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 No | Plastic index value | Consistency |
|---|---|---|
| 1 | 0-3 | Non plastic |
| 2 | 4-15 | slightly plastic |
| 3 | 16-30 | Medium plastic |
| 4 | Greater than 30 | Highly 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).
