In this research, using of classical plasticity and critical state framework a unified two-surface constitutive model developed to simulate nonlinear behavior of various soil such as loose and dense sands, normally consolidated and over consolidated clays, during different kinds of monotonic loadings. The model describes strain-softening and stress-dilatancy with using non-associated flow rule that implements dilatancy parameter. The comparison of experimental data with the results obtained from numerical simulation have shown high performances of the proposed model to simulating drained and undraind responses of soils under in triaxial as well as soil response under drained laterally confined (K 0 ) condition. In the second part of the research a simple and efficient bounding surface plasticity model taking account to fabric change was used to simulate cyclic behavior of saturated sands. Having the model modified, the new model can be used to predict the soil liquefaction phenomenon under earthquake loading. Should this model be implemented in a FEM model, hysteretic damping and stiffness degradation which have very important role on soil-structure interaction under cyclic loading, could accounted accurately.
