The middle surface concept and its application to the elasto-plastic behaviour of saturated sands

An elasto-plastic constitutive model for the monotonic behaviour of saturated sands under triaxial loading conditions is presented in this paper. This model is capable of simulating the behaviour of sands under different densities and confining pressures using a single set of model parameters. The critical-state concept and the state parameter concept for sands are used in developing the model. A novel modelling technique, named the middle surface concept, is proposed to properly incorporate the state parameter into the model within the framework of critical-state soil mechanics. The underlying concept of this new modelling technique is that the true stress-strain response is a combined effect of two pseudo responses. One pseudo response incorporates the concept of the state parameter and the other pseudo response incorporates the concept of the critical state. The true yield surface is defined by the two pseudo yield surfaces and lies in between them: hence the name 'middle surface concept'. By dividing the task between two pseudo yield surfaces, the problems faced by other constitutive models that use both the state parameter and critical-state concepts are avoided. The performance of the model under various conditions is first discussed. Comparisons between the model predictions and experimental results for Nevada sand and Toyoura sand are then presented; good agreement is achieved.