Dynamic Behavior And Constitutive Model Of Soft Clay Under Equivalent Earthquake Loading

In China,80%of the cities are located in the seismic fortification zone.The seismic research of underground engineering is of great significance to the development of large-scale urban underground space in China.In the seismic response analysis of site and structure,soft soil foundation has amplifying effect and filtering effect on earthquake loading.The site seismic damage will directly affect the stability and safety of the structure.In this thesis,the soft clay foundation and subway station structure were taken as the research objects,the shear strength,dynamic behaviors and dynamic parameters of soft clay under earthquake loading in different consolidation states were studied through monotonic triaxial,bidirectional cyclic triaxial,resonance column and bending element tests.By introducing the equivalent cohesion,the Mod-SANI04 constitutive model suitable for clayey soil has been improved.Through numerical simulation,the deformation and failure law of subway station structure in soft clay site under strong earthquake loading and the collapse process of subway station structure were analyzed.The main research contents and research results of this thesis include the following six aspects.(1)The undrained shear strength of the saturated soft mucky clay of layer No.4 in Shanghai was studied by monotonic triaxial tests.The effects of different consolidation states on the stress-strain relationship,stress path and excess pore pressure of soft clay were analyzed.With the consolidation stress increasing,the peak shear stress of the specimen increases.The critical state stress ratio of soft clay is 1.073.As the consolidation ratio increases,the deviatoric stress increment of the specimen decreases.With the increase of the overconsolidation ratio OCR,the peak strength of shear stress appears earlier and the strength value decreases.Based on the critical state theory and the introduction of the Skempton excess pore pressure coefficient,the critical state stress increment ratio was proposed,which can calculate the post strength of specimens with different consolidation ratios.The critical state stress increment ratio of soft clay decreases gradually with the increase of the consolidation ratio of the specimen,and increases with the increase of the excess pore pressure coefficient.The excess pore pressure of soft clay specimens increases with the increase of consolidation stress,decreases with the increase of consolidation stress ratio,and decreases with the increase of over consolidation ratio OCR.The excess pore pressure-shear strain development curves of soft clay specimens with different consolidation states can be well fitted by a hyperbolic model.(2)The principle and loading method for simulating earthquake loading by bidirectional cyclic triaxial test were analyzed.By controlling the amplitude and phase angle of the dynamic deviatoric stress and dynamic confining stress,the normal stress on theο/4 inclined section of the specimen remains unchanged and the shear stress changes alternately,thereby simulating the earthquake shearing effect.The dynamic strength test of soft clay specimens was carried out by bidirectional cyclic loading.The larger the amplitude,the fewer vibrations to reach the failure strain.Based on the soft clay failure strain standard,the equivalent shear stress amplitudes under different consolidation stresses were calculated by the critical stress ratio.Combined with the earthquake equivalent number of vibration for sand liquefaction,an equivalent simulation method of earthquake loading suitable for soft clay was proposed.The dynamic behaviors and post-earthquake shear strength of soft clay under equivalent earthquake loading were studied by bidirectional cyclic loading tests.The hysteresis curve of the specimen gradually develops to the right and becomes sparse with the increase of the equivalent number of vibration.The excess pore pressure increases in a horizontal direction with the development of shear strain.The post-earthquake shear strengths of specimens with different consolidation stresses decrease,while the post-earthquake critical shear strengths of specimens with different consolidation stress ratios increase.According to the variation of effective average principal stress,a calculation model of undrained shear strength of soft clay after earthquake was established.The anisotropic stiffness of soft clay was investigated by step-amplitude cyclic loading tests.The anisotropic stiffness ratio of the specimens increases with the increase of consolidation stress,and the average value is 1.25.