Analysis Of Bearing Behavior Of Bridge Pile Based On Displacement Method Under Lateral Surcharge Loads

The large surcharge load adjacent to buildings or bridges can have a great influence on the stress and deformation of the pile foundation.Surcharge load causes great settlement of the soil around the pile,resulting in negative friction resistance to the pile body.The lateral deformation of the soil around the pile squeezes the adjacent pile foundation,resulting in horizontal lateral displacement and flexural deformation of the pile body,which seriously affects the bearing capacity of the pile and even the destruction of the superstructure.This kind of pile is called "passive pile".The analysis of passive pile is a typical and complicated three-dimensional space problem,and there is "shadowing effect" between pile groups.In this paper,the mechanical and deformation performance of passive pile groups under surcharge load is studied by laboratory model test,numerical simulation and theoretical calculations,and the shadowing effect of passive pile groups is calculated,which provides a basis for calculation and design of passive pile.(1)The stress and deformation of passive pile group under lateral surcharge load is studied by laboratory model test.Model tests of 2 × 2 pile groups and 3 × 3 pile groups under different surcharge load and surcharge distance are carried out.The test results show that the the internal force of pile(bending moment and axial force)and lateral displacement increases with the increase of pile load.With the increase of surcharge distance,the distribution of the negative friction resistance of the pile body is closer to the lower middle part.The pile group effect coefficient is about 15% ～ 25%,and it increases with the increase of the surcharge load,and decreases with the increase of the surcharge distance.The influence of the surcharge distance on the pile group effect coefficient is larger than the surcharge load,the effect of shadowing effect on lateral displacement is smaller than that on bending moment.(2)Plaxis 3D is used to conduct three-dimensional numerical simulation analysis of pile groups,and the internal force and lateral displacement of the pile body basically agree with the results of the model test.The effects of pile stiffness,soil elastic modulus,soil layer thickness,loading modes,pile cap and vertical load on passive pile characteristics are mainly discussed.The results show that the displacement of the pile decreases gradually with the increase of the stiffness of the pile,which shows the deformation characteristics of the rigid pile;with the increase of the elastic modulus of the soft soil layer,the deformation and stress of the pile body become smaller,and the zero point of the bending moment also gradually moves down;as the thickness of the soft soil increases,the displacement of the lower part of the pile body gradually increases,and the position of the maximum lateral displacement also gradually moves down,showing the deformation characteristics of the flexible pile;when rigidly loaded,the lateral displacement of the pile body and the lateral deformation of the soft soil layer are smaller than that under flexible loading;the rear piles are connected into a whole,and the deformation of the group piles is much smaller than that of the single-row piles;when the vertical load is applied first,the vertical load is helpful to reduce the lateral movement and bending moment of the pile body,and when the vertical load is applied later,the lateral displacement and bending moment of the pile body increase with the increase of the vertical load,resulting in a secondary bending moment effect.(3)The two-stage analysis method based on the displacement method is used to calculate the pile deformation,and the influence of the shadowing effect is considered in the calculation.Firstly,based on the Boussinesq solution,the free displacement field of the soil under the lateral surcharge load is analyzed.Combined with the Mindlin solution in the elastic system and the difference equation of the pile,the force and displacement of the passive single pile under the lateral surcharge load are analyzed.The pile-soil-pile interaction soil displacement is transferred to consider the pile foundation shadowing effect to analyze the influence of pile groups under lateral surcharge load.The corresponding finite difference method calculation program is used to analyze the bending moment and lateral displacement of the pile under lateral surcharge load,and the results of the two-stage analysis method are compared with the numerical simulation results.The results agree well.