Approaches To The Triple-Shear Unified Bounding Surface Models For Unsaturated Clays In Normal Consolidation

The development of transportation has brought many conveniences to people,but it has also had an adverse impact on the natural environment:the track foundation soil has undergone irrecoverable cumulative deformation under long-term dynamic loads.Reviewing the relevant literature,scholars established few constitutive models based on natural unsaturated soil to solve the problem of soil deformation caused by static and dynamic loads in the past.This article conducts research based on this innovation.The research content is divided into the following five parts:(1)On the basis of the triple-shear strength theory,this paper introduces unsaturated soils single stress variable theory and double stress variable theory,proposes unsaturated soils triple-shear strength criterions of single stress variable theory and double stress variable theory,and carries out ? plane limit line analysis and meridian analysis.At the same time,based on the proposed criteria,this paper uses the equivalent substitution method and coordinate translation method to derive four kinds of failure stress ratios that can reflect the characteristics of intermediate principal stress effect,full stress state,and effective cohesion intercept,and replaces the failure stress ratio in the modified cambridge model with the derived one.Thereby,it overcomes the shortcoming of stable failure stress ratio in the modified cambridge model.(2)By replacing the yield stress of the saturated soil with the yield stress of the soil in the unsaturated state,the modified cambridge model of the unsaturated soil is obtained.Then,by substituting four kinds of failure stress ratios into the model,which are derived under single stress variables and double stress variables by using the equivalent substitution method and coordinate translation method,the corresponding four elliptic equations are further derived.Combined with the bounding surface model,the triple-shear bounding surface elastoplastic constitutive model of unsaturated soil is established,and the corresponding elastoplastic stiffness matrix and the specific expressions of each element in the matrix could be derived according to the elastoplastic theory.(3)This article takes unsaturated remolded red clay in a certain area of Nanchang as the experimental research object,and conducts relevant geotechnical test,compression rebound test and consolidation quick shear test to determine the required parameters in the model.Besides,unsaturated triaxial tests were performed under consolidated drainage conditions to obtain the stress-strain relationship of soil samples.At the same time,this paper uses Fortran software to program the stress-strain relationships of the four proposed models and obtains numerical simulation results.By comparing the simulation results with the experimental results,it is shown that the model has better applicability under static load.(4)According to the characteristics of unsaturated cyclic dynamic triaxial tests,the numerical calculation program is written on the Fortran software by using the four kinds of stress-strain relations established by the equivalent replacement method and the coordinate translation method under the single stress variable and the double stress variable.At the same time,the experimental data of unsaturated soils under cyclic dynamic loads in relevant literatures are extracted and compared with four numerical simulation data.The results also show that the model can better describe the stress-strain law of unsaturated soils under cyclic dynamic load conditions.(5)According to the established elastoplastic constitutive model,the corresponding true triaxial numerical simulation test program is written.Then,this paper studies the effects of changing the influence coefficient of intermediate principal stress and matrix suction on the stress and strain of unsaturated soils in a true triaxial monotonic loading simulation test.At the same time,the effects of changing minimum principal stress,intermediate principal stress,and amplitude on stress and strain of unsaturated soils are also studied in a true triaxial cyclic loading simulation test.Finally,the simulation effects of four elastoplastic constitutive models under true triaxial stress are compared and analyzed.