<P> The activity (A) of a soil is the PI divided by the percent of clay - sized particles (less than 2 μm) present . Different types of clays have different specific surface areas which controls how much wetting is required to move a soil from one phase to another such as across the liquid limit or the plastic limit . From the activity, one can predict the dominant clay type present in a soil sample . High activity signifies large volume change when wetted and large shrinkage when dried . Soils with high activity are very reactive chemically . </P> <P> Normally the activity of clay is between 0.75 and 1.25, and in this range clay is called normal . It is assumed that the plasticity index is approximately equal to the clay fraction (A = 1). When A is less than 0.75, it is considered inactive . When it is greater than 1.25, it is considered active . </P>

Determination of liquid limit plastic limit and plasticity index