Classification of Soil
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Classification of Soils
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byPranamesh Chakraborty
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Purpose
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Purpose
Arrange various types of soils into groups
according to their engineering or various other
characteristics.
Soil possessing similar characteristics can be
placed in the same group.
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Soil Classification Systems
1. Particle Size Classification
2. Textural Classification
3. Highway Research Board (HRB) Classification
4. Unified Soil Classification System (USCS)
5. Indian Standard Classification System (ISCS)
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Particle Size Classification
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Soils arranged according to grain sizes.
Terms such as gravel, sand, silt and clay are used to
indicate grain sizes.
These terms are used only as designation of particle
sizes.
They do not signify naturally occurring soil types.
Naturally occurring soil are mixture of particles of
different sizes.
Introduction
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IS Classification of Grain Size
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Textural Classification
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Soil classification of composite soils exclusively
based on Particle Size Distribution.
Most popular is the Triangular Classification of U.S.
Public Roads Administration.
Classification based on % of sand, silt and clay size.
More suitable for describing coarse grained soil.
Clay soil properties are less dependent on particle
size distribution.
Introduction
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Textural Classification Chart
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Example
30% sand30% silt40% clay
Soil TypeClay
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Highway Research Board (HRB)Classification
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Based on both particle-size composition as well as
plasticity characteristics.
Mostly used for pavement construction.
7 primary groups (A-1, A-2,…., A-7).
A-1 subdivided into 2 subgroups.
A-2 subdivided into 4 subgroups.
Introduction
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Used to describe performance of soil when used for
pavement construction.
Not used to place a soil in a particular group.
It means rating the value of soil as a subgrade material
within its own group.
Higher the value of GI, poorer is the quality of material.
Group Index (GI)
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Group Index of a soil depends on:
1. Amount of material passing 75 micron IS sieve.
2. Liquid Limit.
3. Plastic Limit.
Group Index (GI) determination
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HRB Classification Table
For A-7-5, IP<= wL-30 For A-7-6, IP > wL-30 16Pranamesh Chakraborty
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HRB Classification Procedure
Proceed from left to right on the chart.
Correct group will be found by process of elimination.
The first group from left into which the test data will fit will be the correct
classification.
Group Index should be rounded off to nearest integer and placed in parenthesis,
such as, A-2-2 (6).17Pranamesh Chakraborty
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Example
56% passes 75 micron sieve.
Plastic Limit (wP) =23%
Liquid Limit (wL)=36%
Plasticity Index (IP) = 36-23=13%
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Example
Passing 75 micron>35% IP < 40% wL>10%
A-6
wL=36%
IP =13%
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Example
Group Index Calculation
a= 56-35=21
b=56-15=41, Take 40
56% passes 75 micron sieve.
Plastic Limit (wP) =23%
Liquid Limit (wL)=36%
Plasticity Index (IP) =13%
c=0
d=13-10=3
GI=5.4=5 A-6 (5)20Pranamesh Chakraborty
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Unified Soil Classification SystemUSCS
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Origin of USCS
First developed by Professor A. Casagrande(1948) for the purpose of airfield constructionduring World War II.
Afterwards, it was modified to enable thesystem to be applicable to dams, foundations,and other construction .
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Four Major Divisions
Course Grained
Fine Grained
Organic Soil
Peat
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Classification Groups
•The soil is classified into 15 groups.
•Each group is designated a symbol consisting of two capital letters.
•The first letter is based on main soil type.
•The second letter is based on gradation and plasticity .
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Classification Group Symbols
Main Soil Type
Prefix Subgroup SuffixClassification
Group symbols
Gravel G
Well-gradedPoorly-graded
SiltyClayey
WPMC
GWGPGMGC
Sand S
Well-gradedPoorly-graded
SiltyClayey
WPMC
SWSPSMSC
Silt MLL < 50%LL > 50%
LH
MLMH
Clay CLL < 50%LL > 50%
LH
CLCH
Organic OLL < 50%LL > 50%
LH
OLOH
Peat Pt Pt25Pranamesh Chakraborty
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Unified Soil Classification System (Contd.)
Coarse-grained soils: Fine-grained soils:
NO.200
0.075 mm
Well or Poor Graded based on Cu and Cc
50 %
NO. 4
4.75 mm
50%
wL >50
wL <50
Gravel (G) Sand (S) Silt (M) Clay (C)
Based on wL and IP
(Plasticity Chart)
For G, use W if Cu > 4 and 1<Cc <3
For S, use W if Cu > 6 and 1<Cc <3 26Pranamesh Chakraborty
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Plasticity Chart
Below A-line, use M (Silt) or O (Organic)
Above A-line, use C - Clay
High Plasticity use H - wL >50
Low Plasticity use L - wL <50
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Silt (M) Clay (C)Organic (O)
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Plasticity Chart
“The soil’s liquid limit (wL) after oven drying is less than 75 % of its liquid
limit before oven drying.” If the above statement is true, then it is Organic Soil
(OL or OH). Otherwise, it is Inorganic Soil (ML or MH)28Pranamesh Chakraborty
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Plasticity Chart
When IP and wL are in the hatched portion of the plasticity chart, the soil is
given dual symbol (CL-ML).
Soil possessing properties of more than one group are termed as boundary
soil and designated by dual group symbol.29Pranamesh Chakraborty
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USCS at a glance
(Santamarina et al., 2001)
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Group Symbols and Group Names
Group Symbol Typical Name
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.
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Group Symbol Typical Name
CL Inorganic clays of low plasticity.
ML Inorganic silts with slight plasticity.
OL Organic soil of low plasticity.
CH Inorganic clays of high plasticity.
MH Inorganic silts with high plasticity.
OH Organic soil of high plasticity.
Pt Peat.
Group Symbols and Group Names
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Example
Passing No.200 sieve 30%
Passing No.4 sieve 70 %
LL= 33
PI= 12
PI= 0.73(LL-20), A-line
PI=0.73(33-20)=9.49
Soil Type= SC
Clayey sand
(Santamarina et al., 2001)
Passing No.200 sieve 30 %
Passing No.4 sieve 70 %
LL= 33
PI= 12
Highly
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Indian Standard Classification System (ISCS)IS:1498-1970
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Introduction
Based on USCS systemModifications are:Fine grained soils subdivided into 3 groups (low, intermediate and high plasticity) as against two groups(low and high) in USCS.
wL <35=L, 35<wL <5o=I, wL >50=H
Thus, soil classified into 18 groups as against 15 in USCS.
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Classification Group SymbolsMain Soil
TypePrefix Subgroup Suffix
Classification Group symbols
Gravel G
Well-gradedPoorly-graded
SiltyClayey
WPMC
GWGPGMGC
Sand S
Well-gradedPoorly-graded
SiltyClayey
WPMC
SWSPSMSC
Silt MLL < 35%
35<LL<>50LL > 50%
LIH
MLMIMH
Clay CLL < 35%
35<LL<>50LL > 50%
LIH
CLCICH
Organic OLL < 35%
35<LL<>50LL > 50%
LIH
OLOIOH
Peat Pt Pt 36Pranamesh Chakraborty
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Plasticity Chart
High Plasticity use H - wL >50
Intermediate Plasticity use I – 35< wL >50Low Plasticity use L - wL <35
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Below A-line, use M (Silt) or O (Organic)
Above A-line, use C - ClayPranamesh Chakraborty
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Thank You
Pranamesh Chakraborty