Discrete Element Analysis Of Shear Characteristics Of Pile-soil Interface

Pile foundation is widely used in high-rise buildings,bridges,highways and other fields because of its high bearing capacity and good seismic performance.The study of mechanical properties of pile-soil interface is the premise and foundation to solve the problem of pile-soil interaction,which has important theoretical research and practical application significance.In this paper,the discrete element software,PFC,is used to simulate the direct shear test and single pile compression test of the contact surface.The mechanical properties of the pile-soil contact surface are analyzed from the macro and micro aspects,and the formation mechanism of the shear band and the distribution characteristics of the micro variables inside and outside the shear band are studied.The main achievements are as follows:（1）The direct shear test model of the contact surface corresponding to the indoor test is established by using PFC software,and the values of the micro parameters are determined.The simulation results fit the indoor test results very well,which shows that the model can accurately reflect the interaction between the pile and the soil.（2）The distribution characteristics of micro variables such as particle movement,porosity and coordination number were analyzed.The results show that the influence area of shear on soil will be concentrated near the contact surface,thus forming a shear band,and the porosity of particles near the contact surface will be obviously increased,and the coordination number will be significantly reduced;the development of coordination number has a significant correlation with its strength;the shear effect will mainly cause the change of tangential contact force between particles.（3）The effects of different mesoscopic parameters on the mechanical properties of contact surface and particle motion were studied.The results show that with the increase of pile friction coefficient,the friction angle in the contact surface increases gradually,the cohesion changes little,and the particle displacement increases gradually;with the increase of particle friction coefficient,the friction angle and cohesion in the contact surface increase gradually;with the increase of porosity,the friction angle and cohesion in the contact surface decrease gradually,the particle horizontal displacement increases slightly,and the vertical displacement increases gradually.With the increase of the normal stiffness of particles,the angle of internal friction increases,the cohesion decreases,the horizontal displacement of particles changes little,and the vertical displacement increases.（4）Based on the analysis of the horizontal displacement of the particles,the thickness of the shear zone of the contact surface is determined,and the influence of different micro parameters on the formation characteristics of the shear zone of the pile-soil contact surface is studied.The results show that the normal stress and particle stiffness have little effect on the thickness of the shear band,which is about 5.6d50.With the increase of pile friction coefficient,particle friction coefficient and soil porosity,the thickness of the shear band increases gradually.（5）The effects of different mesoscopic parameters on the micro variables（porosity,coordination number）in and out of the shear band are analyzed.The results show that when the shear swelling phenomenon occurs in the model,the porosity of soil in the shear zone increases,the coordination number decreases,and the porosity and coordination number outside the shear zone change little;the critical porosity in the shear zone and outside the shear zone decreases with the increase of normal stress,and increases with the increase of pile friction coefficient,particle friction coefficient and particle stiffness;the critical porosity is not affected by the initial porosity of the model influence.（6）In the compression pile model,after the settlement reaches 1mm,the friction resistance has basically remained stable,and the load that continues to increase is mainly borne by the pile end resistance;the thickness of the shear band is about6.9D₅₀-8.2D₅₀,which is basically consistent with the results of the direct shear test,and the porosity of the soil in the shear band increases.