A critical state two-surface plasticity model for sands

Within the critical state soil mechanics framework, the two-surface formulation of plasticity is coupled with the state parameter to construct a constitutive model for sands in a general stress space. The operation of the two-surface model takes place in the deviatoric stress-ratio space, and the state parameter is used to define the peak and dilatancy stress ratios of sand. The model is capable of realistically simulating stress-strain behaviour of sands under monotonic and cyclic, drained and undrained loading conditions. It includes features such as the softening of sands at states denser than critical as they dilate in drained loading and softening of sands looser than critical in undrained loading, and the pore-water pressure increase under undrained cyclic loading. Most important, all these simulations are achieved by a unique set of model constants at all densities and confining pressures of engineering relevance for a given sand. The numerical implementation of the model is particularly easy and efficient due to the very simple formulation. Calibration of model constants is done straightforwardly on the basis of triaxial experiments and measurements of well-known characteristics of sand stress-strain behaviour. Possibly the most attractive feature of the model is its simplicity and its foundation on concepts and data which are well established and understood by the geotechnical engineering community, with basic reference to critical state soil mechanics.