Within the critical state soil mechanics framework , the two-surface formulation of plasticity is... more 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 de®ne the peak and dilatancy stress ratios of sand. The model is capable of realistically simulating stress±strain behaviour of sands under mono-tonic 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 con®ning pressures of engineering relevance for a given sand. The numerical implementation of the model is particularly easy and ef®cient 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 geotech-nical engineering community, with basic reference to critical state soil mechanics.
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