Study On Dynamic Charactistis And Seismic Ground Motion Effect Of Soft Soil

The nonlinear characteristics and ground motion amplification effect of soft soil under earthquake dynamic action has been a hot issue of concern and research in the earthquake engineering field. Because currently, the strong earthquake observation data available is scarcely any ground condition data on the coverage of soft soil, which makes analysis results on the soft soil site seismic response can not be inspected by the actual strong earthquake records, the seismic design of soft soil area exists potential risks. Specific to the dynamic characteristics of soft soil in recent years and some scientific problems and uncertainties existing in the free field seismic response analysis, this paper carries out theoretical analysis and experimental study embracing such four aspects as dynamic characteristics and dynamic strength characteristics under dynamic loading; and nonlinear characteristics and expression behavior of soft soil in the earthquake; and dynamic centrifugal model test analysis of the soft soil free field; comparison and verification of centrifuge simulation and numerical simulation and so on by means of theoretical analysis, numerical simulation and experimental study. It’s goal is to reveal the characteristics of dynamic parameters of soft soil and expression behavior in the earthquake, evaluate seismic performance and dynamic catastrophe process of the soft soil site, reasonably estimate seismic effect of the soft soil site, compensate the condition that verification can’t be made with theoretical calculation due to lack of strong earthquake observation data, promote the research of soft soil site seismic effect field, and provide offer reference for seismic fortification of Chinese engineering in the soft soil area.On the basis of fully absorbing and summarizing the achievements of the predecessors, this paper uses the littoral facies soft soil in Tianjin area as the research object, and embracing the scientific issue, the main work and innovations are as follows:1. Carry out indoor test research of macroscopic dynamic characteristics and microstructure of the soft soil, analyze dynamic characteristics, the dynamic strength characteristics and the quantitative results of the micro pores of soft soil.Specific to the typical soft soil, cyclic dynamic triaxial tests, micro electron microscope scanning test and other indoor tests describing macroscopic dynamic characteristics and microstructure are carried out under the condition of the routine tests of physical and mechanical properties and strain control and stress control. Research on the dynamic characteristic parameters corresponding to the consolidation time, effective confining pressure, consolidation ratio and other factors are carried out under the cyclic loading, and a fitting method with a set of optimized high fitting accuracy test data is proposed and G/Gmax-γ and λ-γ experience curve of the soft soil in Tianjin is given; the relationship between cumulative plastic strain and double amplitude strain and cyclic shake frequency of soft soil under different dynamic stress amplitudes is analyzed according to the dynamic strength test results; electron microscopy scanning technique is used to carry out the microstructure analysis of soft soil unit body sample and quantitative description of the microstructure characteristics of soft soil.2. The large geotechnical centrifuge shaking table test for the typical soft soil and hard soil free field is carried out, and the complete record of the acceleration is obtained, data analysis for the key earthquake parameters is made, and the setting seismics site effect is given through the simulation of strong earthquake station.The centrifugal model of both soft soil and hard soil homogeneous layer grounds is designed and constructed, centrifuge shaking table test is carried out which is used to artificially simulate strong earthquake observation stations, and the natural seismic wave and artificial seismic wave are input as the base, and through adjusting the input peak acceleration, seismic waves of different intensity are simulated, and acceleration and displacement sensors are used at the middle of model box and boundaries, to obtain the acceleration peak value, duration, response spectrum, lateral displacement, settlement and other test results of soft soil at monitoring points of different depths and different horizons, to achieve the test target and effect of using centrifugal model test to simulate strong earthquake stations and obtain a complete record. By comparing the two centrifugal model test results, the results show that compared to the hard soil, the soft soil has strong ability to filter the wave and nonlinear characteristics, and during earthquake motion, energy and the amplitude has certain loss and attenuation from the bottom to the surface, and the peak acceleration presents the trend of increasing first and then decreasing on the whole. Through comparing the test results with the theoretical calculation, the purposes of verifying the centrifugal model test and numerical simulation each other can be achieved.3. Prepare one-dimensional time domain nonlinear seismic response analysis program which can embed a variety of nonlinear hysteretic constitutive models, through restoring centrifugal model to the prototype site, carry out the soft soil free field nonlinear seismic response analysis.Prepare one-dimensional time domain nonlinear seismic response analysis program package which can embed three kinds of hysteretic constitutive model by using the existing mature three kinds of soil nonlinear dynamic constitutive models(Davidenkov skeleton curve model, "damping degradation coefficient” model and “dynamic skeleton curve” model), combining multiple transmitting boundary condition and time-domain recursive push format, to form the numerical simulation calculation program based on the different constitutive models. Based on the centrifugal model, restore the homogeneous layer soft soil and hard soil site prototype, dynamics parameters consistent with centrifugal model test soil are presented through dynamic triaxial test, thus to establish soft soil and hard soil free field dynamic calculation model, carry out the earthquake response numerical simulation of a variety of algorithms, and calculate peak acceleration, duration, reaction spectrum, earthquake motion amplification coefficient and so on of given typical free field and subsurface horizons.4. The mutual comparison and verification for dynamic centrifuge model test and a variety of numerical simulation results are carried out, and rationality of selecting dynamic constitutive model and feasibility of numerical simulation method are verified through centrifugal model test of simulation strong earthquake station.On the basis of the above research results, the careful comparison and analysis for the differences between dynamic centrifugal model test and a variety of numerical simulation results are carried out, which focuses on the peak acceleration, response spectrum, earthquake motion amplification coefficient and other measured results of surface and subsurface horizons, and comparison and analysis of the effectiveness for the time domain nonlinear constitutive model of the soil body is carried out, and the dynamic constitutive model relatively suitable for the nonlinear characteristics of soft soil is selected, thus to achieve the purpose of mutual verification with the numerical simulation, to provide new ideas and technical support for carrying out soft soil site seismic response analysis in the future.The academic contributions and application value of this dissertation is to provide a new path for the study of soft soil dynamics and provide experience for dynamic centrifugal model test on soft soil provides which has important application value for engineering anti-seismic in the area by giving dynamic parameters of soft soil in coastal area of Tianjin.