The strength and deformation characteristics of rockfill materials are related to the initial void ratio and confining pressure. In this study

an elastoplastic constitutive model for rockfill materials is established to predict the mechanical response of rockfill materials under different confining pressures and initial densities. Based on the framework of the Cambridge-type models

the proposed model can account for the transition from dilation to contraction shear response of rockfill materials as the void ratio or the confining pressure increases. The proposed model adopts a micromechanically-based yield function to characterize the evolution of the microstructure with the non-associative flow. A new hardening parameter is derived based on the fact that the virgin compression curves are not unique for rockfill materials. To characterize the state-dependent behavior

a unique critical state surface of rockfill material is assumed. The mathematic requirements for a general Cambridge-type constitutive model are summarized by considering the state dependence; furthermore

both the dilatancy equation and the hardening parameter are modified. A particle swarm optimization method is established to determine the model parameters

and the performance of the proposed constitutive model is verified by comparing with experimental results.