Re-examination of Undrained Strength at Atterberg Limits Water Contents

Most of the testing procedures to determine liquid limit and plastic limit are strength based with the assumption that, irrespective of the soil type, the strengths at these limiting water contents are considered to be unique, being equal to 1.7 and 170 kN/m², respectively. Based on this, the plastic limit has been redefined as the water content at which there is a 100-fold increase in undrained strength as compared to that of liquid limit water content, and the range of water contents producing this strength variation as the plasticity index. However, published data from the various literature sources clearly show that the variation of undrained shear strength at the liquid limit water content is observed to be nearly sixty times, and that at plastic limit is more than seventeen times, and hence, no unique value of undrained strength can be assigned either at the liquid limit or plastic limit of soils. The variation of undrained strength with water content has been well documented in literature. Thus, uniqueness of strength at liquid limit or plastic limit, which is nothing but water holding capacity of soils at different state of consistencies, is not tenable.