Abstract: A constitutive model for unsaturated soils? coupling of the mechanical behavior and hydraulic hysteresis is proposed. The degree of saturation is introduced into the hardening equation to reflect the hydraulic effect. Meanwhile, the plastic strain is introduced into the soil water characteristic curve to reflect the mechanical effect. Therefore, the model could well reproduce the influence of hydraulic behavior on the stress-strain behavior and vice visa. An implicit integration algorithm for integration of the nonlinear constitutive model is introduced. Consistent tangent matrix that describes the relationship between stress and strain and the one that describes the relationship between stress and suction are both derived. A corresponding program code is developed, which can be then embedded into existing finite element code for numerical simulation. In order to verify the correctness of the code, it is used to simulate existing test data under different loading paths including drying-wetting cycle test and triaxial shear test. Results indicate that the model accurately predicts the coupling effect of mechanical and hydraulic behaviors of unsaturated soils.

Key words: unsaturated soils, constitutive model, implicit integration algorithm