Classification of Soil i

SOIL MECHANICS (CE –40)

NOTE-2 (BY MOYUKH DE)

INRODUCTION:

In present day soil engineering practice as applied to various fields of engineering such as highways, airports, dams, foundations, and the like, there are several different methods of soil classification in use. Most of them attempt to classify soils into groups on the basis of their engineering properties; that is, the manner in which they will perform as part of an engineering structure under a given set of conditions. The purpose of this is largely to aid the design engineer in deciding how the soil is apt to behave when it is put into one of these structures or a structure is placed on top of it. The procedures in classifying soils involve a number of tests which are mostly made in the Laboratory. The first objective should be to identify and describe soils in such a way that others will understand exactly what is meant; that is, if a silt loam is mentioned, that name should convey essentially the same meaning to each person. Other terms may elaborate on it and describe it more fully. The complete description would then convey a very definite picture of that particular soil. The following sections are devoted to classifying and describing soil materials in detail. An understanding of the classification procedures will aid the engineer and inspector in the field in identifying soils, in evaluating the engineering properties, in applying better soil selection and in performing the field test more accurately.

CLASSIFICATIO CLASSIFICATION CLASSIFICATION N OF SOIL

TEXTURAL CLASSIFICATI CLASSIFICATION ON

UNIFIED SOIL CLASSIFICAT CLASSIFICATION ION IS CLASSIFICATION SYSTEM

PARTICLE SIZE CLASSIFICATION

HIGHWAY RESEARCH BOARD


Particle size classification is based preliminary on the particle size of the soil grains .These sizes are basically a simple designation of particle size only so do not signify the naturally occurring soil types, because they are a mixture of particles of different sizes & exhibit definite characteristics. The Indian classification classification based on

0.002 Clay

0.075 silt

fine

0.425

2

med.

Sand

particle size classification classification iis follows: s as follows:follows:-

4.75 Coarse

20 fine

80 coarse

300 cobble co bble

bolder

gravel

IS, CLASSIFICATION (IS: 1498-1970) (all dimensions are in mm)

TEXTURAL CLASSIFICATION: Naturally occurring soil consists of particles of different sizes .Now, some composite soil properties are based on particle size distribution (specifically coarse grained soils). So, a soil classification of composite soil exclusively based on the particle size distribut distribution ion is known as textural classification. Clay soil properties are less dependent on particle size distribution so this system cannot be effectively used in their classification a thee way key is constructed with the help of a triangle where every side represents the % age of 3 different soil constitutes which are as follows   

sand(2—0.05mm) clay(< 0.0 05mm) silt (0.05-0.005 (0.05-0.005mm) mm)


TEXTURAL CLASSIFICATION CHART (ADAPTED FROM U.S. PUBLIC ROADS ADMINISTRATION)



ORGANIC SOILS: Soils containing greater than 5 percent organic material by weight are defined as organic soils. Organic soils include plant material in various stages of decay from a condition where the stem and leaf structures can still be detected to a state where the plant tissue has lost its identity and an indefinite mass of organic material exists. Organic soils can be classified as follows: Organic Content Classification by weight % Non-organic <2 slightly organic* 2-5 organic* 6 - 10 highly organic* 11 - 25 Peat-woody, fibrous, decomposed, etc. >25 *Insert specific soil type, e.g. slightly organic, Silt Loam, Peaty loam, etc.

SOME CLASSIFICATIONS BASED ON THE PARTICLE SIZE CLASSIFICATION SYSTEM:

INTERNATIONAL CLASSIFICATION

ULTRA CLAY (colloids)

F

C

F

CLAY

C SILT

F

C MO

F

M

C

V.C.

SAND

GRAVEL

(MAJLA)

U.S. BUREAU OF SOILS AND PRA CLASSIFICATION

CLAY

SILT

(SIZE)

(SIZE)

V.F.

FINE

MEDIUM SAND

COARSE

FINE GRAVEL

GRAVEL


HIGHWAY RESEARCH BOARD CLASSIFICATION This classification is done by a parameter defined as group index (G.I.) . the group index can be defined as follows:Group index depend upon:1 Material pass 75m is sieve 2 Liquid limit 3 Plasticity index Lower the G.I. better is the soil Group index is used to describe the performance of soils when used for pavement construction.

