Foundation Settlement
Settlement is the vertical component of soil deformation deformation beneath the load under unde r consideration. All imposed loads on soils will cause some settlement due to elastic compression of the foundation soils. This settlement occurs relatively relatively rapidly and is termed elastic or immediate settlement .
Settlement is the vertical component of soil deformation deformation beneath the load under unde r consideration. All imposed loads on soils will cause some settlement due to elastic compression of the foundation soils. This settlement occurs relatively relatively rapidly and is termed elastic or immediate settlement .
or those that take take place as the load is applied or within a time period of about 7 days. The water water in the voids is expelled simultaneously with the application of load and as such the immediate and consolidation settlements settlements in such soils are rolled into one. 2. C onsolidation onsolidation , or those that are timedependent and take months to years to develop. The Leaning Tower of Pisa in Italy has been undergoing consolidation settlement for over 700 years. 1. I mmediate mmediate ,
Foundation
settlements must be estimated with great care for buildings, bridges, towers, power plants , and similar high-cost structures. The stress change (q from from this this added added load load produces a time-dependent accumulation of particle rolling, rolling, sliding, crushing, and elastic distortions in a limited influence zone beneath the loaded area. The statistical accumulation of movements in the direction of interest is the settlement. In the vertical direction the settlement will be defined as (H.
Many
engineers seemed to have the misconception that any footing designed with an adequate factor of safety against a bearing capacity failure would not settle excessively. Independent settlement analyses also need to be performed
Settlement frequently controls controls the design of spread footings, especially when B is large, and that the bearing capacity analysis is, in fact, often secondary.
In saturated silts and clays, particularly those which are normally consolidated, the settlement will be dominated by consolidation, as water slowly drains from these soils to reduce the pore water pressures to the original levels.
Settlement of cohesionless soil primarily occur from the re-arrangement of soil particles due to the immediate compression from the applied load
To enable settlements to be calculated we have to calculate the change in stresses within a soil mass, due to imposed external loads on the soil.
Elastic stress distributions within the soil are usually based on the theory of Boussinesq and so methods of computing elastic settlements usually assume that Boussinesq theory is applicable.
Causes of Settlement i. Static loads, such as those, imposed by the weight of a structure or an embankement. ii. Dynamic or transient loads, such as those produced by machinery or moving loads on roads or airfield pavements, pile driving, blasting, etc iii. Changes in moisture content, for example from seasonal fluctuations in the water table
iii
Rainfall, and evaporation or the absorption of the water by the rots of larger trees.
iv
the effects of nearby construction(e.g. excavation, pile driving, subsidence of mines and dewatering) may also be significant.
v
Ground movement on earth slopes, e.g. surface erosion, landslide or slow creep.
Components of Settlement I mmediate (or undrained) settlement , which
occurs immediately upon application of the load, and which in a saturated soil arises from shear deformations under constant volume conditions. ( without change of water content) C onsolidation settlement , which occurs primarily because of the dissipation of excess pore pressures in the soil and is therefore time dependent. This component of settlement arises mainly from volumetric deformation although shear deformations are also involved.
C reep settlement(frequently termed
secondary consolidation) which most
frequently manifest itself as a time dependent settlement after the completion of excess pore pressure dissipation, however, significant creep settlements can also occur undrained conditions. Creep settlements generally involve both shear and volumetric deformations. (only for clay)
Total Settlement or Final Settlemnt ST F = Si + ScF + SSF
Where ST F = Total final settlement Si = Immediate settlement ScF = Final consolidation settlement SsF = secondary consolidation
Or VF = Vi + Vc + Vs
In case of foundation on medium dense to dense sands and gravels , the immediate and consolidation settlements are of relatively small order and take place almost simultaneously and a high portion of settlement is almost completed by the time the full loading comes on the foundations.(High permeability) Similar in the case of loose sands, where as the settlements on the compression clays are partly immediate and partly long term movements. The later takes long time(period of years) and is of greater proportion. (low permeability)
Settlement of foundation are not necessarily confined to very large and heavy structures. In soft clays and silts appreciable settlements can occur under light loadings. (may be in two storey building cracks can occur or are observed). Differential or relative settlements are of greater importance to the stability of the structure.
If a uniform settlements occur under the whole area of foundation, it may not be dangerous, but if differential settlement takes place, the stresses will develop, serious cracks or even collapse of the structure will occur if differential settlements are excessive. Skempton and McDonald have divided damages resulting from settlements into three categories.
ELASTIC SETTLEMENT B ENEATH THE
CORNER OF A UNIFORMLY LOADED FLEXIBLE AREA BASED ON THE THEORY OF ELASTICITY
The net elastic settlement equation for a flexible surface footing may be written as, S e
!
1 q B n
Where S e B
E s
!
!
2
I
f
elastic settlement width of foundation
E s
!
modulus of elasticity of soil
!
