Study On The Quasi-elastic Behavior Of Sand Based On Deformation Steady State Under Cyclic Loading And Its Application

The elastic response of soil is an important parameter to evaluate the stability of geotechnical foundation,which is a hot topic in soil mechanics.It is hard to separate the elastic components since the deformation of soil is elastoplastic,consequently,the study on the elastic response of soil is still insufficient.The literature shows that the elastic behavior of soil is accompanied by energy dissipation,and it is believed that the elasticity of soil is not completely elastic but quasi-elastic.At present,it’s hard to obtain the continuous quasi-elastic response that extends with increasing load by using of wave propagation method to evaluate the elastic response or using unloading-reloading with small strain amplitude to probe the quasi-elastic response.Recently,the quasi-elastic deformation behavior of soil under cyclic loading has attracted the attention of researchers,which has the potential to evaluate the quasi-elastic response.Therefore,motived by the need to fill the gap in the continuous quasi-elastic response of soil extending with increasing load,this thesis investigates the continuous quasi-elastic behavior of soil based on the(deformation)steady state achieved by sand under cyclic loading as well as the evolution of elastic behavior with cyclic stress history.Combination with discrete element numerical simulation(DEM),the microscopic characteristics of quasi-elastic behavior of sand is explored.The applications of quasi-elastic properties of sand in axial strain accumulation and evaluation of plastic shakedown limit under cyclic loading are carried out.The main research contents are as follows:(1)Study on the deformation steady state of sand by different laboratory tests.The conventional cyclic triaxial test,the periodic cyclic triaxial compression test with constant stress,as well as the periodic cyclic confined compression test with constant stress are conducted on different sands.The steady state characteristics of sands under different cyclic loading conditions are studied as well as the corresponding quasi-elastic behavior of sands.The empirical models of elastic modulus(axial)and strain ratio are proposed,respectively.The dependence of elastic modulus on stress state can be characterized by axial stress,which can be expressed as a linear form.With the increasing of deviator stress,the deviation between Poisson’s ratio and strain ratio is observed,indicating that in the quasi-elastic domain,the traditional definition of Poisson’s ratio can no longer describe the conversion relationship between radial strain and axial strain.Therefore,cautions should be taken when using the concept of Poisson’s ratio in soil mechanics for any conversion of elastic parameters.(2)Combination with the 3D-DEM numerical simulation and laboratory experiment,the evolution of quasi-elastic response with cyclic stress history is studied.Numerical simulation of the cyclic triaxial test is implemented,and the evolution of quasi-elastic behavior with the load cycles is presented through the implementation of the elastic probe test.Furthermore,the multi-stage cyclic triaxial test is carried out to compare the difference between the quasi-elasticstress-strain curve in the previous stage and the quasi-elastic-stress-strain curve in the latter stage with a higher cyclic stress level.The results show that the quasi-elastic behavior of sand is not sensitive to the cyclic stress history.(3)Exploring the microscopic characteristics of quasi-elastic response using 3D-DEM.The characteristics of sand in the quasi-elastic domain are discussed at the particle scale.The results show that the sand does not have a completely elastic domain,which is always accompanied by sliding contacts and energy dissipation.In particular,by tracing the movement trajectory of particle contact,the mechanism of dilatancy in reloading and volume shrinkage in unloading are explained at the particle scale.(4)Study on the plastic stress-strain relation by separating elastic and plastic strains.According to the obtained quasi-elastic curve,the stress-strain curve is separated,and the corresponding plastic-stress-strain relationship is explored,consequently,the axial plastic modulus is studied.The axial plastic modulus decays linearly with the axial stress.Combined with the empirical models of elastic modulus and plastic modulus(axial),the hysteretic stressstrain relationship of sand in cyclic loading is predicted,which is comparable with the test results.(5)Applications on strain accumulation and evaluation of plastic shakedown limit.According to the nature of quasi-elasticity,the axial plastic cumulative strain increment is projected to the monotonic stress-strain curve,and the strain cumulative behavior is studied by using the projected coordinate system.Combination with the empirical formula of elastic modulus and plastic modulus,a complete axial strain accumulation model is proposed.According to the degree of energy dissipation in the extended quasi-elastic domain,the degree of quasi-elasticity of sand is discussed.The elastic upper limit of the extended quasi-elastic domain is quantified,and the relationship between the elastic upper limit and the plastic shakedown limit is discussed.A method to indirectly determine the plastic shakedown limit by determining the elastic upper limit is proposed and verified by test results.