Soil Classification (AASHTO and USCS)

CE 240
Soil Mechanics & Foundations
Lecture 3.2
Engineering Classification of Soil
(AASHTO and USCS)
(Das, Ch. 4)

Outline of this Lecture
1. Particle distribution and Atterberg Limits
2. Soil classification systems based on
particle distribution and Atterberg Limits
3. American Association of State Highway and
Transportation Officials System (AASHTO)
4. The Unified Soil Classification System
(USCS)

Objective
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.
Simple indices
GSD, LL, PI
Communicate
between
engineers
Classification
system
(Language)
Use the
accumulated
experience
Estimate
engineering
properties
Achieve
engineering
purposes

Classification Systems
• Two commonly used systems for soil
engineers based on particle distribution
and Atterberg limits:
• American Association of State Highway
and Transportation Officials (AASHTO)
System (for state/county highway dept.)
• Unified Soil Classification System (USCS)
(preferred by geotechnical engineers).

Soil particles
The description of the grain size distribution of soil
particles according to their texture (particle size,
shape, and gradation).
Major textural classes include, very roughly:
gravel (>2 mm);
sand (0.1 – 2 mm);
silt (0.01 – 0.1 mm);
clay (< 0.01 mm).
Furthermore, gravel and sand can be roughly
classified as coarse textured soils, wile silt and clay
can be classified as fine textures soils.

Cobbles or Boulders
Grain Size Distribution Curves
GRAVEL SAND FINES

Atterberg limits
Atterberg limits are the limits of water content used to
define soil behavior. The consistency of soils according
to Atterberg limits gives the following diagram.

American Association of State
Highway and Transportation
Officials system (AASHTO)
Origin of AASHTO: (For road construction)
This system was originally developed by
Hogentogler and Terzaghi in 1929 as the Public
Roads Classification System. Afterwards, there are
several revisions. The present AASHTO (1978)
system is primarily based on the version in 1945.
(Holtz and Kovacs, 1981)

Definition of Grain Size
No specific grain
size-use
Atterberg limits
Boulders Silt-Clay
Gravel Sand
75 mm
No.4
4.75 mm
Coarse Fine
No.40
0.425 mm
No.200
0.075
mm

Classification
(Proceeding from left to right against the columns)
Das, Table 4.1, 2006

Note:
Classification (Cont.)
The first group from the left to fit the test data is the
correct AASHTO classification.
Das, Table 4.1, 2006

Group Index GI
200
200
[ ]
GI = ( F − 35) 0.2 + 0.005( LL −40)
+ 0.01( F −15)( PI −10)
(4.1)
For Group A-2-6 and A-2-7
200
The first term is determined by the LL
The second term is determined by the PI
GI = 0.01( F −15)( PI −10) (4.2) use the second term only
F200: percentage passing through the No.200 sieve
In general, the rating for a pavement subgrade is
inversely proportional to the group index, GI.

Some Explanations of Group Index GI
1, if Eq. 4.1 gives a negative value then GI=0;
2, round up the value calculated by Eq. 4.1 to
an integer;
3, there is no upper limit for GI;
4, the GIs for soil groups A-1-a, A-1-b, A-2-4,
A-2-5, and A-3 are always zero (0).

(PI)
(LL)
Das, Figure 4.1

General Guidance
– 8 major groups: A1~ A7 (with several subgroups) and
organic soils A8
– The required tests are sieve analysis and Atterberg
limits.
– The group index, an empirical formula, is used to
further evaluate soils within a group (subgroups).
A1 ~ A3
Granular Materials
≤ 35% pass No. 200 sieve
Using LL and PI separates silty
materials from clayey materials (only
for A2 group)
A4 ~ A7
Silt-clay Materials
≥ 36% pass No. 200 sieve
Using LL and PI separates silty
materials from clayey materials
– The original purpose of this classification system is
used for road construction (subgrade rating).

