This document provides an overview of soil classification systems, focusing on the Unified Soil Classification System (USCS) and the American Association of State Highway and Transportation Officials (AASHTO) system. It defines key aspects of each system such as grouping soils by grain size and plasticity. Examples are provided to demonstrate how to classify soils using index properties and test results based on the criteria of each system.

1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. •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

8. 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

9. 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

10. 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

11. Unified Soil Classification System (USCS)
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
Dr. Abdulmannan Orabi IUST 11

12. 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
Dr. Abdulmannan Orabi IUST 12

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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 ( % )
PlasticityIndex
OL
Plasticity Chart
Dr. Abdulmannan Orabi IUST 18

19. 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.
Dr. Abdulmannan Orabi IUST 19

20. 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)
Percentfiner
#10 #200#60
10
20
30
40
100
0
0.01110
50
60
70
80
90
D30 D10D60
Dr. Abdulmannan Orabi IUST 20

21. 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 ).
= =
0.6
0.09
= 6.67 =
∗
= 0.74
Dr. Abdulmannan Orabi IUST 21

22. 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

23. 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

24. 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

25. 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

26. 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
Dr. Abdulmannan Orabi IUST 26

27. 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

28. 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

29. AASHTO Classification System
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
Dr. Abdulmannan Orabi IUST 29

30. AASHTO Classification System
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
Dr. Abdulmannan Orabi IUST 30

31. 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-7-5
A-2-6
A-6
Liquid Limit, LL
PlasticityIndex,PI
PI< LL – 30
PI>LL – 30
Dr. Abdulmannan Orabi IUST 31

32. 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

33. 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 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.
Dr. Abdulmannan Orabi IUST 33

34. 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)
Dr. Abdulmannan Orabi IUST 34

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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
Dr. Abdulmannan Orabi IUST 40

41. 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
Dr. Abdulmannan Orabi IUST 41

42. 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

43. 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