A critical state model for saturated and unsaturated interfaces

Based on the concept of independent stress state variables to consider the impact of unsaturated conditions, an elasto-plastic critical state constitutive model for saturated and unsaturated interfaces is introduced in this paper. The proposed model is capable of predicting many characteristics of unsaturated interface behavior, such as the dependence of initial tangent modulus, peak shear stress, dilatancy, and ultimate strength on matric suction, net normal stress, and the interface state measured with respect to the critical state line. To this aim, two distinct yield mechanisms are employed in the model. While change in stress ratio generates plastic deformation in the first mechanism, plastic deformations are due to an increase in net normal stress, decrease in matric suction, or both, in the second mechanism. The presence of appropriate state dependent ingredients enables the model to provide realistic predictions over a wide range of variations of density, net normal stress, and matric suction. By direct comparison of the model predictions with experimental data, the predictive capacity of the proposed model is evaluated.