This standard details the laboratory methods for determining the liquid limit and plastic limit (Atterberg limits) of fine-grained soils. These parameters are essential for soil classification, estimating the plasticity index, and predicting the engineering behavior of soils under varying moisture conditions.
Describes methods for determining the liquid limit and plastic limit of fine-grained soils.
Quick Reference Values
Sieve size for soil preparation425-micron IS Sieve
Casagrande apparatus drop rate2 revolutions per second
Liquid limit standard blows25 drops
Standard groove closure length12 mm
Plastic limit thread diameter3 mm
Oven drying temperature105 to 110 °C
Key Formulas
PI = LL - PL — Plasticity Index
If = (w1 - w2) / log10(N2/N1) — Flow Index
Practical Notes
Soil must be thoroughly mixed with distilled water and allowed to mature (curry) for a sufficient time (up to 24 hours for highly plastic clays) to ensure uniform moisture distribution.
When rolling threads for the plastic limit test, apply just enough pressure to reduce the soil thread diameter to exactly 3 mm before it crumbles.
The Casagrande apparatus base and cup should be strictly calibrated and checked for wear, as variations can significantly alter the liquid limit blow count.