The activity ratio, defined as the ratio of plasticity index to the percentage by weight of material finer than 0.002 mm (clay sizes), has been found to be an important indicator of soil behavior.

The activity of soil gives information about the type and effect of clay minerals present in the soil. It is known that for a given amount of clay mineral present in the soil the plasticity resulting in the soil will vary for the different types of clays. For soil of a specific origin, the activity is constant. The plasticity index increases as the amount of clay fraction increases. Further highly active clay minerals, such as montmorillonite, can produce a large increase in the plasticity index even when present in small quantities.

Plastic index is defined as the difference between the liquid limit and the plastic limit.

I_P = w_L - w_P

Where,

w_L = Liquid Limit

w_P = Plastic limit