In order to study the mechanical behavior of soil-structure interface under the constant stress increment ratio stress paths, the simple shear tests of interface between high plasticity clay and concrete face were carried out, from the tests, the stress-strain relation curves under different stress paths were obtained. Five main phenomena were found, firstly, the high plasticity clay could well suitable to large deformation, and had strain-hardening behavior obviously, as well as, the little dilatancy; secondly, there was no correlation between stress paths and shear strength but the stress-strain relations were influenced by stress paths; thirdly, the little stress increment ratio, the higher increase speeding of shear stress and breaking strength under the condition of same initial normal stress; the fourth, the relations between normal strain and normal stress in simple shear test was consistent with which in the single-direction compression test, if there was no dilatancy; the last, the curve of relation between stress ratio and shear strain could be well simulated by hyperbola. Based on the test result and generalized potential theory, a bi-potential surface elasto-plastic model for soil-structure interface was created, in which the soil-structure interface problem was regard as a two-dimensional mathematical question in stress field, and plastic state equations were used to replace the traditional field surface. Studying shown that the proposed model which could reflect the infection of stress paths and initial normal stress on mechanical behavior of interface had a universal character and could be used in FEM, so it was good to be promoted and applied in the interface studying field.
