Plasticity Index and Liquidity Index
Two numbers from the Atterberg limits place a clay on the map: the plasticity index says what the soil is, the liquidity index says what state it is in right now.
The equation
PI = LL – PL, LI = (w – PL) / PI
- LL and PL are the liquid and plastic limits
- w is the natural water content
- LI near 0 means stiff, near 1 means soft as its liquid limit
Worked example
A clay with liquid limit 42 and plastic limit 21 has PI of 21, plotting above the A line, so it classifies as CL. At a natural water content of 28 percent its liquidity index is 0.33, a firm to stiff state.
When to use it
PI drives classification, swell potential, and strength correlations. An LI approaching 1 warns of a sensitive soil that may lose most of its strength when disturbed, the signature of quick clay failures.
Related Calculators and Reference
- USCS Soil Classification Tool: classify soils from lab results.
- Slope Stability Risk Calculator: run stability with uncertainty included.
- ISSMGE: the international society for soil mechanics.