Activity of clay soil defines its water retaining capacity and we know that swelling and shrinkage of soil depends on the water within soil. Thus both phenomena of volume change depend on activity of soil.
In our previous post we have discussed about method of evaluation of activity. Now we will classify soil depending on activity. Soils are classified as three types as follows:
a. Inactive soil
b. Normal soil
c. Active soil
Soil having activity less than 0.75 is considered as inactive soil.
Soil having activity in between 0.75 and 1.25 are taken as normal soil.
Soil that has activity greater than 1.25 is considered as active.
Identifying clay minerals depending on activity:
Soil having activity greater than 4 have significant amount of montmorillonite.
Soil having activity less than 1 has mainly kaolinite
Soil having activity in between 1 and 2 have significant amount of illite.
Thus activity furnishes information about effect and type of mineral in clay soil. We conclude following points about activity:
a. Activity remains constant for a soil from specific origin. In such case, plasticity index reduces as quantity of clay fraction reduces and vice versa.
b. In case of highly active clay minerals like montmorillonite, shows great increase in plasticity index under small quantity of clay fraction.