Study On Ratcheting Behavior Experiment And Constitutive Model Of Clay Under Passive Confining Pressure

With the rapid development of highway in China,more and more attention has been paid to the traffic safety of highway.How to ensure the long-term dynamic stability of subgrade is the key issue in the design,construction,and post-maintenance.The long-term dynamic stability of subgrade mainly refers to the volume change and plastic deformation accumulation of subgrade soil subjected to the cyclic loading of vehicle in the design life cycle of highway,which leads to the cumulative additional settlement of subgrade.When the additional settlement of subgrade reaches a certain degree,the dynamic instability and collapse of subgrade will occur.In order to study the cumulative deformation of subgrade soil under cyclic loading,the ratcheting behaviors of clay with different moisture contents under different passive confining pressures were studied by the artificial remodeling of clay.Then,the subloading loading surface theory and modified Cam-Clay model are combined to establish a cyclic plastic constitutive model for simulating the ratcheting behavior of the clay under cyclic pressure.The research results can provide theoretical guidance for the analysis of subgrade cumulative settlement and related issues.The detailed contents of the research are as follows:(1)The clays with water contents of 16%,18% and 20% were artificially remolded,and the cyclic loading experiment with the passive confining pressure was carried out by using the developed passive confining pressure fixture.The results show that,the water content of 18% is the best,and the ratcheting strain evolution rate is the slowest;The ratcheting strain of clay is closely related to the upper and lower limits of loading,average force and force amplitude: the ratcheting strain decreases with the increase of loading lower limit,and increases with the increase of loading upper limit;the ratcheting strain increases with the increase of the applied average force and force amplitude;under the optimum water content,the ratcheting strain gradually reaches a stability state with the increase of average loading force.(2)By introducing the subloading surface theory,the framework of cyclic plastic constitutive model was established,and its material parameters c,X,u were studied.It is found that the size of hysteresis loop and ratcheting strain decrease with the increase of parameter c;When the parameter X increases,ratcheting strain increases;the parameter u can significantly affect the slope of stress-strain curve,and the slope is proportional to u.Then,the subloading surface theory was introduced into the modified Cam-Clay model,and the cyclic plastic constitutive model of clay was established.(3)By using the closest point projection and cutting plane methods,the numerical implementations of the proposed cyclic plastic constitutive model were carried out based on the implicit stress integration algorithm,respectively.By comparing with the simulated results of modified Cam-Clay model in ABAQUS,the accuracies of the numerical implementation of two algorithms were verified.It is found that the efficiency and accuracy of nearest point projection method which needs to calculate the partial derivative to the yield function are better than that of the tangent plane one.Furthermore,the ratcheting behavior experiments under different loading conditions were simulated.It is shown that the experimental results are in good agreement with the simulated ones,indicating that the proposed cyclic plastic constitutive model based on the subloading surface theory can be used to predict the ratcheting behavior of clay.