The plasticity index (PI) is a measure of the plasticity of a soil. The plasticity index is the size of the range of water contents where the soil exhibits plastic properties. The PI is the difference between the liquid limit and the plastic limit (PI = LL-PL). SOILS with a high PI tend to be clay, those with a lower PI tend to be silt, and those with a PI of 0 tend to have little or no silt or clay.

PI is used to relate to that how expansive the clays were. A PI lower than 20 to 24 was GENERALLY a safe area...but higher than that and we would then have to respond to swelling clay conditions.

It is a measure of the cohesive qualities of the BINDER resulting from the clay content. Also, it givessome indication of the amount of swelling and shrinkage that will result in the wetting and drying of that fraction tested. If some soils do not have SUFFICIENT mechanical interlock they require AMOUNTS of cohesive materials to give a satisfactory performance. A deficiency of clay binder may cause raveling of gravel wearing courses during dry weather and excessive permeability