Experimental And Bounding-surface Modeling Study On The Static And Dynamic Behavior Of Soft Clay Considering The Effects Of Cyclic Loading And Reconsolidation

Soft clay with high water content,large void ratio and low strength can be generally found in Chinese coastal areas.Such clay applied as the foundation of structures are always subjected to the long-term cyclic loading.The stability of the soft clay foundation under cyclic loading such as wind waves is one of the most important issues needed to be solved in the practical engineering.In the previous studies on the post-cyclic shear behavior of soft clay,the most common experimental method was to apply the completely undrained conditions.A few scholars carried out some experimental studies on the condition of full reconsolidation after cyclic loading.However,in practical engineering,the clay beneath the structure is actually between the states of the completely undrained condition and the condition of full reconsolidation after cyclic loading.The tests with full and no reconsolidation cannot demonstrate the post-cyclic shear characteristics of clay in practical engineering.In addition,the real marine clay is subjected to an alternating effects of the wave load and reconsolidation process.However,there is no research on the shear behavior of the clay under the effects of multiple cycling-reconsolidation.Moreover,the previous strengthdegradation bounding-surface model is often achieved by introducing the parameter of plastic deviator strain length in the hardening criterion.Although this model can predict the cyclic failure of clay,the post-cyclic strength degradation cannot be reasonably evaluated when the cyclic stability is achieved.Besides,the shear strength after multiple cycling-reconsolidation cannot be effectively predicted by the model.Therefore,in this paper,a large number of static and dynamic triaxial tests were carried out on Yantai Port silty clay.The effects of different degrees of reconsolidation and multiple cyclingreconsolidation process on the dynamic and shear characteristics of clay were investigated under complex loading conditions.Based on the traditional no-elasticregion bounding-surface model,a model which can reasonably consider the strength degradation of the clay and predict the shear strength after multiple cyclingreconsolidation process was proposed.The modified model was developed in ABAQUS to make it possible to be applied in finite element numerical analysis.The research content and conclusions of this paper are as follows:(1)A series of dynamic triaxial tests with different cyclic stress ratios and different confining pressures have been carried out.It demonstrates that a critical dynamic stress ratio is existed for Yantai port silty clay.The state of cyclic failure and cyclic stability can be achieved when the cyclic stress ratio is above and below respectively.Besides,the characteristics of accumulative plastic strain,accumulative cyclic pore pressure,dynamic elastic modulus and dynamic strain amplitude of the soil above and below the critical dynamic stress ratio was further investigated.Based on the test results,a dynamic strength model which can consider the effect of critical stress ratio on silty clay was established.(2)By changing the back pressure of the soft clay during the post-cyclic reconsolidation process,the post-cyclic reconsolidation degrees of the soil were controlled,based on which the shear behavior of silty clay were investigated.Three kinds of tests including no reconsolidation,full reconsolidation and different reconsolidation degrees were conducted to investigate the shear characteristics of soil.The effects including confining pressure,cyclic stress ratio,consolidated deviator stress ratio and reconsolidation degrees were considered.Based on the Yasuhara equivalent overconsolidation model,a semi-empirical shear strength prediction model considering the different post-cyclic reconsolidation degrees was established.The model prediction results are in good agreement with the experimental results,which verifies the validity of the model.(3)The experimental investigation on the cyclic and shear characteristics of silty clay after multiple cycling-reconsolidation were carried out,considering the effects of confining pressure,dynamic stress ratio,number of cycling-reconsolidation process,and degrees of reconsolidation and so on.Considering the influence of multiple factors,the effect of multiple cycling-reconsolidation process on the dynamic characteristics including plastic strain,cyclic accumulative pore pressure,dynamic elastic modulus and cyclic dynamic strain of soft clay,and shear characteristics including the post-cyclic shear stress path,stress-strain relationship and the shear strength.It is found that after a certain number of cycling-reconsolidation process,the dynamic modulus,cyclic accumulated axial strain and shear strength tend to be stable and will not grow.(4)A bounding-surface model which can consider the post-cyclic strength degradation was established by introducing a variable of cyclic memory parameter based on the traditional bounding-surface model with no elastic region.The model can consider the contraction of the bounding-surface during the loading and unloading process,so that the strength degradation can be reasonably evaluated after the cyclic loading.In addition,a bounding-surface model which can consider the effects of multiple cycling-reconsolidation process on the shear strength was established by introducing a parameter related to the position in the stress plane of the stress point.By comparing the model prediction results with the experimental results,the validity and correctness of the model were verified.(5)Using the user-defined subroutine Umat interface of ABAQUS,the fully implicit radial return algorithm was adopted,and the numerical implementation of the modified bounding-surface model in the finite element calculation was achieved.It can be applied in finite element dynamic analysis to facilitate the calculation in the dynamic finite numerical element analysis of soil-structure interaction.This model was used to simulate the laboratory model experiments of Das and Shin by finite element numerical simulation,which verified the validity of the model and the effectiveness of the development of the model in the ABAQUS.