Abstract: In this paper, the loading wet yield surface (LC yield surface) in the SFG model and the suction increasing yield surface (SI yield surface) proposed in the BBM model are unified into a single smooth yield surface. The corresponding elasto-plastic constitutive model is derived under the framework of elasto-plastic theory. The model calculation parameters of Jiangxi red clay were determined by unsaturated consolidation direct shear test. The calculation parameters are entered into the model program and the stress path of the unsaturated triaxial test is calculated by the program and compared against experimental results in a published paper. Based on stress-strain curves calculated from the comparison model and test results, we draw a conclusion that the calculation results of the unsaturated triaxial test model under different stress paths are in good agreement with the experimental results. It is verified that the model can better simulate the volumetric strain behavior of unsaturated soil under various stress paths than other models. Uniform yield surface simplifies the coupling analysis between loading wet yield surface and suction increasing yield surface. Our model solves the problem of inconvenience caused by the discontinuity of the double yield surface, and broadens the applicability of the SFG model.

Key words: unsaturated soil, SFG constitutive model, uniform yield surface