Measurement And Characterization Of Dynamic Soil Properties In Centrifuge Model Test

Earthquake refers to the ground motion caused by the seismic wave released by the source in the geotechnical medium.It can generally be expressed by the time function of the acceleration,velocity or displacement of the ground particle.As a sudden natural disaster,many problems currently faced in geotechnical engineering are closely related to earthquakes,and earthquakes not only induce damage to sites and buildings but also cause great harm to the normal production activities of human society.The analysis of the failure law and dynamic response characteristics of the soil under the action of earthquake is the basis of the earthquake disaster of the soil in the study site.Transient monitoring of soil structure evolution during the earthquake plays a direct support role in understanding the catastrophic law under the earthquake action of the soil.The physical and mechanical behaviors of different types of soils are different,the state of the soil before earthquake and its dynamic response characteristics have a great influence on the study of seismic action.Only by studying the influencing factors of soil dynamic characteristics can we reveal the soil in different sites.And the response law and failure criterion under earthquake action provide accurate dynamic characteristics parameters for numerical simulation analysis and geotechnical seismic design.Dynamic characteristics of the soil can be further studied through unit test,geotechnical centrifuge test,on-site investigation and analysis,and numerical analysis.Centrifuge test technology is an emerging physical simulation method in the field of soil mechanics and geotechnical engineering.The gravity field of the rock and soil can be restored by the ultra-gravity centrifuge,and the ground motion input is realized at the bottom of the model through the vibration table of the centrifuge.And many monitoring methods can directly obtain the dynamic deformation and stability characteristics of the rock and soil structure caused by the earthquake.Considering the advantages and disadvantages of various means,the centrifuge test has obvious advantages in studying the dynamic response behavior of the soil.In this paper,theoretical analysis and centrifuge shaker test are used to analyze the elastic wave velocity combined test results of the model soil under hyper gravity conditions,and the accuracy is theoretically verified.And the corresponding dynamic and elastic parameters of soil are obtained by use the elastic wave velocity.By the dynamic centrifuge test,the dynamic response of the site is obtained,and the shear stress response time history of the soil is obtained by inverse calculation.Then the law of the dynamic modulus and damping ratio of the soil changes with the shear strain level is analyzed.And seismic design provides precise dynamic parameters that are instructive for practical engineering problems.Specific research content includes:1.Based on the results of the unit test of the participating institutions of the International Parallel Test of Earthquake Liquefaction Centrifuge(referred to as LEAP)in 2015 and 2017,summarizing the physical and mechanical properties of Ottawa F56 sand.In addition,according to analysis a large number of dynamic triaxial test data of the LEAP team,the dynamic modulus and damping ratio of Ottawa F65 sand under different shear strain levels were obtained,the result provides a basis for the correctness of the dynamic parameters in the full strain section obtained by the centrifugal model test.2.The application of elastic wave velocity testing technology in centrifugal model tests is very challenging for the particularity and complexity of the working environment of the centrifuge.Two sets of static supergravity centrifugal model tests,including dry sand and saturated sand horizontal site model,were carried out using the existing elastic wave test technology of centrifuge test and the optimized bending element and bending disk.By studying the elastic wave monitoring and analysis method of soil under the condition of supergravity,the elastic wave propagation law in the supergravity environment is obtained,and the elastic parameters of the soil are calculated,including the shear modulus G,the Yang's modulus E and the Poisson's ratio v.However,the result of the damping ratio D is quite different from the actual one.Comparing the experimental test results of elastic wave velocity with the results obtained by theoretical analysis,the reliability and accuracy of the current wave velocity testing technology are verified,which lays a foundation for the wide application of this technology.3.For in-depth studying the nonlinear dynamic characteristics of the soil,centrifuge shake table test is an effective research tool.A set of centrifuge shaker tests were carried out on an inclined field.Through multiple vibration events,including the input of unequal amplitude sine waves and step waves,the dynamic response characteristics of the saturated soil,such as acceleration and pore pressure,were obtained.The shear stress response time history of the soil in the site is inversely calculated by linear method,then the dynamic modulus and damping ratio parameters of the soil in a certain strain range are obtained.Finally,the model test results are compared with the results of the unit soil dynamic triaxial test.The research shows that the various influencing factors in the model test will cause the difference between the model test results and the unit test results,and verify the reliability of the method for estimating the dynamic modulus and damping ratio of soil by model test.Based on the above research contents,the reliability and accuracy of the soil elastic wave velocity monitoring technology under super-gravity conditions are demonstrated,and the monitoring of major geotechnical engineering disasters is an effective means.The rationality of the method for estimating the nonlinear characteristics of soil(including the variation of soil dynamic modulus and damping ratio with shear strain)by centrifugal simulation test is also verified,and the research method for monitoring and characterizing the dynamic parameters of soil in the full strain section in a super-gravity environment is established.The above research content is of great value to reveal the dynamic response principle and failure mechanism of soil under earthquake action,and provides a reliable scientific basis and important parameters for numerical simulation analysis and geotechnical seismic design.What is more,providing important technical means and research ideas for further research on such soil dynamics and seismic engineering issues.