Atterberg Limits

The Atterberg limits are the water contents at which a fine-grained soil changes consistency state — the liquid limit (LL, soil flows), the plastic limit (PL, soil can no longer be rolled into a 3 mm thread) and the shrinkage limit (SL, no further volume change on drying). They are determined per IS 2720 Part 5 (LL by Casagrande cup or cone penetrometer, PL by thread-rolling) and Part 6 (SL).

They are the foundation of soil classification (IS 1498), correlating with compressibility, swelling, strength and the suitability of a soil as subgrade or fill. A high liquid limit signals a highly plastic, compressible clay (e.g. black cotton soil, LL often > 50). The plasticity index PI = LL − PL feeds the IS 1498 plasticity chart that names the soil (CL, CH, ML, MH etc.) and predicts behaviour like swell potential and CBR.