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AASHTO Soil Classification System
Classification of soils for road construction purposes

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, but has been revised several times since.

AASHTO Soil Classification System (from AASHTO M 145 or ASTM D3282)
General ClassificationGranular Materials (35% or less passing the 0.075 mm (No. 200) sieve)Silt-Clay Materials (>35% passing the 0.075 mm (No. 200) sieve)
Group ClassificationA-1A-3A-2A-4A-5A-6A-7
A-1-aA-1-bA-2-4A-2-5A-2-6A-2-7A-7-5 A-7-6
Sieve Analysis, % passing
2.00 mm (No. 10)50 max
0.4255 mm (No. 40)30 max50 max51 min
0.0755 mm (No. 200)15 max25 max10 max35 max35 max35 max35 max36 min36 min36 min36 min
Characteristics of fraction passing 0.425 mm (No. 40)
Liquid Limit40 max41 min40 max41 min40 max41 min40 max41 min
Plasticity index6 maxN.P.10 max10 max11 min11 min10 max10 max11 min11 min1
Usual types of significant constituent materialsstone fragments, gravel and sandfine sandsilty or clayey gravel and sandsilty soilsclayey soils
General rating as a subgradeexcellent to goodfair 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.

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See also

References

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