plastic index and plasticity index of a soil

Definition of Plastic index

The plasticity index of a soil is the numerical difference between its liquid limit and its plastic limit, It is a dimensionless number as both the liquid and plastic limits are moisture contents.

Calculation of plastic index of soil

Plasticity Index = Liquid Limit – Plastic Limit Different P.I Values ranges and their interpretation is considered as under:

S NoPlastic index valueConsistency
10-3Non plastic
24-15slightly plastic
316-30Medium plastic
4Greater than 30Highly plastic

NOTE: There are certain circumstances under which the plasticity index cannot be determined.

  • When either the liquid limit or plastic limit cannot be determined, report the plasticity index as NP (non-plastic).
  • When the soil is extremely sandy, the plastic limit test shall be done before the liquid limit test. If the plastic limit cannot be determined, then report the plasticity index as NP (non-plastic).
  • When the plastic limit is equal to or greater than the liquid limit, report the plasticity index as NP (non-plastic).

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