Poissn' s ratio ,
qn
!
net foundation pressure,
I f
!
influence factor
Evaluation of Undrained
modulus of Deformation of Elasticity
Eu = 500 Su (soft sensitive clay Nc) 1000 Su (firm to stiff clay OCR< 2) 1500 Su (very stiff clay OCR> 2) Approximately. Si = 0.1Sc for N.C Si = 0.5Sc for O.C
Settlement of Saturated
Clays(NC)
Sdc = Qg Sc
Sdc = corrected consolidation
settlement Qg = correction factor for geological
conditions Sc = settlement calculated from
consolidation.
Sc = mv x ( W z x H Where mv = average coefficient of volume compressibility obtained from the effective pressure increment in the particular layer under consideration. ( W z =average effective vertical stress imposed on the particular layer resulting from the net foundation pressure qn H = Thickness of the particular layer under consideration.
OR
H
S c
!
H
!
e1
!
1 e1
( e1
e 2 )
As defined above initial void ratio , corres pond ing to initial
overburden pressure pod at the center of the layer
read from e e2
!
log p curve
final void ratio corres pond ing to a pressure
pod z .
OR S c
H !
1 e1
C c log 10
pod z pod
Where pod Initial effective overburden pressure . !
C c
!
compression index
slope of virgin
compression curve e
logp.
Calculation of Cc (empirical eqs.) C c
!
C c
!
C c
!
C c
!
0 .007 L . L 10%
for N .C .C
0 .009 L . L 10%
for remolded clay
1 .15e o
0 .35
for all clays
0 .50
soils with low
0 .75e 0
plasticity
C c
!
0 .141G s
1 .2
¨ 1 eo ¸ © ¹ ª G s º
C c = 0.0115 w N
2 .38
for all clays
Consolidation Settlement S c S c S c
!
!
!
C c H c 1 eo C s H c 1 eo C s H c
po log
pav
po log
pav
pc
log 1 eo po
for N .C .C
po
for O .C .C with po
po
C c H c 1 eo
for O .C .C with po or po
po log
pav "
pc
po
pc
pav
pc "
pav
po
pav
pc
Where po = Average effective pressure on the clay layer before the construction of the foundation. (pav = Average increase of pressure on the clay layer caused by the foundation construction. pc = pre-consolidation pressure. eo = initial void ratio of the clay. Cc = compression index. Cs = swelling index Hc = thickness of clay layer.
(pav
= 1/6 ((pt + 4(pm + (pb)
Where (pt ,(pm and (pb are the pressure increases at the top ,middle and bottom of the clay layer caused by the foundation construction.
Settlement of Cohesioless soil Settlement occurs immediate @
Total settlement = immediate settlement
Cone Penetration Test (CPT) S !
H C
2 .3 l og 10
pod z
W ere c
!
co sta t of com ressi ility
c
!
1 .5
qc
!
qc p od
static co e
e etratio
p od! Effective over
r e
of meas reme t, k a qc
!
p od
to 2
B
resista ce , k a ress re at
oi t
Standard Penetration Test for Shallow
Foundations sat rated sands and gravel S ! 0 .96 ere q
n
N
! N et
N !
B
qn fo
q B } n N press re k P a
ndation
verage corrected SPT bl o s t e seat of sett le ment .
For si l ty sands q S ! 2
n
B
N If D " 4 B
( Dee p Fo
S ! 1 2
qn
ndation s )
B
N
it i n
C c C c
!
!
eo
e
! 1d
l og 10 od
e l og 10
com pressi on i ndex re presents t e sl o pe of t e li near portion of t e press re void ratio c rve , and remai ns cons tan t for fair l y l arg e ran ge of press re .
C oeff i ci ent of com pressi bili ty ! av ! e !
C oeff i ci ent of volume c an ge ! m 3
e
! d
v
o d
e
!
!
1
v
1 eo
av , mv
e
eo
d
av 1 eo
When the soil is laterally confined, the change in the volume is proportional to change in thickness (H and the initial volume is proportional to initial thickness Ho , Hence
H 1 mv ! . H o d @
H ! mv v H o
v d
Compressibility of Various Types of Clays Type
Qualitative Descripti on
Coeff icient of volume compressibility, mv - m2 /MN
Heavily over consolidated boulder clay
Very low compressibility Below 0.05
Normally consolidated alluvial clays
High compressibility
0.3- 1.5
Very organic alluvial clays and peats
Very high compressibility
Above 1.5
Estimation of Rate of Consolidation May
be required to know the rate of settlement of foundation during the long process of consolidation. This is normally calculated as the time period required for 50% or 90 % of the final settlement. The time required is given by 2
t
!
T v d cv
Or e
resse
i m/years 2
t years !
T v d
v 10
3 .154 v c v
its
7
m
2
/ s
T v =Time factor(Theoretical time factor, a pure
number that has been determined for all conditions of importance and is given in terms of u d = H (Thickness of compressible stratum
measured from foundation level for point which W z is small say 10 to 20 kN/m2 for
Drainage in one direction. Or d = H/2 for drainage at top and bottom of clay stratum. C v = Average coefficient of consolidation over
the range of pressure involved.
cv U
U "
or
k
mv
or
1 e
k
w
av
o
v w
2
60 %
¨ U ¸ T v ! / 4© ¹ ª 100 º
60 %
U ¸ ¨ T v ! 0 .9332 l og 10 © 1 ¹ 0 .851 ª 100 º
T v
!
