Explain the AASHTO system of soil classification.

American Association of state highway and transportation officials classification system is useful for classifying soils for highway this classification is based on  following three soil properties:

• particle size distribution

• liquid limit

• plasticity index

This system consists of seven group of inorganic soil, A-1 to A-7 with 12 subgroups in all. A group Index is introduced to identify completely soils containing appreciable fine-grained materials. The higher the value of group index the power is the quality of the material.Group index is given by the following equation:

Group Index,`G.I=0.2a+0.005ac+0.01bd`

Where,

a=The portion of percentage of soil particles passing NO. 200 sieve Greater than 35 and not exceeding 75 expressed as a positive number (0 to 40)=F-35.

b=Portion of percentage of soil particles passing number 200 sieve  greater than 15 not exceeding 55 expres.... Show More

American Association of state highway and transportation officials classification system is useful for classifying soils for highway this classification is based on  following three soil properties:

• particle size distribution

• liquid limit

• plasticity index

This system consists of seven group of inorganic soil, A-1 to A-7 with 12 subgroups in all. A group Index is introduced to identify completely soils containing appreciable fine-grained materials. The higher the value of group index the power is the quality of the material.Group index is given by the following equation:

Group Index,`G.I=0.2a+0.005ac+0.01bd`

Where,

a=The portion of percentage of soil particles passing NO. 200 sieve Greater than 35 and not exceeding 75 expressed as a positive number (0 to 40)=F-35.

b=Portion of percentage of soil particles passing number 200 sieve  greater than 15 not exceeding 55 expressed as a positive number (0 to 40)=F-15

c =the  portion of percentage of  liquid limit greater than 40 in not exceeding 60 expressed as a positive number (0 to 20)=`W_l-40`

d= the portion of the plasticity index greater than 10 in not exceeding 30 expressed as a positive number (0 to 20)=`I_p-10`

The group index is a means of rating the value of a soil as a subgrade material within its own group. It is not used in order to place a soil in a particular group, that is done directly from the results of Sieve analysis, the liquid limit and plasticity index. The higher the value of the group index, the poorer is the quality of the material. The group index is a function of the amount of material passing through the No. 200 sieve, the liquid limit and plasticity index.

If the pertinent index value for soil falls below the minimum limit associated with a, b, c or d, the value of the corresponding term is zero and the term drops out of the group index equation. When the values of a, b, c, d are more than the prescribed maximum values, then the highest value of 40 or 20 should be assigned.The group index value should be shown in parentheses after group symbol as A-6(12)  where 12 is the group index .

The classification procedures can be explained from following table: