A Modified Two-Surface Plasticity Model for Saturated and Unsaturated Soils

A modified two-surface critical state plasticity model for saturated and unsaturated soil is presented in this study. The key modification in new model is inclusion of an alternative yield surface used to simulate the behavior of unsaturated soils in addition to corresponding saturated conditions. Moreover, a numerical technique is used to obtain an incremental stress–strain response from loading curves. Modification is applied continuously in each incremental step to return the final stress states and hardening parameters to the yield surface. Results revealed that the adopted modeling approach can predict two independent sets of laboratory unsaturated experiments under various conditions satisfactorily. The model can well predict the augmentation of shear strength due to a rise in suction under different net stress levels. In addition, the influence of net stress on volumetric tendency was reliably simulated under drained conditions. The distinct feature of this model in capturing the suction-induced hardening and volumetric transition from dilation to contraction should be highlighted. It is also important to note that according to the results of hypothetical simulations, the stress–strain behavior of saturated coarse-grained soils can be rationally captured for a wide range of confining stresses and densities under both drained and undrained conditions.