1 .781 0 .933 l og 10 100 U %
Estimation of Final Settlement Vf = Vi Voed
+ Vc
B
= mv x Wz xH
qn
= mv x 0.55q x1.5 B + immediate settlement
1.5 B
Average
pressure in the center of layer = 0.55 qn
0.1qn
1. Structural damages which involves only frame, i.e.
stanchions and beams. 2.
Architectural damage involving only the panel walls, floors
or finishes. 1. Visual appearance 2. Serviceability or function 3. Stability 3.
C ombined structural and architectural damage .
A study has shown that structural damage is likely to take place when the angular distortion( (/L) of the span(l) between adjacent column or along a given length of load bearing walls exceeds 1/150 and that architectural. Damage is likely to occur when the angular distortion exceeds 1/300
Differential settlement
Total settlement (
l
l
(/l =
angular distortion
Influence of structural rigidity on differential settlement(a) very flexible structure has little load transfer, and thus could have larger differential settlements; (b) a more rigid structure has greater capacity for load transfer, and thus provides more resistance to excessive differential settlement
Skempton and MacDonald(1956)
Soviet Code of Practice(1955)
Bjerrum [27] recommended the following limiting angular distortion ($max) for various structures
Grant et al.[28] correlated ST( max) and $max for sev er al bui ld in gs w ith the following results.
TABLE 5.20
Recommendation of European Committee for Standardization on Diff erential Settlement Parameters
Table 9.1 Tolerable
diff erential settlement of buildings, in inches, recommended maximum values in parentheses l
l
s
S max H
(Uniform settlement)
S min
S min
(Tilt) S max
(Nonuniform settlement)
(
s = smax- smin = diff. settlement
Angular distortion =
s
l
!
l
Causes of diff erential settlements 1. V ariation in soil strata one part of structure may be founded on a compressible soil and the other part on incompressible material. Like (i) glacial deposits. Lenses of clay in sandy materials. (ii) Irregular bed rock surface (good rock, weathered compressible rock) (iii) Wind laid or water laid deposits of sands and gravels varying in density.
2.
V ariation
in foundation loading:
Some parts heavy load and other light. For example, (i) Building consists of high central tower, low projecting wings, (ii) factory- heavy and light items of machinery. 3. Large loaded areas on flexible foundations.
(i) Large flexible raft foundation
Requires rigid raf t Dense Gravel
Compressible soil Diff erential
settlement
Bowl shape
4.
Difference in time of construction of
adjacent parts of structure .
This is the case when extension of a structure is to be done after many years.(then the completion of original). Long term consolidation settlement of built structure may be complete, but the new structure(if of the same foundation loading as the original) will eventually settle an equal amount. Special provisions in the form of vertical joint are needed to prevent distortion and cracking.
5. V ariation
in site conditions (History)
one part of building area may be occupied by heavy structure which had been demolished or on sloping site it may be necessary to remove considerable thickness of overburden to form a level site. This variation results in different stress conditions.
Following
are the major causes of settlement:
(1) Changes in stress due to: a. Applied structural load or excavations. b.
Movement
of ground water table.
c. Glaciation; and d. Vibration due to machines and earthquake etc
(2)
Desication due to surface drying and/or plant life.
(3)
Changes due to structure of soil (secondary compression)
(4)
Adjacent excavation
(5)
Mining
(6)
Swelling and Shrinkage
(7)
Lateral expulsion of soils
(8)
Land slides.
subsidence
Compression of foundations soils under static loads. Compression of soft clays due to lowering ground water table. Compression of cohesionless soils due to vibrations Compression of foundation soils due to wetting. Shrinkage of cohesive soils caused by drying Loss of foundation support due to erosion. Loss of foundation support due to excavation of adjacent ground
Loss of support due to formation of sink holes
Loss of support due to thawing of permafrost
Loss of support due to partial or complete liquefaction.
Down drag on piles driven through soft clay.
Methods of Preventing Excessive Diff erential Settlement
Remedial Measures Philosophy of remedial measures is to (a) reduce or eliminate settlement (b) design structures to withstand the settlement. (a) Reduction of Settlement
To reduce or eliminate settlement, consider following: 1. Reduce the contact pressure. 2. Reduce compressibility of the soil deposits using various ground improvement techniques(stabilization, precompression, vibroflotation etc.)
(3) (4)
(5) (6)
Remove soft compressible material such as peat, muck etc Build slowly on cohesive soils to avoid lateral expulsion of a soil mass, and to give time for pore pressure dissipation. Consider using deep foundations (piles and piers) Provide lateral restraint or counterweight against lateral expulsion.