Example 4.1, Soil B
Passing No.200 86%
LL=70, PI=32
LL-30=40 > PI=32
GI
= ( F
200
+ 0.
01(
F
=
33.
47
− 35)
200
≅
33
[ 0.
2 + 0.
005(
LL − 40)
]
−15)(
PI −10)
Round off
A-7-5(33)

This is the example
of Das, Example 4.1
for the AASHTO
system classification

USCS Classification System
• Originally developed for the United
States Army Corps of Engineers
(USACE)
• The method is standardized in ASTM D
2487 as “Unified Soil Classification
System (USCS)”
• USCS is the most common soil
classification system among
geotechnical engineers

Unified Soil Classification System
Origin of USCS:
(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.
Four major divisions:
(1) Coarse-grained
(2) Fine-grained
(3) Organic soils
(4) Peat

Boulders
Definition of Grain Size
Cobbles
300 mm
75 mm
Gravel Sand
Coarse Fine Coarse Medium Fine
No.4
4.75 mm
19 mm No.10 No.40
2.0 mm
No specific grain sizeuse
Atterberg limits
0.425 mm
No.200
0.075
mm
Silt and
Clay

(Das, Table 4.2)

• Soil symbols:
• G: Gravel
• S: Sand
• M: Silt
• C: Clay
• O: Organic
• Pt: Peat
Example: SW, Well-graded sand
SC, Clayey sand
SM, Silty sand,
MH, Elastic silt
The Symbols
• Liquid limit
symbols:
• H: High LL (LL>50)
• L: Low LL (LL

USCS System (cont.)
• A typical USCS classification would be:
Group
Symbol
-
SM Silty sand with gravel
Group
Name

Classification of Soils
• From sieve analysis and the grain-size
distribution curve determine the percent
passing as the following:
– > 3 inch – Cobble or Boulders
– 3 inch - # 4 (76.2 – 4.75 mm) : Gravel
– # 4 - # 200 (4.75 - 0.075 mm) : Sand
– < # 200: Fines
• First, Find % passing # 200
• If 5% or more of the soil passes the # 200
sieve, then conduct Atterberg Limits (LL &
PL)

Classification of Soils
• If the soil is fine-grained (≥ 50% passes
#200) follow the guidelines for fine-grained
soils
• If the soil is coarse-grained (

50%
General Guidance
Coarse-grained soils:
Gravel Sand
•Grain size distribution
•C u
•C c
NO. 4
4.75 mm
50 %
NO.200
0.075 mm
Required tests: Sieve analysis
Fine-grained soils:
Silt Clay
Atterberg limit
•PL, LL
•Plasticity chart
LL>50
LL

Coarse-grained Soils

Fine-grained Soils

Gravel
98-62 = 36%
Sand
62-8 = 54%
Fines = 8%
Example – Soil A
gravel sand fines
Soil A: D 60 = 4.2 mm , D 30 = 0.6 mm, D 10 = 0.09 mm
Cu = 46.67
Cc = 0.95

Grave = 36%
Sand = 54%
Fines = 8%
Cu = 46.7
Cc = 0.95
LL = 42
PL = 31
PI = 42-31 = 11
GO TO Plasticity
Chart
Example – Soil A (Cont.)

Example – Soil A (Cont.)
• Soil A is then classified as
SP-SM – Poorly-grades sand with silt and
gravel

LL = 42
PL = 31
PI = 42-31 = 11
Example – Soil A (Cont.)
ML

PI
The Plasticity Chart
L
H
LL
(Holtz and Kovacs, 1981)
• The A-line generally
separates the more
claylike materials
from silty materials,
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,
they should be
rechecked.

Procedures for Classification
Coarse-grained
material
Grain size
distribution
Fine-grained
material
LL, PI
Highly
(Santamarina et al., 2001)

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
SC
(≥15% gravel)
Clayey sand with
gravel
Passing No.200 sieve 30 %
Passing No.4 sieve 70 %
Highly
LL= 33
PI= 12
(Santamarina et al., 2001)

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.

This is the Figure 4.7 of Das’ textbook, the scanning
electron micrographs (SEM) for 4 peat samples.

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.

Borderline Cases (Dual Symbols, cont.)
–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

Borderline Cases (Summary)
(Holtz and Kovacs, 1981)

Reading Assignment:
Das, Ch. 4
Homework:
Problem 4.3