Lecture 4 classification of soil

INTERNATIONAL UNIVERSITY
FOR SCIENCE & TECHNOLOGY
‫وا‬ ‫م‬ ‫ا‬ ‫و‬ ‫ا‬ ‫ا‬
CIVIL ENGINEERING AND
ENVIRONMENTAL DEPARTMENT
303322 - Soil Mechanics
Classification of Soils
Dr. Abdulmannan Orabi
Lecture
2
Lecture
4

Dr. Abdulmannan Orabi IUST 2
Das, B., M. (2014), “ Principles of geotechnical
Engineering ” Eighth Edition, CENGAGE
Learning, ISBN-13: 978-0-495-41130-7.
Knappett, J. A. and Craig R. F. (2012), “ Craig’s Soil
Mechanics” Eighth Edition, Spon Press, ISBN: 978-
0-415-56125-9.
References

A soil classification system-
It is the arrangement of different soils with
similar properties into groups & subgroups
based on their application or to their probable
engineering behavior.
Introduction
A classification for engineering purposes
should be based mainly on mechanical
properties: permeability, stiffness, strength.

Most of the soils classification systems that
have been developed for engineering purposes
are based on simple index properties such as
particle size distribution & plasticity.
Introduction
The aim of a classification system is to
establish a set of conditions which will allow
useful comparisons to be made between
different soils.

Classification System
SOIL CLASSIFICATION BASICS
Commonly based on grain size and soil
consistency. Several classification systems
exist:
1. Unified Soil Classification System (USCS)
(ASTM D2487-11).
2. American Association of State Highway and
Transportation Officials (AASHTO) (ASTM
D3282-09).

Unified Soil Classification System (USCS)
According to the USCS, the soils are divided into
two major categories:
1. Coarse-grained soil that are gravelly and sandy in
nature with less than 50 % passing through the No
200 sieve ( that is F200 < 50%).
2. Fine-grained soils with 50% or more passing
through the No. 200 sieve ( that is F 200 > 50%).

•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 .

The following Symbols are used in this system:
1. Coarse-grained
P – Poorly graded
W–Well graded
M – Silty
C – Clayey
G – Gravel
S – Sand
2. Fine-grained soil
M – Silt, C – Clay, O – Organic
L – Low Plastic H – High Plastic
First Letter
Second Letter

Group Symbol Typical Name
Sw or Gw Well graded soil (sand or gravel )
SP or GP Poorly graded soil ( sand or gravel )
GM Silty gravel or Silty gravel with sand
GC Clayey gravel or Clayey gravel with sand
SM Silty sand or Silty sand with gravel
SC Clayey sand or Clayey sand with gravel
1. Coarse-grained

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.
2. Fine-grained soil

The classification is based on material passing a
75mm sieve:
Coarse–grainedSoil
Fine–
grained
SoilR 200 > 50
F200 > 50
Gravel
Sand
Silt & Clay
Sieve No. 4
Sieve No. 200
F200
R200
F4
R4
4.75 mm
0.075 mm

F 200 = percent passing No. 200 sieve
( % of fines )
R4 = percent retained above No.4 sieve.
( % of Gravel )
R200 = percent retained above No.200 sieve.
F4 = percent passing No. 4 sieve

Worked Example
F200 = 14%
R4 = 18 %
R200 = 86%
F4 = 82%
Gravel = 18 %
Sand = 68 %
Silt and Clay= 14%
Gravel
Sand
Silt & Clay
Sieve No. 4
Sieve No. 200
F200
R200
F4
R4
4.75 mm
0.075 mm

If R4/ R200 > 0.5, the soil is a gravel ( G )
1. Coarse-grained soil
Symbol Criteria
GW F200<5 % Cu > 4 , 1< Cc < 3
GP F200<5 % Not meeting the GW criteria of Cu & Cc
GM F200>12 % PI < 4, or plots below A-Line ( Plasticity Chart )
GC F200>12 % PI > 7, and plots on or above A-Line
GW-GM 5<F200<12 Satisfies Cu & Cc crit. of GW, meets PI for GM
GW-GC 5<F200<12 Satisfies Cu & Cc crit. of GW, meets PI for GC
GP-GM 5<F200<12 Doesn't satisfy Cu & Cc crit, of GP, meets PI,GM
GP-GC 5<F200<12 Doesn't satisfy Cu & Cc crit, of GP, meets PI,GM
GM-GC F200>12 % PI plots in the hatched area ( Plasticity Chart )

