Research On The "Sudden Drop" In The Critical Shear Stress Ratio Of Clay And Its Influence

Clay under high-pressure working conditions has relatively small pores e in its critical failure state.Previous experimental research and relevant literature analysis by the research team have shown that when approaching a small boundary value,the critical shear stress ratio M of clay may significantly decrease within a narrow range,indicating a "sudden drop" in shear impedance.However,the experiment that revealed this phenomenon itself has shortcomings and cannot guarantee the objectivity and universality of the phenomenon of critical shear stress ratio M "sudden drop" in clay.To further verify,this article takes the deep clay in East China as the research object in the above experiments.Firstly,in order to address the "false phenomenon" caused by the inappropriate shear rate and end constraints that may cause uneven distribution of pore water pressure inside the soil sample,triaxial undrained shear test simulation and direct shear test simulation under different stress levels are carried out,To verify the objectivity of the phenomenon of critical shear stress ratio M "sudden drop";Secondly,taking other types of clay different from the above tests as the research object,theoretical formulas are first used to predict the boundary pore ratio,and then direct shear tests are carried out from low stress levels to high stress levels centered on the consolidation stress corresponding to the boundary pore ratio,in order to reveal the universality of the phenomenon of critical shear stress ratio M "sudden drop";Finally,taking the stability analysis of deep drilling hole walls as the research object,the impact characteristics of overestimated Mvalues on the analysis results are explored,providing reference for engineering design.The main research content and achievements of this article are as follows:Firstly,based on ABAQUS finite element software,a seepage stress coupled isotropic consolidated triaxial undrained shear test model was established.The SCLAY1 constitutive model considering induced anisotropy(M values determined by atmospheric pressure tests)was used to simulate undrained shear tests under different end constraints and shear rate conditions within the range of 0.2～6.4 MPa;The simulation results indicate that the end friction and shear rate in the previous experiments of the research group have an impact on the distribution of pore water pressure and q-εa The influence of curve a is minimal and will not simulate and predict smaller values determined by high-pressure tests.This indicates that the "sudden drop"ofMphenomenon in the early triaxial shear test is not a "false phenomenon" caused by improper shear rate and end friction,but an objective existence.Secondly,establish a seepage stress coupling direct shear numerical model,and address the significant shear distortion of the shear band by setting gaps between the upper and lower shear boxes to meet the needs of studying the effects of shear rate,permeability coefficient,and other factors on determining shear strength.Conduct direct shear test simulations under different shear rate conditions within the range of 0.2～6.4 MPa.The simulation results show that the remolded deep clay studied in this thesis,Under a consolidation pressure of 1.6 MPa,its permeability coefficient has decreased to 10-12 m/s.Using a 0.5 times recommended upper limit slow shear rate,the obtained internal friction angle is close to the measured undrained shear index;As the shear rate increases,the internal friction angle determined by the simulation results decreases,and becomes more pronounced as the consolidation pressure increases or the permeability coefficient decreases.This is because faster shear rates and lower permeability coefficients increase the excess pore water pressure on the shear band;The envelope of shear strength under different shear rates has good linearity,indicating that the internal friction angle is a fixed value,and will not simulate and predict the smaller M value determined by high-pressure test.It is thus confirmed that the "sudden drop"of M phenomenon in the previous direct shear test is not a "false appearance" caused by improper shear rate,but an objective existence.Then,selecting new clay for basic physical parameter measurement and using an improved consolidation pressure chamber for K0 test research,combined with theoretical formulas to predict the boundary pore ratio and consolidation stress,a total of six pressure levels of direct shear tests were carried out from low stress to high stress levels.The results show that the boundary void ratio calculated by the theoretical formula is 0.5786,corresponding to a consolidation pressure around 3.0MPa;When the consolidation pressure is higher than 3.0MPa,the compression index of saturated clay is significantly lower than that of consolidation pressure lower than 2.0MPa.The compression curve can be represented by segmented double lines,with the boundary point between 2.0～3.0MPa;The shear strength of saturated clay increases with the increase of vertical pressure.When the consolidation pressure is higher than 3.0MPa,the internal friction angle is significantly smaller than when the consolidation stress is lower than 2.0MPa.The two are 8.95° and 11.16°,respectively,with a difference of 2.21°.The shear strength envelope can be represented by segmented double broken lines,and the intersection point of the double broken lines is within the range of 2.0MPa～3.0MPa.The decrease in internal friction angle of new clay reveals that the M"sudden drop" in critical stress ratio is not an accidental phenomenon and has a certain universality.Finally,taking the stability analysis of deep borehole walls as the engineering object and studying the impact characteristics of overestimated M values on the analysis conclusions as the research objective,a numerical model for borehole unloading is established based on the MCC and S-CLAY constitutive models;The simulation results indicate that when taking low pressure of M test values,the influence of boundary effects can be ignored when the soil radius is 10 times the drilling radius,which is 10.The plastic zone range of soil in both MCC and S-CLAY1 models is[0,1.21 a0];When taking high pressure test values of M,the soil radius should be taken as 20 to ignore the influence of boundary effects;The plastic zone range of the MCC model is[0,1.62 a0],while the plastic zone range of the S-CLAY model is[0,2.45 a0];For the MCC model,the hole wall shrinkage when taking high pressure test values of M is 6 times higher than when taking low pressure test values of M;For the S-CLAY model,the hole wall shrinkage when taking high pressure test values is 20 times higher than when taking low pressure test values.The above indicates that of M when the soil is at a high stress level,overestimated values have a significant impact on actual engineering problems and are worthy of attention.