Research On Key Problems Of Back Calculation Methodology For Soil Constitutive Relation By Using Centrifuge Model Test

Dynamic shear modulus and damping ratio are the core indicators to characterize soil deformation and strength characteristics,and are also necessary conditions for site seismic response analysis and foundation performance seismic design.At present,the empirical knowledge and expressions of dynamic shear modulus and damping ratio are almost all derived from unit tests such as resonance column,dynamic triaxial,direct shear,and compression.However,compared with the actual engineering conditions,the unit test has significant differences in stress boundary,drainage conditions,loading methods,etc.,and has error factors such as size effect and rubber membrane embedded effect.The establishment of mathematical models and expressions often has great uncertainty in engineering applications.In contrast,the soil stress boundary,drainage conditions,loading methods,etc.of the dynamic centrifugal test and the shaking table test are more consistent with the actual engineering conditions.However,the reconstructed soil stress of the latter is often very limited.For example,the Japan E-Defense,the world’s largest shaking table,can only reconstruct the soil stress of 2-3m.Therefore,the acquisition of soil shear stress-strain response and dynamic shear modulus damping ratio characteristics of dynamic centrifugal test has important scientific value and significance for the development of geotechnical engineering theory and behavioral seismic design methods.The shear stress-strain inversion method based on acceleration time history is an economical and practical method to obtain the seismic deformation and strength characteristics of soil by model tests and strong earthquake observations.However,the existing shear stress-strain hysteresis loops generally have problems such as non-smooth,non-closed,discontinuous,etc.,which are difficult to reflect the real soil deformation characteristics,let alone obtain the regular response of the dynamic shear modulus and damping ratio.Although many scholars have carried out a lot of research work on the shear beam distribution function and applicable conditions of the inversion method in the past,and obtained many valuable research results.However,the important link of acceleration integral displacement reliability is often overlooked,and it has become a key obstacle and unknown factor to obtain reliable dynamic shear modulus damping ratio characteristics by current inversion,and relevant basic research work is urgently needed.Focusing on the importance of obtaining dynamic shear modulus damping ratio characteristics based on acceleration time history inversion for geotechnical earthquake prevention and disaster mitigation engineering,the goal is to reveal the reliability of acceleration integral displacement and obtain reliable dynamic shear modulus damping ratio.The displacement accuracy and key issues of the existing acceleration integration methods are analyzed.A new phase lossless robust integral displacement method based on separation variables is proposed.The effect of the integral displacement method on the inverse shear stress-strain response is clarified.The characteristics and laws of the inversion dynamic modulus damping ratio are discussed.The main completed work and research results are as follows:1.The sandy slope-rocker-mass block shaking table model test under seismic loads of different intensities and bandwidths has been designed and carried out.Abundant and grouped measured acceleration and displacement data are obtained under various working conditions such as dynamic displacement,permanent displacement,and base deflection.Three representative acceleration integral correction methods are selected,and the accuracy and applicability of the existing integral methods for dynamic displacement and permanent displacement are given.The key problem that the high-pass filtering process leads to the phase out of phase between the integral and the measured displacement time history is revealed and it is proved that the influence of the acceleration integral displacement method and the data processing process cannot be ignored.2.Based on the basic idea of the separation variable method,a new phase lossless robust integration approach for evaluating displacement from acceleration record(PLRI method)is developed,which adaptively corrects the static,dynamic periodic and aperiodic integral amplification errors,respectively.It solves the problems of time-history asynchrony,experience parameters and human factors interference of the existing method of integral displacement.The applicability and robustness of the new method under different frequency and intensity dynamic loads and different sensor types are verified through group shaking table tests and blind test tests.The results show that compared with the existing methods,the average accuracy of the new method’s integral dynamic displacement is improved from 73.65% to 90.74%.3.The cyclic shear stress-strain inversion analysis method based on acceleration array is briefly described,and its basic assumptions and applicable conditions are summarized.Through the sand dynamic centrifugal model test,the influence characteristics and laws of various factors on the inversion of cyclic shear stress and shear strain are compared and analyzed.The results show that the shear stress and shear strain are very sensitive to the integral displacement method.The new method solves the continuity,smoothness and closure of the hysteretic loop,and can obtain the hysteretic loop response and the development trend of the modulus damping ratio consistent with the existing knowledge.Combined with the test results and applicable conditions,the selection basis and application suggestions of the integral displacement method and the shear beam distribution function are given.4.A parallel test of dry sand/saturated sand versus dynamic centrifugal model was designed and carried out.The characteristics and laws of acceleration time history variation of the seismic response of dry sand/saturated sand layers are discussed.The characteristic differences in the dry-sand/saturated-sand shear stress-strain hysteresis loop responses are clarified,and the two-model shear strain ranges are indicated.The variation trend of the dynamic shear modulus damping ratio of dry sand/saturated sand is inversely analyzed,and the response characteristics and laws of the maximum dynamic shear modulus with burial depth,shear strain,vibration order,and relative compactness are discussed.Combined with classical constitutive relation and empirical knowledge,the accuracy and reliability of inversion analysis of dynamic shear modulus damping ratio of dry sand/saturated sand are evaluated.