A new three-dimensional cone-cap limit state surface yielding and kinemtic hardening model is proposed for fully and partially saturated soils. Only the modified net stress defined by a variable air pressure and constant air entry suction is taken as the stress variable, and thus the transition between saturated and unsaturated conditions takes place without discontinuity, and numerical integration for constitutive equation becomes simple as that of saturated soils. The nonlinear behavior of the normally consolidation curve of the partially saturated soil in specific volume versus log modified net stress plane can be considered in the hardening law coupled with SWCC model. Matsuoka-Nakai failure criterion equations are adopted as the cone yield functions, and a new cap associated with Matsuoka-Nakai cone is developed accordingly. A non-associated flow rule is considered. Parameters involved in the proposed model can be simply determined from laboratory oedometer compression and SWCC experiments.
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