Liquid Limit, Plastic Limit & Plasticity Index — Part 5
Part 5 determines the Atterberg limits — liquid limit (Casagrande percussion or cone penetrometer), plastic limit, and the plasticity index PI = LL − PL. With grain size, these classify fine-grained soils, indicate compressibility and swelling/shrinkage potential (expansive clays), and are central to assessing a soil's suitability for foundations and earthworks.
Key Requirements
•Liquid limit by the specified method (Casagrande or cone penetrometer — state which)
•Plastic limit by thread-rolling; plasticity index PI = LL − PL
•High LL/PI → high compressibility, swelling/shrinkage (expansive clay) risk
•Used with grain size for soil classification and earthwork/foundation suitability
•Representative, properly prepared sample (IS 2720 Part 1)
Formulas
PI = LL − PL
Plasticity index from liquid and plastic limits
PI = plasticity index (%)LL = liquid limit (%)PL = plastic limit (%)
Practical Notes
✓High plasticity index is the headline expansive-soil warning — black-cotton/expansive clay foundations need special design (under-reamed piles IS 2911 Part 3, soil replacement).
✓State the liquid-limit method: Casagrande and cone-penetrometer values are not always identical.
Common Mistakes
⚠Not reporting which liquid-limit method was used.
⚠Ignoring high PI as an expansive-soil / swelling-pressure red flag.
⚠Classifying fine soil without both grain size and Atterberg limits.