Settlement, 3 7 , 4 1 , 72 Sewer line, 155, 157, 185,232 Sewer pipe, 77, 122, 155 Shale bedrock, 33, 64, 74, 82 Shear failure, 203, 204, 207 Shear ring, 73 Shear strength, 76, 88, 175 Shear strength reduction, 88 Shear stress, 26 Shearing resistance, 83, 86 Sheet rock 131, 134 Shrinkage, 3 7 , 1 5 5 , 1 8 9 crack, 33, 121, 124 limit, 1 9 , 2 0 , 2 1 , 2 2 , 4 4 , 1 7 1 , 175,228 mechanics of, 37 test, 38 Side shear, 73 Sidewalk, 3 7 , 1 2 1 , 129, 131,134, 146,143,207 Silt, 28, 34 Siltstone, 63, 64, 131,146 Site investigation, 61 Skin friction, 73, 74, 77, 83, 86,195, 257 Slabs exterior, 129 floating, 127 interior floor, 121, 128 movement, 49, 90, 111, 121,122, 128,223 patio, 129,132, 134 prestressed, 125 reinforced, 121 structural, 121, 125,170,273 Slab-bearing partition wall, 135,136, 171, 189,194, 222,232,260 Slab-on-ground, 1,44, 4 9 , 6 6 , 121,122, 127, 129, 160,170, 199 Slip joint, 127, 129,130,132, 134,136,170, 194, 233,252,260 Slope stability, 151 Soil engineers, 26, 34, 37, 52, 55, 6 1 , 72, 73,169, 175,191,193 Soil lattice, 11, 17 Soil profile, 40 Soil replacement, 110, 166, 169 depth of, 167,168 extent of, 169,170 Soil stabilization, 159 Soil suction, 16, 2 1 , 24, 26, 34, 35 Soil test, 184 Soil-water equilibria, 39

Soil-water system, 14, 37,173 Sonotube, 8 6 , 9 4 , 1 2 6 , 1 2 7 South Africa, 1, 3, 6, 33, 104, 149,161 Spain, 3, 6 Specific gravity, 44 Specific surface, 11 Split level, 104, 111,213 Sprinkling system, 155, 193 Staircase wall, 131, 134, 140, 255 Stiffened slab, 111, 112, 113,115,120, 125 Stratum thickness, 5, 165 Stress history, 27 Stress release, 26 Structure engineer, 189 Structural fill, 234 Stud wall, 132, 135 Subdrain, 49, 145, 153,154,192, 194, 272 Subdrainage system, 124,152, 156,184,194, 213, 247,260, 272 Subsoil condition, 64 Suction test, 26, 29 Sump pump, 125, 152,194, 213, 244 Support index, 111, 112, 113, 114,120 Surcharge load, 27, 46,163 Surcharge pressure, 2 1 , 3 8 , 4 0 , 4 5 , 4 6 , 1 6 5 Surface drainage, 4 1 , 145,146, 151, 155,192,247, 272 Surface geology, 61 Surface water, 33, 77, 82, 101, 122,151,152,197, 218,229 Surficial geology, 62 Swell index, 2 2 , 2 5 , 3 8 , 1 1 2 percent of, 26, 112 test, 39,67, 112,168,200 total, 168 Swelling characteristic, 18, 38, 29,43 definition of, 38 mechanics of, 18,43, 82 potential, 2, 16, 18, 19, 20, 24, 26, 27, 28, 38, 64,171,199,225,229 pressure, 6, 14, 26, 27, 39, 4 0 , 4 3 , 4 5 , 4 6 , 4 7 , 50, 52, 55, 56, 60, 75, 82,94, 106,110, 126,165,199,203,225,229,255 soil, 9, 64,145 Τ

Tensile stress, 94, 95 Tension crack, 8 4 , 9 5 , 9 7 , 1 9 0 , 203, 221 Temperature crack, 122 Test hole, 64, 72,73 Test pit, 6 1 , 6 3 , 6 4 , 1 9 0 , 245 Texas, 6, 86, 160, 162 Texas A & M, 1,3, 112 Texas Highway Department, 147, 160,171

280

Texture, 17 Thermal gradients, 34,122 Thermal-osmosis, 159 Thermal transfer, 37, 145 Time element, 39 Topographie feature, 33, 6 1 , 62 Topographie survey, 61 Total heave, 40, 41 Transpiration, 34 Triaxial shear strength, 72, 88 U

Ultimate settlement, 41,42 Unconfined compressive strength, 72, 86, 88, 106 Underpin, 194, 232,233 Under-reamed pier, 83 Underslab gravel, 124, 190 Underslab soil, 128, 131, 148, 173, 175 Undisturbed sample, 2 1 , 28, 34, 38, 39, 40, 55, 112 Undrained shear test, 88 United States, 3, 6, 33,111 Unsaturated soils, 24, 35 Unwetted zone, 83,93 Uplift, 44, 88, 168,257 coefficient of, 76, 82 differential, 47 movement, 47, 214, 245 tolerable, 47 Uplifting force, 76, 80, 81, 83, 89, 215, 216 Uplifting pressure, 67, 75, 8 1 , 94, 96, 99,108, 128, 129, 194, 198,203,215,252 U.S. Bureau of Reclamation, 65, 161 U.S.B.R. method, 20, 21 Utility lines, 35, 122, 136, 185,231,273 Utility trench, 149, 185, 192 V

Van der Waals' force, 13

INDEX

Vane shear test, 88 Van't Hoff equation, 14 Vapor barrier, 145 Vapor pressure, 154 Vapor transfer, 34, 37, 41,154 Vegetation, 155 Vermiculite, 9 Verticel,97,98,99,125, 126 Void-ratio, 4 1 , 42 Void-space, 89, 96, 107, 109, 189, 194, 227 Volume change, 2 1 , 28, 38, 3 9 , 4 3 , 4 7 , 48, 50, 6 7 , 1 1 1 , 1 6 1 , 175

W

Waffle slab, 113,125 Walkout door, 104, 218, 243 Wall paneling, 131, 135, 140 Water level, 33,62 Water line, 131,157,259,273 Water main, 77 Water table, 27, 28, 35, 63, 65, 152, 183, 184, 218,228,256,259 Water transfer, 33 Water vapor, 34 Weathered claystone, 214 Withholding force, 75, 83, 89, 217 Wyoming, 6 , 6 3 , 183 X X-ray diffraction, 11,16, 17, 227 Ζ Zone of aeration, 151 Zone of wetting, 77 Zone unaffected by wetting, 83, 84, 85, 86

FU HUA CHEN (Eds.) Foundations on Expansive Soils 1975

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