Characteristics and development of Tsinghua Elasto-plastic Model for soil

Unlike other elasto-plastic models of soil,Tsinghua Elasto-plastic Model was developed in a different way,in which the plastic potential surface and the function of the model was directly determined from experimental data without any other assumptions.According to the Drucker’s postulation,the yield function was identical with plastic potential function by selecting proper hardening parameter.The yield locus of the model,formed by 2 circular arcs on π plane,was developed by using true triaxial tests and plane strain tests.A three-dimensional Tsinghua Model was then proposed and the three-dimensional flow rules were presented.Based on the wetting test results of Xiaolangdi rock fill,it was found that as a type of plastic strain,the wetting strain vector was normal to the yield locus of the model.Provided that the model parameters of dry soil and saturated soil and the wetting volumetric strain in isotropic compression test were determined,the whole stress-strain relationship during wetting of the sample could be simulated by the model.An unsaturated soil model was also developed following the similar approach to the wetting Tsinghua Model.In the triaxial test of unsaturated soil,some ice crumbs were mixed into the dry sample before test and the ice crumbs were melted at determined stress states.From the test results it was found that yield surfaces of soil with different moisture contents were the same and hardening parameter was the function of plastic strain and Moisture content.In the Tsinghua Model for unsaturated soil,the strain increment produced by increments of stress and moisture content could be calculated conveniently without appearance of matrix suction.The whole stress-strain relationship in triaxial tests with increasing moisture contents was predicted by the model reasonably.Additionally,based on the behavior observed in triaxial compression tests with dense Yongding River sand,the model was still developed to simulate strain-softening characteristics of the sand,in which the hardening parameter was assumed to be the function of plastic work.The model was verified in computing the displacement vs.load of a shallow footing on dense sand by hydraulic gradient model tests.If the rotational kinematic hardening is applied in the model instead of isotropic hardening,it could be used to model the soil behavior with large stress reversals,by which stress strain relationship during load decreasing or cyclic loading can be simulated.It is thought that Tsinghua Elasto-plastic Model is a promising model for soil.