III.

Soil Classification

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Outline

1. Purpose
2. Classification Systems
3. The Unified Soil Classification System (USCS)
4. American Association of State Highway and
Transportation Officials System (AASHTO)
5. Suggested Homework

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 1. Purpose
Classifying soils into groups with similar behavior, in terms
of simple indices, can provide geotechnical engineers a
general guidance about engineering properties of the soils
through the accumulated experience.

Communicate
between
engineers

Simple indices Classification Estimate Achieve

system engineering engineering
GSD, LL, PI (Language) properties purposes
Use the
accumulated
experience
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2. Classification Systems

Two commonly used systems:

• Unified Soil Classification System (USCS).

• American Association of State Highway and

Transportation Officials (AASHTO) System
• Craig’s Soil Mechanics use BS

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3. Unified Soil Classification System
(USCS)
Origin of USCS:
This system was first developed by Professor A. Casagrande
(1948) for the purpose of airfield construction during World
War II. Afterwards, it was modified by Professor Casagrande,
the U.S. Bureau of Reclamation, and the U.S. Army Corps of
Engineers to enable the system to be applicable to dams,
foundations, and other construction (Holtz and Kovacs, 1981).
Four major divisions:
(1) Coarse-grained
(2) Fine-grained
(3) Organic soils
(4) Peat
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 3.1 Definition of Grain Size
No specific
grain size-use
Atterberg limits

Gravel Sand Silt and

Boulders Cobbles Clay
Coarse Fine Coarse Medium Fine

300 mm 75 mm No.4 No.200

4.75 mm 0.075
19 mm No.10 No.40 mm
2.0 mm 0.425 mm

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3.2 General Guidance
50 %
Coarse-grained soils: Fine-grained soils:
Gravel Sand Silt Clay
50% NO. 4 NO.200
4.75 mm 0.075
mm

•Grain size distribution •PL, LL LL>50

LL <50
•Cu •Plasticity chart
•Cc
Required tests: Sieve analysis

Atterberg limit
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3.3 Symbols

Soil symbols: Liquid limit symbols:

G: Gravel H: High LL (LL>50)
S: Sand L: Low LL (LL<50)
M: Silt Gradation symbols:
C: Clay W: Well-graded
O: Organic P: Poorly-graded
Pt: Peat Well  graded soil
Example: SW, Well-graded sand 1  Cc  3 and C u  4
(for gravels)
SC, Clayey sand
1  Cc  3 and C u  6
SM, Silty sand,
(for sands )
MH, Elastic silt
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 3.4 Plasticity Chart
L H
• The A-line generally
separates the more
claylike materials
from silty materials,
PI
and the organics
from the inorganics.
• The U-line indicates
the upper bound for
general soils.

Note: If the measured

limits of soils are on
the left of U-line,
LL they should be
rechecked.
(Holtz and Kovacs, 1981)

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3.5 Procedures for Classification

Coarse-grained
material
Grain size
distribution

Fine-grained
material
LL, PI

Highly

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3.7 Organic Soils

• Highly organic soils- Peat (Group symbol PT)

 A sample composed primarily of vegetable tissue in various stages of
decomposition and has a fibrous to amorphous texture, a dark-brown
to black color, and an organic odor should be designated as a highly
organic soil and shall be classified as peat, PT.

• Organic clay or silt( group symbol OL or OH):

 “The soil’s liquid limit (LL) after oven drying is less than 75 % of its
liquid limit before oven drying.” If the above statement is true, then
the first symbol is O.
 The second symbol is obtained by locating the values of PI and LL
(not oven dried) in the plasticity chart.

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3.8 Borderline Cases (Dual Symbols)
For the following three conditions, a dual symbol should be
used.
 Coarse-grained soils with 5% - 12% fines.
 About 7 % fines can change the hydraulic conductivity of the coarse-
grained media by orders of magnitude.
 The first symbol indicates whether the coarse fraction is well or poorly
graded. The second symbol describe the contained fines. For example: SP-
SM, poorly graded sand with silt.
 Fine-grained soils
with limits within the shaded zone. (PI
between 4 and 7 and LL between about 12 and 25).
 It is hard to distinguish between the silty and more claylike materials.
 CL-ML: Silty clay, SC-SM: Silty, clayed sand.
 Soil contain similar fines and coarse-grained fractions.
 possible dual symbols GM-ML

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3.8 Borderline Cases (Summary)

(Holtz and Kovacs, 1981)

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6. References
Main References:
Holtz, R.D. and Kovacs, W.D. (1981). An Introduction to Geotechnical Engineering,
Prentice Hall. (Chapter 3)
Das, B.M. (1998). Principles of Geotechnical Engineering, 4th edition, PWS Publishing
Company. (Chapter 3)

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Chart

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Chart

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Chart
L H

PI

LL

(Holtz and Kovacs, 1981)

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 Passing No.200 sieve 30 % LL= 33
Example Passing No.4 sieve 70 % PI= 12

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
SC
(15% gravel)
Clayey sand with Highly
gravel

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Examples

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Example

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Example

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Example

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Example

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Example

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Example
The results of particle size analyses of four soils A, B, C and D are shown in below figure.
And LL ,PL in below table.

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Example
soil LL PL
A Non plastic ----
B Non plastic

C 26 9
D 42 24

Classify each soil.

Answer
(A:GW, B:SP, C:CL, D:GC)

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Example
Passing No.200 sieve 50 %, Passing No.4 sieve 70 %, LL= 25,PL=20,

, soil classified as SC-CL

Passing No.200 sieve 20 %, Passing No.4 sieve 40 %, LL= 25,PL=20,

, soil classified as GC-CM

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