The Plasticity Index (often abbreviated as PI) is a numerical measure of the plasticity of a soil. It corresponds to the range of moisture contents, expressed as percent water by dry weight of soil, within which the soil has plastic properties.
The Plasticity Index is calculated by subtracting the plastic limit (PL) from the Liquid Limit (LL). i.e., PI = LL - PL. Soils with a high PI tend to be predominatly clay, those with a lower PI tend to be predominatly silt, and those with a PI of 0 tend to have little or no silt or clay.
Plasticity is influenced by the past history of the clay, type of clay, amount of working the mixture has received, the amount of souring, temperature and moisture content.
More precisely the plasticity is effected by the physiochemical relationships between colloidal clay particles and water, so is effected by mineral composition, particle size and particle size distribution, cation-exchange capacity, cations and pH and the surface tension of the water.
Plasticity of natural clays depends on the nature of the very fine fraction, montmorillonites may be found in the fine fraction, but not in the bulk of material.
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