Soil Classification as per AASHTO

The main aim of The American Association of State Highway and Transportation Officials (AASHTO) and the Federal Highway Administration (FHWA) soil classification system is to have a system appropriate for highway sub-grade materials based on their performance. The AASHTO System has seven soil classes including A-1 to A-7. The soil is placed in the classes based on performance characteristics. A-1 is the highest and A-7 is the lowest. A-1 to A-3 are sands and gravels, A-4 to A-7 are silts and clays. Table below illustrates AASHTO soil classification system.

Table:

ASSHTO Soil Classification System (from [AASHTOM145] or [ASTMD3282])

General Classification	Granular Materials (35% or less passing the 0.075 mm sieve)							Sil-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
Group Classification	A-1-a	A-1-b	A-3	A-2-4	A-2-5	A-2-6	A-2-7	A-4	A-5	A-6	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 max	...	...	...	...	...	...	...	...
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		clayes soils
General rating as a subgrade	excellent to good							fair to poor

Note (1): 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