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