Study On Dynamic Constitutive Relation Of Saturated Clay Based On Elastoplasticity Bounding Surface Theory

Many offshore engineering structures,such as pile foundations,suction anchors,and large-diameter bucket foundations,need to be built on the seabed of saturated clay.The clay ground are subjected static loads from structures as well as cyclic loads from earthquake or waves.It is very important to evaluate the deformation stability of the foundation under the atrocious marine environment.Therefore,the constitutive relationship of saturated clay under cyclic loading was studied in this paper.The main work includes the following aspects.The incremental elastic-plastic constitutive model was developed based on the bounding surface theory and the existing research for describing the stress-strain response of saturated clay under cyclic loading,and then was extended to three-dimensional stress space according to the generalized Mises criterion.The model is mainly composed of bounding surface equation,anisotropic hardening criterion,radial mapping rule with movable mapping center,associated flow rule and incremental elastoplastic stress-strain relation.In this model,a parameter was introduced into the isotropic hardening relationship of bounding surface to describe the effect of loading rate on cyclic stress-strain response;the failure line involving cohesion was adopted to reflect the influence of cohesion due to consolidation on the strength of clay.In view of the fact that all model parameters were calibrated as constants in previous bounding surface models,it is difficult to quantitatively describe cyclic stress-strain responses of soils under different stress levels.The variation of model parameters with stress levels is emphatically analyzed,and the relationship between model parameters and static stress ratio and cyclic stress ratio was established.Using the bounding surface model and the calibrated model parameters in this paper,the static and the cyclic triaxial test results of k₀consolidated undrained saturated clay were simulated.The results show the stress-strain relationship of soil are well predicted for static triaxial compression specimens under different confining pressures;the stress-strain and the accumulative strain-cyclic number relationship predicted by the model are in good agreement with the test results of cyclic triaxial specimens with different static stress ratios and cyclic stress ratios.In order to further verify the applicability of the model and model parameters calibrated according to triaxial test results for predicting stress-strain responses of soil elements under different stress states,the static and cyclic torsional shear tests with different stress paths and different stress levels were carried out.Then the torsional shear test results were predicted.The comparison between the predicted results and the test results were made.The results show that the constitutive model established in this paper and the model parameters determined based on triaxial test can be well used to predict the stress-strain response of static torsional shear test,and can be also used to predict the axial and circumferential stress-strain response of cyclic torsional shear test with complex stress paths.