A unified constitutive model for both clay and sand with hardening parameter independent on stress path

A unified constitutive model for both clay and sand under three-dimensional stress conditions is derived from the modified Cam-clay model, by taking the following two points into consideration. First, a transformed stress tensor based on the SMP (spatially mobilized plane) criterion is applied to the Cam-clay model. The proposed model consistently describes shear yielding and shear failure and combines critical state theory with the SMP criterion for clay. Secondly, a new hardening parameter, which is independent of the stress path, is derived in order to develop a unified constitutive model for both clay and sand. It not only describes the dilatancy for lightly to heavily dilatant sand, but also reduces to the plastic volumetric strain for clay. The validity of the hardening parameter is confirmed by the test results of triaxial compression and extension tests on sand under various stress paths. Only five conventional soil parameters are needed in the proposed model.