This standard prescribes the methods for determining the liquid limit and plastic limit (Atterberg limits) of soils. These limits are essential for classifying fine-grained soils and evaluating their engineering properties such as compressibility, shear strength, and shrink-swell potential.
Specifies methods for determining the liquid limit and plastic limit of soils, which are key Atterberg limits.
Quick Reference Values
sieve size for sample preparation425-micron IS Sieve
standard blows for liquid limit25 blows
blow rate for casagrande device2 revolutions per second
thread diameter for plastic limit3 mm
cone penetration for liquid limit20 mm
oven drying temperature105 to 110 °C
Key Formulas
Plasticity Index (PI) = Liquid Limit (LL) - Plastic Limit (PL)
Flow Index (If) = (w1 - w2) / log10(N2 / N1) — slope of the flow curve
Practical Notes
Ensure the soil paste is allowed to mature (cure) for at least 24 hours before testing, particularly for highly plastic clays, to ensure uniform moisture distribution.
The Casagrande device must be mounted on a firm, unyielding surface to prevent loss of blow energy which can skew results.
When rolling the soil thread for the plastic limit test, apply uniform pressure to prevent premature crumbling before the exact 3 mm diameter is reached.