Study On Shear Vibrational Fluidization Characteristics Of Cohesionless Dry Granular Soils Based On DEM

In the field of civil engineering,granular soils,as the carrier of a large number of engineering structures,are frequently subjected to dynamic load,such as the rock and soil mass around the subway tunnel,the subgrade of high-speed train,and the slope under the action of earthquake.The granular soils may undergo a transition from solid state to liquid（fluid）state under dynamic loads with the decrease of bearing capacity,leading to catastrophic consequences.Domestic and foreign scholars have done a lot of research on the liquefaction of wet sand,but there is still a lack of in-depth understanding of the phenomenon of vibrational fluidization,which is related to the interaction mechanism of soil particles.Based on the theory of vibrational fluidization and the results of vibrational direct shear test conducted by Dr.Taslagyan,the discrete element software PFC^3D is used as the platform for secondary development.Through a large number of numerical simulations,research on the vibrational fluidization characteristics of cohesionless dry granular soils is carried out.The main research contents are as follows:（1）Based on the related experiment and theory of vibrational fluidization,the three-dimensional discrete element simulation models of vibrational direct shear test on glass beads and coarse sand are conducted.In order to ensure the accuracy of the numerical tests,the mesoscopic parameters of the particles are calibrated.The vibration of the upper shear box is simulated by dividing one period of vibration into several parts and applying different horizontal velocity to the wall in different parts.（2）The results obtained from the numerical tests are compared with the experimental data.By analyzing the shear stress,vertical displacement and shear strength envelope,the macroscopic mechanics and deformation characteristics of the vibrational fluidization are studied.（3）The reliability of the numerical simulation results is the premise of the mesoscopic analysis.In view of the size effect of the model,the ratio of the minimum size of the model to the maximum particle size is considered to carry out the comparative modeling analysis.（4）According to the local body strain distribution,the model is divided into several analysis zones.Various visualization methods are used to show the evolution process of mesoscopic parameters,and the variation rules of various parameters and the interaction mechanism of particles in the process of fluidization are discussed.The results show that:in the three-dimensional direct shear model,the method of applying horizontal velocity in different parts of a single period can effectively simulate the vibration process.Choosing a smaller model width according to particle size can not only ensure the reliability of simulation results,but also significantly improve the computational efficiency of the model;the vibrational fluidization is accompanied by the decrease of residual shear strength,internal friction angle and vertical displacement.The vibrofluidizational limit is an important parameter describing the vibrational fluidization characteristics,which can reflect the anti-fluidization ability of granular soils;in the process of vibrational fluidization,all the mesoscopic parameters show typical strain localization characteristics.The main occurrence area of fluidization is called fluidized shear band,in which the local volume strain and void ratio increase while the local coordination number decrease during vibration.The special area has a thickness of about 13.3 to 14.5 times the median diameter d50.