Abstract：In classical soil mechanics，dilatancy is often considered as a monodrome function of the stress ratio for triaxial tests，then plastic flow direction is decided by uniquely and independent of material¢s internal state. This obviously contracts the observed facts. For two different examples of a kind of sand，one in loose state and the other in dense state，the loose sand contracts while the dense sand dilates when they are subjected to the same shear stress. These two different responses are corresponding to a same ，however，the dilatancies are different，one being positive and the other being negative. If the dilatancy is supposed as a monodrome function of ，the dissimilar characters for cohesionless soil in various initial densities and consolidation pressures cannot be simulated by a unified model. On the other hand，Drucker Postulate cannot satisfy all the conditions for soils，and a new energy dissipation mechanics is needed as the base of constitutive relation. The first and second law of thermodynamics，as the universal rules in the nature，can be invoked to describe the energy dissipation. Starting with this point，the corresponding yield locus and flow rule may be deduced in a systematic manner in terms of the free energy function and dissipation potential function. Within the framework of critical state soil mechanics，a modified model that combines the theory which treats the dilatancy as a state-dependent quantity with constitutive models is presented based on the first and second law of thermodynamics. This model accounts for the dependence of dilatancy on the material internal state during the deformation history，and guarantees the consistence of the dilatancy function with the yield locus because of the thermodynamics basis involved. The simulative capability of model is shown by computing different features under various initial densities and confining pressures in sand triaxial undrained shear test，using the modified model with a unified set of model parameters and the hardening rule suggested.
