AASHTO Soil Classification System

The AASHTO Soil Classification System was developed by the American Association of State Highway and Transportation Officials, and is used as a guide for the classification of soils and soil-aggregate mixtures for highway construction purposes. The classification system was first developed by Hogentogler and Terzaghi in 1929,[1] but has been revised several times since.

AASHTO Soil Classification System (from AASHTO M 145 or ASTM D3282)
General Classification Granular Materials (35% or less passing the 0.075 mm sieve) Silt-Clay Materials (>35% passing the 0.075 mm sieve)
Group Classification A-1 A-3 A-2 A-4 A-5 A-6 A-7
A-1-a A-1-b A-2-4 A-2-5 A-2-6 A-2-7 A-7-5 A-7-6
Sieve Analysis, % passing
2.00 mm (No. 10) 50 max
0.425 (No. 40) 30 max 50 max 51 min
0.075 (No. 200) 15 max 25 max 10 max 35 max 35 max 35 max 35 max 36 min 36 min 36 min 36 min
Characteristics of fraction passing 0.425 mm (No. 40)
Liquid Limit 40 max 41 min 40 max 41 min 40 max 41 min 40 max 41 min
Plasticity index 6 max N.P. 10 max 10 max 11 min 11 min 10 max 10 max 11 min 11 min1
Usual types of significant constituent materials stone fragments, gravel and sand fine sand silty or clayey gravel and sand silty soils clayey soils
General rating as a subgrade excellent to good fair to poor

Plasticity index of A-7-5 subgroup is equal to or less than the LL - 30. Plasticity index of A-7-6 subgroup is greater than LL - 30.


  1. ^ Hogentogler, C.A.; Terzaghi, K. (May 1929). "Interrelationship of load, road and subgrade". Public Roads: 37–64.

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