Classification of Soils

by
Dr . Sridhar

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Purpose

Pranamesh Chakraborty
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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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 Introduction
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.
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IS Classification of Grain Size

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Textural Classification

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 Introduction
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.
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Textural Classification Chart

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Example

30% sand
30% silt
40% clay

Soil Type
Clay

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Highway Research Board (HRB)
Classification

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 Introduction

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.

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 Group Index (GI)

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.

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 Group Index (GI) determination

Group Index of a soil depends on:

1. Amount of material passing 75 micron IS sieve.
2. Liquid Limit.
3. Plastic Limit.

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 HRB Classification Table

For A-7-5, IP<= wL-30 For A-7-6, IP > wL-30

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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). Pranamesh 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%

wL=36%
IP =13%

A-6

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 Example
Group Index Calculation 56% passes 75 micron sieve.
 Plastic Limit (wP) =23%
Liquid Limit (wL)=36%
Plasticity Index (IP) =13%

a= 56-35=21 c=0
b=56-15=41, Take 40 d=13-10=3
GI=5.4=5 A-6 (5) Pranamesh Chakraborty
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Unified Soil Classification System
USCS

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 Origin of USCS

 First developed by Professor A. Casagrande

(1948) for the purpose of airfield construction
during World War II.

 Afterwards, it was modified to enable the

system to be applicable to dams, foundations,
and other construction .

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 Four Major Divisions

Course Organic
Grained Soil

Fine
Peat
Grained

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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 Classification
Prefix Subgroup Suffix
Type Group symbols

Well-graded W GW
Poorly-graded P GP
Gravel G
Silty M GM
Clayey C GC

Well-graded W SW
Poorly-graded P SP
Sand S
Silty M SM
Clayey C SC

LL < 50% L ML
Silt M
LL > 50% H MH
LL < 50% L CL
Clay C
LL > 50% H CH
LL < 50% L OL
Organic O
LL > 50% H OH
Peat Pt Pt
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 Unified Soil Classification System (Contd.)

50% 50 %

Coarse-grained soils: Fine-grained soils:

Gravel (G) Sand (S) Silt (M) Clay (C)
NO.200
0.075 mm
NO. 4 Based on wL and IP
4.75 mm (Plasticity Chart)

Well or Poor Graded

based on Cu and Cc wL >50
wL <50
For G, use W if Cu > 4 and 1<Cc <3
For S, use W if Cu > 6 and 1<Cc <3
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 Plasticity Chart

Silt (M)
Clay (C)
Organic (O)

Below A-line, use M (Silt) or High Plasticity use H - wL >50

O (Organic)
Low Plasticity use L - wL <50
Above A-line, use C - Clay
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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)
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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
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soil and designated by dual group symbol.
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 Symbols and Group Names
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.

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 Passing No.200 sieve 30 % LL= 33
Example Passing No.4 sieve 70 % PI= 12

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
Highly

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(Santamarina et al., 2001) 33
Indian Standard Classification
System (ISCS)
IS:1498-1970

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 Introduction
Based on ISCS system
Modifications are:
Fine grained soils subdivided into 3 groups (low,
intermediate and high plasticity) as against two
groups(low and high) in ISCS.

wL <35=L, 35<wL <5o=I, wL >50=H

Thus, soil classified into 18 groups as against 15

in ISCS.

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 Classification Group Symbols
Main Soil Classification
Prefix Subgroup Suffix
Type Group symbols
Well-graded W GW
Poorly-graded P GP
Gravel G
Silty M GM
Clayey C GC

Well-graded W SW
Poorly-graded P SP
Sand S
Silty M SM
Clayey C SC
LL < 35% L ML
Silt M 35<LL<>50 I MI
LL > 50% H MH
LL < 35% L CL
Clay C 35<LL<>50 I CI
LL > 50% H CH
LL < 35% L OL
Organic O 35<LL<>50 I OI
LL > 50% H OH
Peat Pt Pt
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 Plasticity Chart

Below A-line, use M (Silt) or High Plasticity use H - wL >50

O (Organic)
Intermediate Plasticity use I – 35< wL >50
Above A-line, use C - Clay Low Plasticity use L - wL <35
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Thank You

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