Centrifuge Modeling And Numerical Analysis On Seismic Site Response And Catastrophes Of Deep Clay Deposits

Considering that the seismic loading applied to building structure is transmitted from the ground,the structural failure caused by the earthquake is closely related to the geotechnical engineering problems.In China,the soil foundations consisting of soft clay deposits carry more than half of the country's total population and GDP.Thus,studying the seismic response characteristics of clay site and optimizing the seismic performance of the overlying structure is one of the key tasks for disaster pre-vention and mitigation.Nonlinear site response of deep offshore clay deposit plays an important role in changing the characteristics of ground motions when subjected to strong shaking,especially when different amplification behavior of acceleration and displacement are considered.Previous studies reveals considerable amplification of surface ground accelerations for low bedrock accelerations of the order of 0.05 g to 0.10 g and significant attenuation of accelerations in soft soil sites subjected to large earthquakes,which seems favorable for the traditional force-based seismic design.However,less data is available to evaluate the effect of deep deposits of clay on amplification or attenuation of ground deformations.To address these problems,the present study carried out both physical modeling and numerical simulation on seismic response of deep soft clay deposits subjected to earthquake loadings.Through systematic parametric analysis,the influence of different factors on the amplification effect of the site was discussed.The main research work and results of this paper are as follows:1.This paper attempted to optimize the design of the present piezoelectric sensors,so as to meet both requirements of sensor miniaturization and excitation energy maximization.According to a series of compression wave velocity measurements performed in different transmission medium,the manufactured bending disks showed a good performance and sufficient testing accuracy:the first arrival in the received signal was clear and easily distinguished,and the bending disks could capture the slight changes in stress level and saturation state of the experimental sand.Based on a set of centrifugal model test to simulate the rainfall infiltration of loess site,the performance of the manufactured bending disks in high-gravity environment is verified,and the preliminary study on the realization of combined characterization of the soil state in centrifuge test is conducted.2.The paper describes a series of centrifuge shaking table tests and numerical simulations to investigate the behavior of deep offshore clay deposits subjected to earthquake loadings.The centrifuge model tests were performed on a slightly overconsolidated clay deposit at UC Davis.A suite of shaking tests were conducted including impulse step-wave testing,frequency sweeps,a small "elastic" earthquake event and a "ductility" level large earthquake event.Accelerations,displacements and pore pressures were measured in the soils(free field)throughout the test program.For the elastic level excitations,about a 33%amplification was observed between the input acceleration at the base and the measured accelerations near the top of the clay deposit.However,for the ductility-level earthquake excitation,which had peak input acceleration at the base of 0.46 g,acceleration near the top of the deposit was reduced to 0.07 g.These observations are correctly simulated by DEEPSOIL with proper soil models and parameters back analyzed from the centrifuge model test.Then systematic 1-D site response analyses are performed on synthetic clay deposit models with 30 to 120 m depths using the validated DEEPSOIL program,and the amplification characteristics of acceleration and deformation induced by base excitation with different intensities and frequencies are analyzed in both time and frequency domains.The results reveal that for deep soft offshore clay deposits subjected to large earthquakes,significant acceleration attenuation may occur near the top of deposit due to soil nonlinearity and even local shear failure;however,significant amplification of displacement at low frequencies are expected regardless the intensities of base motions,which suggests that for displacement sensitive offshore foundations and structures,such amplified low-frequency displacement response will play an important role in seismic design.3.In the present study,undrained triaxial compression tests on reconsolidated samples of a soft sensitive marine clay were conducted,and a soil disturbance dependent simple constitutive model is proposed for describing post-peak nonlinear strain softening behavior of sensitive clays based on experimental results,and then is implemented in QUIVER code to perform one-dimensional seismic response analysis of mild infinite slopes,to address the relative importance of soil disturbance effect on the site response and sliding displacement of submerged slopes during seismic loading.Detailed laboratory test descriptions and results of a soft sensitive marine clay,model formulations and comprehensive computational efforts are presented.Parametric study of site response analysis focusing on the effects of soil disturbance degree and depth on generic clay grounds with varying sloping angles subjected to harmonic and earthquake loadings are performed.The predicted acceleration amplification,maximum shear strains and permanent displacements provide insight of earthquake performance of submarine sensitive clay slopes.The main findings are drawn based on several key concepts.First,undisturbed sensitive clay is typically stiffer and stronger than disturbed clay consolidated at the same stress level and hence,unless strain softening is triggered,structured sensitive clay will perform better than insensitive clay of the same residual strength.Second,post-peak softening at shallow depth will lead to strain localization on a thin layer,which is the main cause of sudden change of upward wave propagation characteristics and large permanent displacement at seabed surface of mild submarine clay slope.Thirdly,soil disturbance degrees and depths have very marked effects on seismic response and displacement performance of sensitive clay slopes,and such effect will be further enhanced by slope angle or earthquake intensity,which should be cautiously treated in engineering design.