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INTERNATIONAL UNIVERSITY FOR SCIENCE & TECHNOLOGY
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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.
Dr. Abdulmannan Orabi IUST 3
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.
Dr. Abdulmannan Orabi IUST 4
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).
Dr. Abdulmannan Orabi IUST 5
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%).
Dr. Abdulmannan Orabi IUST 6
•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 .
Unified Soil Classification System (USCS)
Dr. Abdulmannan Orabi IUST 7
Unified Soil Classification System (USCS)
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
Dr. Abdulmannan Orabi IUST 8
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
Unified Soil Classification System (USCS)
1. Coarse-grained
Dr. Abdulmannan Orabi IUST 9
Unified Soil Classification System (USCS)
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
Dr. Abdulmannan Orabi IUST 10
Unified Soil Classification System (USCS)
The classification is based on material passing a 75mm sieve:
Coa
rse
–g
rain
ed S
oil
Fin
e –
gra
ined
S
oil
R 200 > 50
F200 > 50
Gravel
Sand
Silt & Clay
Sieve No. 4
Sieve No. 200
F200
R200
F4
R4
4.75 mm
0.075 mm
Dr. Abdulmannan Orabi IUST 11
Unified Soil Classification System (USCS)
� 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
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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
Dr. Abdulmannan Orabi IUST 13
Unified Soil Classification System (USCS)
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 )
Dr. Abdulmannan Orabi IUST 14
Unified Soil Classification System (USCS)
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 )
Dr. Abdulmannan Orabi IUST 15
Unified Soil Classification System (USCS)
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
Dr. Abdulmannan Orabi IUST 16
Unified Soil Classification System (USCS)
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)
Dr. Abdulmannan Orabi IUST 17
Unified Soil Classification System (USCS)
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 ( % )
Pla
sticity In
de
x
OL
Plasticity Chart
Dr. Abdulmannan Orabi IUST 18
Unified Soil Classification System (USCS)
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.
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Example 1
Unified Soil Classification System (USCS)
Using the grain-size distribution curve shown below, determine the uniformity coefficient Cu and classify the soil
Particle diameter (mm)
Per
cen
t fi
ner
#10 #200 #60
10
20
30
40
100
0 0.01 1 10
50
60
70
80
90
D30 D10D60
Dr. Abdulmannan Orabi IUST 20
Unified Soil Classification System (USCS)
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 ).
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���
=0.6
0.09= 6.67 �� =
����
��� ∗ ���
= 0.74
Dr. Abdulmannan Orabi IUST 21
Unified Soil Classification System (USCS)
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
Dr. Abdulmannan Orabi IUST 22
Unified Soil Classification System (USCS)
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
Dr. Abdulmannan Orabi IUST 23
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.
Dr. Abdulmannan Orabi IUST 24
AASHTO Classification System
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.
Dr. Abdulmannan Orabi IUST 25
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.
AASHTO Classification System
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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.
AASHTO Classification System
Dr. Abdulmannan Orabi IUST 27
AASHTO Classification System
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%).
Dr. Abdulmannan Orabi IUST 28
AASHTO Classification System
Granular materials ( F200 < 35% )
Group
classification
A – 1
A - 3A – 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
Dr. Abdulmannan Orabi IUST 29
AASHTO Classification System
Granular materials ( F200 < 35% )
Group
classificationA - 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
Dr. Abdulmannan Orabi IUST 30
AASHTO Classification System
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-7-5A-2-6
A-6
Liquid Limit, LL
Pla
sticity I
ndex,
PI
PI< LL – 30
PI>LL – 30
Dr. Abdulmannan Orabi IUST 31
AASHTO Classification System
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.
Dr. Abdulmannan Orabi IUST 32
AASHTO Classification System
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 group3. 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.
Dr. Abdulmannan Orabi IUST 33
AASHTO Classification System
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)
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AASHTO Classification System
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
Dr. Abdulmannan Orabi IUST 35
The results of the particle-size analysis of a soil are as follows:
Percent passing through the No. 10 sieve = 100
Percent passing through the No. 40 sieve = 80
Percent passing through the No. 200 sieve = 58
The liquid limit and plasticity index are 30 and 10, respectively.
Classify the soil by the AASHTO system.
Worked Examples
Example 3
Dr. Abdulmannan Orabi IUST 36
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
Dr. Abdulmannan Orabi IUST 37
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 )
Dr. Abdulmannan Orabi IUST 38
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
Dr. Abdulmannan Orabi IUST 39
Classify the following soils using Unified classification system
Worked Examples
Example 6
SoilSieve 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
Dr. Abdulmannan Orabi IUST 40
Worked Examples
Example 6
Classify the following soils using USCS and AASHTO systems and give the GI.
Soil
Sieve analysis,
LL PINo.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
Dr. Abdulmannan Orabi IUST 41
0
10
20
30
40
50
60
70
80
90
100
0.010.1110
Worked Examples
Curve – soil A Example 6
Dr. Abdulmannan Orabi IUST 42
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
Dr. Abdulmannan Orabi IUST 43