1. CE 240
Soil Mechanics & Foundations
Lecture 3.2
Engineering Classification of Soil
(AASHTO and USCS)
(Das, Ch. 4)
2. 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)
3. 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.
Communicate
between
engineers
Simple indices Classification Estimate Achieve
system engineering engineering
GSD, LL, PI (Language) properties purposes
Use the
accumulated
experience
4. 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).
5. 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.
7. 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.
8. 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)
9. Definition of Grain Size
No specific grain
size-use
Atterberg limits
Boulders Gravel Sand Silt-Clay
Coarse Fine
75 mm No.4 No.200
4.75 mm 0.075
No.40 mm
0.425 mm
12. Group Index GI
The first term is determined by the LL
GI = ( F200 − 35) [ 0.2 + 0.005( LL − 40)]
+0.01( F200 − 15)( PI − 10) (4.1)
The second term is determined by the PI
For Group A-2-6 and A-2-7
GI = 0.01( F200 − 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.
13. 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).
15. 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 A4 ~ A7
Granular Materials Silt-clay Materials
≤ 35% pass No. 200 sieve ≥ 36% pass No. 200 sieve
Using LL and PI separates silty
materials from clayey materials (only Using LL and PI separates silty
for A2 group) materials from clayey materials
– The original purpose of this classification system is
used for road construction (subgrade rating).
16. Example 4.1, Soil B
Passing No.200 86% GI = (F200 − 35)[0.2 + 0.005(LL − 40)]
LL=70, PI=32 + 0.01(F200 − 15)(PI − 10)
LL-30=40 > PI=32 = 33.47 ≅ 33 Round off A-7-5(33)
17. This is the example
of Das, Example 4.1
for the AASHTO
system classification
18. 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
19. 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.
Four major divisions:
(1) Coarse-grained
(2) Fine-grained
(3) Organic soils
(4) Peat
20. 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
22. The Symbols
• Soil symbols: • Liquid limit
• G: Gravel symbols:
• S: Sand • H: High LL (LL>50)
• M: Silt • L: Low LL (LL<50)
• C: Clay • Gradation symbols:
• O: Organic • W: Well-graded
• Pt: Peat • P: Poorly-graded
Example: SW, Well-graded sand
Well − graded soil
1 < C c < 3 and C u ≥ 4
SC, Clayey sand
(for gravels)
SM, Silty sand,
1 < C c < 3 and C u ≥ 6
MH, Elastic silt (for sands)
23. USCS System (cont.)
• A typical USCS classification would be:
SM - Silty sand with gravel
Group Group
Symbol Name
24. 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)
25. 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% passes
#200) follow the guidelines for coarse-
grained soils
– Find % Gravel & Sand
– Calculate Cu & Cc
– Calculate LL, PL and PI
26. 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
29. Example – Soil A
gravel sand fines
Gravel
98-62 = 36%
Sand
62-8 = 54%
Fines = 8%
Cu = 46.67
Soil A: D60 = 4.2 mm , D30 = 0.6 mm, D10 = 0.09 mm Cc = 0.95
30. Example – Soil A (Cont.)
Grave = 36%
Sand = 54%
Fines = 8%
Cu = 46.7
Cc = 0.95
LL = 42
PL = 31
PI = 42-31 = 11
GO TO Plasticity
Chart
31. Example – Soil A (Cont.)
• Soil A is then classified as
SP-SM – Poorly-grades sand with silt and
gravel
32. Example – Soil A (Cont.)
LL = 42
PL = 31 ML
PI = 42-31 = 11
33. The 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)
36. 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.
37. This is the Figure 4.7 of Das’ textbook, the scanning
electron micrographs (SEM) for 4 peat samples.
38. 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.
39. 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.
– Soilcontain similar fines and coarse-grained
fractions.
− possible dual symbols GM-ML
