Critical appraisal of the cone penetration method of determining soil plasticity

Abstract

Plasticity is a characteristic feature that all plastic soils possess. It is shown that the soil plasticity is mainly due to undrained cohesion. Soil plasticity characteristics obtained from laboratory tests are reasonable for use only when the testing procedures adopted to determine them measure the factors responsible for the soil plasticity. It is shown that this is the case with the percussion method of liquid limit determination and the 3 mm thread rolling method of plastic limit determination. Further, it is also shown that the results obtained from the cone penetration method cannot represent the soil plasticity fully, as the mechanisms that come into play during testing relate to undrained strength due to both undrained cohesion and undrained friction. It is stressed that the percussion and 3 mm thread rolling methods must be the only ones used to determine the plasticity of soils. The circumstance under which the cone penetration method can be used is also indicated.