Classification of Soil

Classification of Soils

1

byPranamesh Chakraborty

Purpose

2Pranamesh Chakraborty

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.


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)


Particle Size Classification


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


IS Classification of Grain Size


Textural Classification


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


Textural Classification Chart


Example

30% sand30% silt40% clay

Soil TypeClay


Highway Research Board (HRB)Classification


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


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)


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


HRB Classification Table

For A-7-5, IP<= wL-30 For A-7-6, IP > wL-30 16Pranamesh Chakraborty

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

Example

56% passes 75 micron sieve.

Plastic Limit (wP) =23%

Liquid Limit (wL)=36%

Plasticity Index (IP) = 36-23=13%


Example

Passing 75 micron>35% IP < 40% wL>10%

A-6

wL=36%

IP =13%


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

Unified Soil Classification SystemUSCS


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 .


Four Major Divisions

Course Grained

Fine Grained

Organic Soil

Peat


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 .


Classification Group Symbols

Main Soil Type

Prefix Subgroup SuffixClassification

Group symbols

Gravel G

Well-gradedPoorly-graded

SiltyClayey

WPMC

GWGPGMGC

Sand S


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

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

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

27

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

Pranamesh Chakraborty

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

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

USCS at a glance

(Santamarina et al., 2001)


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.


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


33

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)



LL= 33

PI= 12

Highly


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


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.


Classification Group SymbolsMain Soil

TypePrefix Subgroup Suffix

Classification Group symbols

Gravel G


SiltyClayey

WPMC

GWGPGMGC

Sand S


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

Plasticity Chart

High Plasticity use H - wL >50

Intermediate Plasticity use I – 35< wL >50Low Plasticity use L - wL <35

37

Below A-line, use M (Silt) or O (Organic)

Above A-line, use C - ClayPranamesh Chakraborty

38

Thank You


Engineering

Classification of Soil