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.
| General Classification | Granular 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 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 | … | … | … | … | … | … | … | … | … | … |
| 0.4255 mm (No. 40) | 30 max | 50 max | 51 min | … | … | … | … | … | … | … | … |
| 0.0755 mm (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.