Symbol Criteria
SW F200<5 % Cu > 6 , 1< Cc < 3
SP F200<5 % Not meeting the GW criteria of Cu & Cc
SM F200>12 % PI < 4, or plots below A-Line ( Plasticity Chart )
SC F200>12 % PI > 7, and plots on or above A-Line
SW-SM 5<F200<12 Satisfies Cu & Cc crit. of SW, meets PI for SM
SW-SC 5<F200<12 Satisfies Cu & Cc crit. of SW, meets PI for SC
SP-SM 5<F200<12 Doesn't satisfy Cu & Cc crit, of SP, meets PI,SM
SP-SC 5<F200<12 Doesn't satisfy Cu & Cc crit, of SP, meets PI,SM
SM-SC F200>12 % PI plots in the hatched area ( Plasticity Chart )
1. Coarse-grained soil
If R4/ R200 > 0.5, the soil is a gravel ( G )

2. Fine – Grained Soil
Silts and Clays: LL < 50 %
ML PI < 4 or plots below A- Line
CL PI > 7 and plots on or above A –Line
CL- ML PI plots in the hatched area
OL PI plots in the OL area

2. Fine – Grained Soil
Silts and Clays: LL > 50 %
MH PI plots above A- Line
CH PI plots on or above A –Line
OH plots in the OH area
Pt High organic matter (Peat)

0
10
20
30
40
50
60
70
0 10 20 30 40 50 60 70 80 90 100
CH or OH
LL
PI
MH
or
OH
CL or
ML or
OL
CL - ML
Liquid Limit ( % )
PlasticityIndex
OL
Plasticity Chart

Plasticity Chart
Note:
If the results of your soil tests fall above
the U – line on the plasticity chart, you
should be suspicious of your results and
repeat your tests.

Example 1
Using the grain-size distribution curve shown below,
determine the uniformity coefficient Cu and classify the
soil
Particle diameter (mm)
Percentfiner
#10 #200#60
10
20
30
40
100
0
0.01110
50
60
70
80
90
D30 D10D60

Solution
From the grain distribution curve, D60 = 0.60 mm, D30 =
0.20 mm and D10 = 0.09 mm,
F200= 0, R4= 3 therefore the coefficients are,
A well-graded soil has a uniformity coefficient greater than
4 for gravels and 6 for sands.
Therefore, this soil is classified as uniform sand. (According
USCS, the soil is SP ).
= =
0.6
0.09
= 6.67 =
∗
= 0.74

Example 2
Laboratory tests were performed on a light-brown
sandy soil, which visually has several pieces of gravel
larger than 6 mm, indicated that PL = 22.6 % and
LL = 33.2 %. Classify this soil using USCS.
Sieve No. % Passing
4 98.0
40 36.5
200 28.5

Solution
F200 = 28.5% < 50% , the soil is coarse grained. R4 =
2%, R4/R200 < 0.5, the soil is sand
F200 = 28.5% > 12% The soil group is SM or SC
PI = 33.2 – 22.6 = 10.6%
Atterberg limits above A-Line
Therefore, this soil can be classified as SC

AASHTO Classification System
The classification system is based on the following
criteria
1. Grain size
a. Gravel: fraction passing the 75 mm sieve and
retained on the No. 10 ( 2mm ) sieve
b. Sand: fraction passing the No. 10 sieve and
retained on the No. 200 U.S. sieve.
c. Silt and clay: fraction passing No.200sieve.

2. Plasticity : the term silty is applied when the fine
frictions of the soil have a plasticity index of 10
or less.
The term clayey is applied when the fine frictions
have a plasticity index of 11or more.
3. If cobbles and boulders are encountered, they are
excluded form the portion of the soil sample which
classification is made.

According to the present form of this
system, soils can be classified according to
eight major groups, A-1 through A-8, based
on their grain size distribution, liquid limit,
and plasticity indices.

Soils listed in groups A-1, A-2, and A-3 are
coarse-grained materials, and those in groups
A-4, A-5, A-6, and A-7 are fine-grained
materials.
Peat, muck, and other highly organic soils are
classified under A-8.
They are identified by visual inspection.

According to the AASHTO, the soils are divided into
two major categories:
1. Coarse-grained soil that are gravelly and sandy
in nature with less than 35 % passing through
the No 200 sieve ( that is F200 < 35%).
2. Fine-grained soils with 35% or more passing
through the No. 200 sieve ( that is F 200 >
35%).

