Direct Simple Shear Experiment On Cyclic Behavior Of Saturated Soft Clay Under Static And Cyclic Coupling Effect

Detailed prospecting of the construction zone goes first in the establishment of an offshore platform,which provides necessary perimeters for the design of offshore structures and energy development projects.Thus,it is of vital importance to investigate the dynamic response of marine deposit soft clay consolidating under k₀ condition.Direct simple shear tests achieve a better simulation of in-situ stress state because samples firstly undergo consolidation under k₀ condition.This paper explores the shear strength recorded during Direct Simple Shear tests and dynamic deformation characteristics of saturated marine soft clay with utilization of dynamic simple shear apparatus.Firstly,the paper gives a detailed introduction of the composition and functional characteristics of GDS Cyclic Direct Simple Shear apparatus,experiment principles and the reason why isometric drained test method is adopted.Then a 3D model of test sample is established in ABAQUS,according to the simulation test data to determine the stress state of the single shear test.Introducing SHANSEP method to study shear strength,this method apply to saturated soft clay of South China Sea.Several stress-controlled cyclic direct simple tests of different static stress ratios?0,0.25,0.5,0.75?and cyclic stress ratios are conducted to investigate the characteristics of simple shear strength saturated marine soft clay in combination of diferent static and cylic stress ratios.The experimental results show that,under the situation of no initial static stress,the cyclic strain inflection point can be obtained,which is a criteria of failure.There is also a threshold of cyclic stress ratio.The soil would never be destroyed when the dynamic stress ratio was less than the value.The cyclic strain would accumulate on the direction of initial strain when initial static stress exists and the accumulation accelerates with either larger static stress ratio or larger dynamic stress ratio.Particularly,dynamic stress ratio exerts a relatively predominant effect on accumulated strain and number of cycles to failure.In addition,the failure envelop when number of cycles to failure equals 1000 is given and a safe combination of static and cyclic stress ratio is offered in the paper.Through an analysis of the changing mode of index and parameter of degeneration,the strain-controlled cyclic direct simple test method is adopted to explore the effect of amplitude of cyclic shear strain,vertical consolidation pressure,over consolidation ratio and frequency on degeneration characteristics of saturated soft clay,and the changing rule of cyclic shear module and damping ratio is also investigated.It is indicated that there exists a threshold value of shear strain: the degradation index and degradation parameter show randomness when the cyclic shear strain amplitude is less than 3.2×10^-3.However,when the cyclic shear strain amplitude exceeds that value,less vertical consolidation pressure and smaller over consolidation ratio will lead to larger degradation index and accordingly,larger degradation parameter,that is,cyclic softening becomes more obvious.Also,it should be noted that over consolidation ratio exerts a predominant effect on soil softening among other factors,while frequency imposes little influence within the study.In addition,through a regression analysis of the tests,an empirical formula is proposed to predict the degradation of saturated soft clay when cyclic loads are applied.