Group Index =0.2a+0.005ac+0.0 =0.2a+0.005ac+0.01bd 1bd Where a=percentage of soil passing 75m greater (other words finer than 75 m) than 35 but not exceeding 75[entered in whole no. 0-40] b=percentage of soil passing75m greater than 15 not exceeding 55[entered in whole no. 0-40] c=numerical liquid limit of soil greater than 40 not exceeding 60[entered as positive whole number 0-20] d=portion of numerical plastic limit greater than 10 not exceeding 30[entered as positive whole number 020] It should be noted that highway research board classification system is also known as

SOIL CLASSIFICATION SYSTEM

AASHTO


UNIFIED SOIL CLASSIFICATION CENTRE DEFINES SOIL IN BASIC 4 GROUPS: Soil

Coarse grained

Fire grained

Organic

Peat

BASIS OF CLASSIFICATION:Coarse grained soils are classified on basis of their grain size distribution Fine grained soils are classified on basis of two basic Atterberg Limits, i.e. plasticity index & liquid limit. THE FOLLOWING SYMBOLS ARE USED FOR THE CLASSIFICATION ARE AS FOLLOWS:


SL NO.

SOIL TYPE

PREFIX

SUB GROUP

SUFFIX

1

GRAVEL

G

WELL GRADED

W

2

SAND

S

POORLY GRADED

P

3

SILT

M

SILTY

M

4

CLAY

C

CLAYEY

C

5

ORGANIC

O

LL<50%

L

6

PEAT

Pt

LL>50%

H

PLASTICITY CHART:The plasticity chart was first de eloped by Cassagrande’s Cassagrande’s , it is an effective effective tool for the classification of soil ,as it help us to distinguish between the clay & silt particles of the fine gra ined soil on the basis of plasticity index & liquid limit. A figure of cassagrandes plastici ty chart has been given below:60

50

40

30

20

10

0 0

10

20

30

40

50

60

70

80

90

100

If after over drying the liquid li it of the sample sample decreases by 30% or more more it is classified as organic soil. Total 15 groups are considered here.Highly organic & visible fibrous matter cont ined soils are put under group peat. (Swelling soils are also contained in the group).


COARSE GRAINED SOILS ARE CLASSIFIED INTO THE FOLLOWING GROUPS:GW (WELL GRADED GRAVELS) • •

GP (POORLY GRADED GRAVELS)

•

GM (SILTY GRAVELS)

•

GC (CLAYEY GRAVELS)

•

SW (WELL GRADED SANDS)

•

SP (POORLY GRADED SANDS)

•

SM (SILTY SANDS)

•

SC (CLAYEY SANDS)

THE FINE GRAINED SOILS ARE DIVIDED INTO THE FOLLOWING GROUPS:-

W L>50%

W L<50%

Highly organic soil ML

MH Pt

CL

CH OH

OL

NOTE: the process of classification has been provided in a separate word document:

INDIAN STANDARD CLASSIFICATION SYSTEM, ISCS (IS: 1498-1970) It is a revised version of USCS of USCS The procedure of classification is same as that of the USCS classification system: COARSE GRAINED SOILS ARE CLASSIFIED INTO THE FOLLOWING GROUPS:GW (WELL GRADED GRAVELS) • •

GP (POORLY GRADED GRAVELS)

•

GM (SILTY GRAVELS)

•

GC (CLAYEY GRAVELS)

•

SW (WELL GRADED SANDS)

•

SP (POORLY GRADED SANDS)


•

SM (SILTY SANDS)

•

SC (CLAYEY SANDS)

FINE GRAINED SOILS ARE CLASSIFIED INTO THE FOLLOWING G OUPS:FINE GRAINED SOILS SILTS & CLAYS : With low compressibility: li uid limit <35%

M C O M C O M C O Pt

SILTS & CLAYS : With medium compressibili ty: 35%>liquid limit<50% SILTS & CLA S: With high compressibility: li uid limit> 50% HIGH ORGANIC OILS:

Differences are as follows :

Total group 18 ( 15 grou ps of USCS+ 3 extra groups) Fine grained on basis of liquid limit classified as low, medium & high. Plasticity chart consider d for the fine grained fraction classification is as follows:

 

60

50

40

30

20

10

0 0

10

20

30

40

50

60

70

80

90

100


Note: If numerically If numerically PI> (LL – 20)*. 73: that’s means the plot should be above the A-LINE If numerically PI< (LL-20)*.73: that’s means the plot should be below the A-LINE

Classification of Soil i

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