Granular materials ( F200 < 35% )
Group
classification
A – 1
A - 3
A – 1 – a A – 1 – b
Sieve analysis
(Percentage
passing )
No. 10 50 max
No.40 30 max 50 max 51 min
No.200 15 max 25 max 10 max
Characteristic of
fraction passing
No. 40
Liquid
Limit
------
Plasticity
index
6 max NP
Usual types of materials Stone fragments, gravel, and
sand
Fine sand

Granular materials ( F200 < 35% )
Group
classification
A - 2
A – 2 – 4 A – 2 –5 A – 2 - 6 A – 2 - 7
Sieve analysis
(Percentage
passing )
No. 10
No.40
No.200 35 max 35 max 35 max 35 max
Characteristic
of fraction
passing No. 40
Liquid
Limit
40 max 41 min 40 max 41 min
Plasticity
index
10 max 10 max 11 min 11 min
Usual types of materials Silty or clayey gravel and sand

Plasticity Chart
0
10
20
30
40
50
60
70
0 10 20 30 40 50 60 70 80 90 100
A-4A-2-4
A-7-6
A-2-5A-5
A-2-7
A-7-5
A-2-6
A-6
Liquid Limit, LL
PlasticityIndex,PI
PI< LL – 30
PI>LL – 30

Group Index ( GI )
To evaluate the quality of a soil as a highway
subgrade material, one must also incorporate a
number called the group index (GI) with the
groups and subgroups of the soil.
Group index of a soil depends on amount of
material passing 75 micron IS sieve, liquid limit ,
and plastic limit.

Group Index ( GI )
1. Used to describe performance of soil when
used for pavement construction.
2. Note used to place a soil in a particular group
3. It means rating the value of soil as a subgrade
material within its own group
4. Higher the value of GI, poorer is the quality
of material. The quality of performance of a
soil as a subgrade material is inversely
proportional to GI.

Group Index ( GI )
The group index is given by the equation:
GI = (F200-35)[0.2+0.005(LL-40)]+0.01(F200-15)(PI-10)
When calculating the group index for soils that
belong to group A – 2 – 6 and A – 2 – 7 , use the
partial group index for PI, or
GI = 0.01(F200 -15) (PI - 10)

A soil having GI of zero is considered as the best.
If the equation gives negative value for GI,
consider it zero.
Always round off the GI to nearest whole number.
GI=0 for soils of groups A-1-a, A-1-b ,A-2-4,
A-2-5, and A-3.
Group Index ( GI )
There is no upper limit for the group index

The results of the particle-size analysis of a soil
are as follows:
Percent passing through the No. 10 sieve = 100
The liquid limit and plasticity index are 30 and
10, respectively.
Classify the soil by the AASHTO system.
Worked Examples
Example 3

F200 = 58% > 35%, the soil is fine- grained ( silt
or clay )
Using plasticity chart(slid No. 24), the soil is A-4.
So, the soil will be classified as A-4 (3)
Worked Examples
Solution
GI = (F200-35)[0.2+0.005(LL-40)]+0.01(F200-15)(PI-10)
GI = (58-35)[0.2+0.005(30-40)]+0 .01(58-15)(10-10)
GI = 3.45

Worked Examples
Example 4
Ninety-five percent of a soil passes through the No.
200 sieve and has a liquid limit of 60%and
plasticity index of 40.
Classify the soil by the AASHTO system.
Solution :
F200 = 95% > 35
According to plasticity chart, this soil falls under group
A-7 , 40 > 60 – 30, GI = 42
The soil is A-7 – 6 ( 42 )

For an inorganic soil, the following grain-size analysis
is given:
For this soil, LL = 23% and PL = 19%. Classify the
soil according to:
a. the AASHTO soil classification
b. the Unified soil classification system
Worked Examples
Example 5
Sieve No. % Passing
4 100
10 90
20 64
40 38
80 18
200 13

Classify the following soils using Unified classification
system
Worked Examples
Example 6
Soil
Sieve analysis, Liquid
Limit
Plasticity
indexNo.4 No.200
A 80 52 30 8
B 79 45 26 4
C 91 80 60 32
D 95 75 41 12
E 82 41 24 2

Worked Examples
Example 6
Classify the following soils using USCS and
AASHTO systems and give the GI.
Soil
Sieve analysis,
LL PI
No.4
No.
10
No.
20
No.
40
No.
60
No.
100
No.
200
0.01
mm
0.002
mm
A 94 63 21 10 7 5 3 - - - NP
B 98 86 50 28 18 14 10 - - - NP
C 100 100 98 93 88 83 77 65 60 63 25
D 100 100 100 99 95 90 86 42 40 55 28
E 100 100 100 94 82 66 45 26 21 36 22

0
10
20
30
40
50
60
70
80
90
100
0.010.1110
Worked Examples
Curve – soil A Example 6

Classify the following soils using the AASHTO system
and give the GI.
Worked Examples
Example 7
Soil
Sieve analysis,
LL PI
No. 10 No. 40 No. 200
A 62 30 8 - NP
B 90 67 35 32 8
C 90 76 34 37 12
D 100 78 8 - NP
E 85 68 45 38 9

Lecture 4 classification of soil

Lecture 4 classification of soil

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