Approaches To The Triple-Shear Unified Elasto-plastic Constitutive Models For Unsaturated Soils

Most of the soil is in an unsaturated state under natural conditions,and the unsaturated nature of the soil has a great influence on the mechanical properties of the soil.Firstly,the single stress variable method and the double stress variable method are used to generalize the triple-shear unified yield criterion from saturated soil to unsaturated,and the three-shear uniform yield criterion of unsaturated soil is obtained.Then,from the actual stress state of unsaturated soil,the matrix suction is introduced into the modified Cambridge model,and the modified Cambridge model of unsaturated soil is derived.However,the modified stress ratio of the modified Cambridge model of unsaturated soil is a fixed value,which cannot reflect the change of stress state or reflect the unsaturated soil.In order to solve this problem,the triple-shear unified yield criterion of unsaturated soil is used as the failure criterion of the new constitutive model,and the triple-shear unified elastoplastic constitutive model of unsaturated soil is established.Both the single stress variable method and the double stress variable method for the triple-shear unified yield criterion of unsaturated soils consider the influence of cohesive force c and matric suction of unsaturated soils,and are more in line with the mechanical properties of unsaturated soils.The model is also three-dimensionally processed to reflect the three-dimensional stress-strain characteristics of unsaturated soils.The non-saturated remolded red soil in Jiangxi Province was used as the test soil sample,and the basic soil parameters and model parameters were obtained through basic geotechnical tests and triaxial tests of unsaturated soil.The relationship between shear stress and axial strain,volume strain and axial strain of unsaturated laterite is obtained,which emphasizes the influence of unsaturated on the strength and deformation of soil.Using the Fortran programming language,a subroutine of the shear unified constitutive model of unsaturated soils is written.The relationship between the stress and strain of the soil under unsaturated triaxial test and true triaxial test and the variation law are simulated.The simulation results of the single stress variable method and the double stress variable method are compared with the experimental results to verify the applicability and rationality of the newly established constitutive model.The influence of the intermediate principal stress influence coefficient b on the constitutive model is also studied.First,the domestic and international status and research methods of constitutive models of unsaturated soils are evaluated and summarized.The focus is on how to deal with the unsaturated properties of unsaturated soils,such as the consideration of matric suction,water content considerations and saturation.Secondly,the single stress variable theory of Bishop unsaturated soil and the double stress variable theory of unsaturated soil such as Fredlund are combined with the triple-shear unified yield criterion to apply the triple-shear unified yield criterion to unsaturated soil.The stress state,represents the failure stress ratio of the single stress variable method of unsaturated soil and the failure stress ratio of the double stress method of unsaturated soil.Finally,the triple-shear unified yield criterion of unsaturated soils established by the two methods was verified by the relevant true triaxial test.Third,in order to apply the modified Cambridge model to unsaturated soil,the yield stress of the saturated soil in the modified Cambridge model is replaced by the yield stress of the unsaturated soil,and the matrix suction is taken as the effective water pressure and considering the matrix suction.The influence of soil cohesion,and the single stress variable method and the double stress variable method three shear uniform yield criterion as the failure criterion,combined with the single stress variable method,the triple-shear unified yield criterion failure stress ratio,so as to obtain the influence of stress state and matrix suction change.the triple-shear unified yield criterion elastoplastic constitutive model of unsaturated soils with single stress variable method and double stress variable method.Furthermore,the three-dimensional processing method of the constitutive model is proposed,which can express the stress-strain relationship of the unsaturated soil under the six-dimensional stress state.Finally,the specific elastoplastic stiffness matrix of the constitutive model is derived.Fourthly,Jiangxi remolded red clay as the experimental research object,in order to obtain the basic mechanical parameters of the red soil,a series of physical and mechanical experiments were carried out.The model parameters required for the numerical calculation of the newly established constitutive model are obtained.Fifth,use Fortran language to write the program of the newly established constitutive model theory solution.It is used to simulate the relationship between shear stress and axial strain and the relationship between body strain and axial strain of soil under different matrix suction and different net confining pressure in unsaturated soil in triaxial drainage experiment.The model theory solution obtained by the single stress variable method and the double stress variable method is compared with the results of the unsaturated triaxial experiment.It can be seen from the comparison of various mechanical property simulations that the double stress variable method model is more consistent with the single stress variable method model.The mechanical properties of saturated soil are practical.By comparing the simulated value with the experimental value,it is proved that the single stress variable method model and the dual stress variable method model have good effects on simulating the mechanical properties of unsaturated soil,and the rationality of the constitutive model established by the two methods is verified and analyzed.The influence of matrix suction on the mechanical properties of the triple-shear unified yield criterion constitutive model of unsaturated soils.The true triaxial test of the triple-shear unified yield criterion constitutive model of unsaturated soil is also simulated,and the simulated values of the single stress variable method and the double stress variable method are compared to analyze the mechanics of unsaturated soil under the condition of true triaxial test.Characteristics,the stress-strain relationship diagram of unsaturated soil under different matrix suction conditions,the variation law of the constitutive model established by two different methods are basically the same,and the influence of the intermediate principal stress influence coefficient b on the constitutive model is studied.