Chapter (4) Instructor : Dr. Jehad Hamad

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) Unified Soil Classification System (USCS).

3 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 (Holtz and Kovacs, 1981)

4 Definition of Grain Size Boulders GravelSand Silt-Clay CoarseFine 75 mm No mm No mm No mm

5 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)

6 Classification Das, 1998

7 Group Index 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

8 Classification (Cont.) Das, 1998 Note: The first group from the left to fit the test data is the correct AASHTO classification.

Example Passing No.10 sieve LL = 3080Passing No.40 sieve PI =1058Passing No.200 sieve A-4(3)

Example Passing No.4 sieve LL = 3776Passing No.10 sieve PI =1234Passing No.200 sieve 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.

Soil Coarse F200 < 50 Gravel G Sand S Fine F200 ≥ 50 Clay C Silt M

USCS (Major Division) cont. For Coarse Soil Aggregates Gradation Well Graded (w) Poorly Graded (P)

Classification Steps Classify soil using Table 4.2, given: (F 200, C u, C c, LL, PI) Find soil name: If soil is coarse use Figure 4.3 Page 90 If soil is fine use Figure 4.4 Page 91 If soil was found to be Organic then use Figure 4.5 Page 92

Step One: Classify Soil

Step Two: Find Soil Name

Gravel

Sand

Fine Aggregates

Sand

Fine Aggregates