American Association of State Highway And Transportation Officials (AASHTO) classification system is useful for classifying soil for highways.
This classification is based on both particle size ranges and plasticity characteristics. The system comprises seven groups of inorganic soils, A-1 to A-7 with 12 sub-groups in all. The system is based on the following soil properties:
(i) particle size distribution
(ii) liquid limit
(iii) plasticity index.
A group index is introduced to further differentiate soils, containing appreciable fine-grained materials.
Group index is given by the following equation:
Group index (G.I.) = 0.2a + 0.005ac + 0.01bd
Where, a is that portion of percentage passing 75 sizes sieve, greater than 35 and not exceeding 75 expressed as a whole number (0 to 40) = (F-35)
b is that portion of percentage passing 75 sizes sieve, greater than 15 and not exceeding 35 expressed as a whole number(0 to 40) = (F-15)
c is that portion of the liquid limit greater than 40 = (WL-40), (0 to 20)
d is that portion of the plasticity index greater than 10 = (Ip-10), (0 to 20)
F is the percent passing No. 200 sieve (0.0075 mm) if F<35, use f-35 =0