Liquid and plastic limits of clays by cone method

Abstract

Undrained shear strength of a soil at the liquid limit water content can be considered to be around 1.7 kPa according to several researchers. Plasticity index of soils has been defined by one school of thought as a range of water content producing a 100-fold variation in their undrained shear strength. This has led to the redefinition of the plastic limit as the water content at which undrained shear strength is 170 kPa. The undrained shear strength-water content relationship has been found to be linear in the log–log plot for a wide range of water contents beginning from around the plastic limit to near the liquid limit. Normalization of undrained shear strength—water content relationship in a log–log plot has led to the conclusion that the water content at the liquid limit and at the plastic limit, obtained by cone penetration, could also be uniquely related. This contradicts the original understanding of Atterberg limits, namely liquid and plastic limits which are two independent parameter not related at all. It is more suitable to call this value of plastic limit from cone method as PL 100 to differentiate it from Casagrande’s plastic limit.