This document discusses different systems for classifying soils, including particle size, textural, Highway Research Board (HRB), Unified Soil Classification System (USCS), and Indian Standard Classification System (ISCS). The USCS system divides soils into four main groups - coarse grained, fine grained, organic, and peat. Each soil is designated a two letter symbol based on grain type and plasticity. The ISCS system is similar but subdivides fine grained soils into low, intermediate, and high plasticity groups. Classification involves analyzing particle size distribution and plasticity characteristics on a chart to determine the appropriate group.

1. Classification of Soils
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by
Pranamesh Chakraborty

2. Purpose
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3. 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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4. 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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5. Particle Size Classification
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6. 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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7. IS Classification of Grain Size
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8. Textural Classification
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9. 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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10. Textural Classification Chart
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11. Example
30% sand
30% silt
40% clay
Soil Type
Clay
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12. Highway Research Board (HRB)
Classification
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13. 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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14. 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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15. 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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16. HRB Classification Table
For A-7-5, IP<= wL-30 For A-7-6, IP > wL-30 16Pranamesh Chakraborty

17. 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).
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18. Example
56% passes 75 micron sieve.
 Plastic Limit (wP) =23%
Liquid Limit (wL)=36%
Plasticity Index (IP) = 36-23=13%
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19. Example
Passing 75 micron>35% IP < 40% wL>10%
A-6
wL=36%
IP =13%
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20. 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

21. Unified Soil Classification System
USCS
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22. 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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23. Four Major Divisions
Course
Grained
Fine
Grained
Organic
Soil
Peat
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24. 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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25. Classification Group Symbols
Main Soil
Type
Prefix Subgroup Suffix
Classification
Group symbols
Gravel G
Well-graded
Poorly-graded
Silty
Clayey
W
P
M
C
GW
GP
GM
GC
Sand S
Well-graded
Poorly-graded
Silty
Clayey
W
P
M
C
SW
SP
SM
SC
Silt M
LL < 50%
LL > 50%
L
H
ML
MH
Clay C
LL < 50%
LL > 50%
L
H
CL
CH
Organic O
LL < 50%
LL > 50%
L
H
OL
OH
Peat Pt Pt
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26. 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
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27. 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)
Pranamesh Chakraborty

28. 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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29. 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.
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30. USCS at a glance
(Santamarina et al., 2001)
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31. 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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32. 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
Pranamesh Chakraborty

34. Indian Standard Classification
System (ISCS)
IS:1498-1970
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35. Introduction
Based on USCS system
Modifications 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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36. Classification Group Symbols
Main Soil
Type
Prefix Subgroup Suffix
Classification
Group symbols
Gravel G
Well-graded
Poorly-graded
Silty
Clayey
W
P
M
C
GW
GP
GM
GC
Sand S
Well-graded
Poorly-graded
Silty
Clayey
W
P
M
C
SW
SP
SM
SC
Silt M
LL < 35%
35<LL<>50
LL > 50%
L
I
H
ML
MI
MH
Clay C
LL < 35%
35<LL<>50
LL > 50%
L
I
H
CL
CI
CH
Organic O
LL < 35%
35<LL<>50
LL > 50%
L
I
H
OL
OI
OH
Peat Pt Pt 36Pranamesh Chakraborty

37. Plasticity Chart
High Plasticity use H - wL >50
Intermediate Plasticity use I – 35< wL >50
Low Plasticity use L - wL <35
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Below A-line, use M (Silt) or
O (Organic)
Above A-line, use C - Clay
Pranamesh Chakraborty

38. 38
Thank You
Pranamesh Chakraborty