This document provides an overview of different soil classification systems including: particle size classification, textural classification, Highway Research Board (HRB) classification, Unified Soil Classification System (USCS), and Indian Standard Classification System (ISCS). It describes the key aspects of each system such as group name designations, plasticity characteristics, grain size thresholds. Examples are provided to demonstrate how to classify soils according to their particle sizes, plasticity properties and assign them the appropriate group symbol under different systems.

1. Geotechnical Engineering
(RCE-501)
3RD YEAR (5th Semester)
CIVIL ENGINEERING DEPARTMENT
I. T. S ENGINEERING COLLEGE, GR. NOIDA
By
Aungkar Bhagat

2. 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)

3. Particle Size Classification

4. 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

5. IS Classification of Grain Size

6. Textural Classification

7. 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

8. Textural Classification Chart

9. Example
30% sand
30% silt
40% clay
Soil Type
Clay

10. Highway Research Board (HRB)
Classification

11. 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

12. 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)

13. 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

14. HRB Classification Table
For A-7-5, IP<= wL-30 For A-7-6, IP > wL-30

15. 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).

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

17. Example
Passing 75 micron>35% IP < 40% wL>10%
A-6
wL=36%
IP =13%

18. 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)

19. Unified Soil Classification System
USCS

20. 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 .

21. Four Major Divisions
Course
Grained
Fine
Grained
Organic
Soil
Peat

22. 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 .

23. 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

24. 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

25. 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
Silt (M)
Clay (C)
Organic (O)

26. 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)

27. 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.

28. USCS at a glance
(Santamarina et al., 2001)

29. 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.

30. 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

31. 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

32. Indian Standard Classification
System (ISCS)
IS:1498-1970

33. 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.

34. 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

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

36. Thank You