(3)The torsional excitation test and the free vibration attenuation test were carried out on the soft clay specimen by resonance column apparatus.The variations of resonance frequency,dynamic shear modulus and damping ratio of soft clay under different consolidation states were analyzed.The resonance frequency of the specimen decreases with the increase of the excitation voltage,increases with the increase of the consolidation stress,decreases with the increase of the overconsolidation ratio OCR,and decreases with the increase of the overconsolidation ratio OCR_Ⅱ.The peak value of shear strain at the specimen resonance frequency is not necessarily the maximum value in the resonance curve.The shear modulus of the specimen decreases with the increase of shear strain,while the damping ratio increases with the increase of shear strain.Both of them increase with the increase of consolidation stress,decrease with the increase of overconsolidation ratio OCR,and increase with the increase of overconsolidation ratio OCR_Ⅱ.(4)The variation of the initial shear modulus of soft clay under different consolidation states was studied by bending element test.Considering the influence of the near-field effect,the slenderness ratio of the sample calculated with the P-wave reflection principle decreases with the increase of Poisson’s ratio.The initial shear modulus measured by the C-S method has a larger dispersion,while the results measured by the P-P method and the C-C method are relatively similar.The result measured by the S-S method is the largest.The initial shear modulus of the specimen increases with the increase of the consolidation stress,increases with the increase of the consolidation ratio,and decreases with the increase of the overconsolidation ratio OCR.Combining the three tests of bending element,resonant column and step-amplitude cyclic triaxial tests,a model for the development of shear modulus with shear strain in a wide strain range was established.The effects of step-amplitude cyclic loading and monotonic shearing on the initial shear modulus of soft clay were compared and analyzed.The initial shear modulus of the specimen decreases with the increase of the step-amplitude.After shearing,the output signal appeared double-peak phenomenon,and the initial shear modulus increases.(5)Based on the transformation relationship between the Mohr-Coulomb ultimate strength and the critical state strength parameters,the equivalent cohesion was introduced into the SANISAND constitutive model by means of stress translation.A sharp angle control parameter was added to the yield surface,and an excess pore pressure model was introduced.The Mod-SANI04 constitutive model which is suitable for clayey soil has been improved.The parameters were calibrated and analyzed by the laboratory test results and the element specimen loading and unloading model.With the increase of the principal stress translation,the strength of the specimen increases.The plasticity parameter has no effect on the ultimate strength of the specimen,but it controls the growth rate of the shear stress.The larger the boundary interface parameter,the smaller the specimen strength and plastic modulus.The dilatancy parameter has little effect on the shear strength of the specimen,but affects the volume strain of the specimen.In the true triaxial numerical simulation,as the principal stress parameter increases from 0 to 1,the length of the stress path gradually becomes shorter,and finally reaches the critical state surface.The monotonic triaxial numerical simulation results are in good agreement with the experimental results.The dynamic triaxial numerical simulation results are basically similar to the experimental results,but there are some differences.(6)Based on the improved Mod-SANI04 constitutive model,the deformation and failure laws of the site soil and the subway station structure under strong earthquake loading were studied through numerical simulation.The central column of the station structure suffers severe bending deformation.The roof and the upper soil collapsed,and the subsidence of the surface soil above the station is the largest.The excess pore pressure of the soil around the bottom of the station is the largest.The site has a certain amplification effect on the earthquake loading.At the surface,the maximum horizontal acceleration amplitude is magnified by about 2.5 times relative to the bottom of the model.Due to the horizontal earthquake loading and MPC constraints of the model,the horizontal displacement of the site’s vertical monitoring path presents an S-shape.The closer it is to the central column,the larger the vertical settlement value of the top plate and the smaller the vertical settlement value of the bottom plate.The vertical velocity and acceleration peaks in the middle of the top and bottom plates are smaller than those at the two ends.The horizontal displacement of the middle position of the central column of the station is the largest,and the peak speed and acceleration are smaller than those of the roof and floor.The horizontal displacement of the top of the side wall is the largest,and the peak speed is larger than that of the top and bottom plates.The peak value of horizontal acceleration in the middle of the side wall is smaller than that at both ends.The thesis has 232 figures,55 tables,and 258 references.