State-dependant dilatancy in critical-state constitutive modelling of sand

A bounding-surface hypoplasticity model is modified to incorporate the basic premises of critical-state soil mechanics and cover both dense and loose sand behavior. The modification consists of rendering the phase-transformation line a function of the state parameter, which measures the difference between the current and critical void ratios at the same value of p, such that when the state parameter is zero, the phase-transformation line becomes identical to the critical-state line in q-p space. As a result the dilatancy depends on the state in a way that yields a zero value at critical state. This dependence allows a realistic modelling of the response of a sand in either loose or dense state, or in the transition from one state to another state. A comparison between model simulations and a sequence of experimental results for drained, undrained, monotonic, and cyclic loading conditions shows that the proposed concept and modelling technique work effectively over a wide range of densities and confining pressures using a unique set of parameters (or parameter dependence) for a given sand.Key words: bounding surface, critical state, dilatancy, phase transformation, soil plasticity, state parameter.