Chapter (4)Chapter (4)
Instructor : Dr. Jehad HamadInstructor : Dr. Jehad Hamad
2011-20102011-2010
Different soils with similar prosperities may be classified into groups and sub-groups according to their engineering behavior. Currently , two elaborate classification systems are commonly used by soil engineers .
American Association of State Highway and Transportation Officials system (AASHTO)
American Association of State Highway and Transportation Officials system (AASHTO)
Unified Soil Classification System (USCS).Unified Soil Classification System (USCS).
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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)
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Definition of Grain Size
Boulders Gravel Sand Silt-Clay
Coarse Fine
75 mm
No.4
4.75 mm No.40
0.425 mm
No.200
0.075 mm
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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).
– The original purpose of this classification system is used for road construction (subgrade rating).
A4 ~ A7A1 ~ A3
Granular Materials
35% pass No. 200 sieve
Silt-clay Materials
36% pass No. 200 sieve
Using LL and PI separates silty materials from clayey materials
Using LL and PI separates silty materials from clayey materials (only for A2 group)
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Classification
Das, 1998
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Group Index
)10PI)(15F(01.0
)40LL(005.02.0)35F(GI
200
200
)10PI)(15F(01.0GI 200
For Group A-2-6 and A-2-7
The first term is determined by the LL
The second term is determined by the PI
In general, the rating for a pavement subgrade is inversely proportional to the group index, GI.
use the second term only
F200: percentage passing through the No.200 sieve
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Classification (Cont.)
Das, 1998
Note :
The first group from the left to fit the test data is the correct AASHTO classification.
Example 4.1 Passing No.10 sieve 100
Passing No.40 sieve 80LL = 30
Passing No.200 sieve 58PI =10
345.3
)10)(15(01.0
)40(005.02.0)35(
200
200
PIF
LLFGI
A-4(3)
Example 4.3 Passing No.4 sieve 90
Passing No.10 sieve 76LL = 37
Passing No.200 sieve 34PI =12 038,0
)10)(15(01.0 200
PIFGI
A-2-6(0)
• The original form of the unified classification system was proposed by Casagrande in 1942 during world war II for use in airfield construction undertaken by the Army Corps of Engineers.At present,it is widly used by engineers.
USCS (Major Division) cont.
For Coarse Soil Aggregates
Classification Steps
Step One: Classify Soil
Step Two:
Find Soil Name
Gravel
Sand
Fine Aggregates
Fine Aggregates
Sand
Fine Aggregates
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