Research On The Interaction Mechanism Between Soft Soil With Lateral Movement Induced By Surcharge Loads And Bridge Pile Foundations Of High-speed Railway And Its Computing Method

With the rapid development of the high-speed railway,the gradual densification of railway lines has caused long-term or temporary surcharge loads along the lines occasionally.For bridge pile foundations of a high-speed railway in deep soft soil areas,the adjacent surcharge load may cause excessive lateral displacement to soft soil,resulting in additional deformation and bending moment of piles,seriously deteriorating into the service performance of the pile foundation,and then leading to the excessive displacement of the pier top,affecting operation quality and even threatening driving safety.However,due to the complexity of the nonlinear deformation behavior(consolidation-creep,large deformation,etc.)of soft soil and the combined load mode(active-passive load mode,etc.)of pile foundation,the interaction mechanism between soft soil with lateral movement and pile foundation is still unclear,and the calculation theory of active-passive pile response is still limited.Relying on the National Natural Science Foundation of China,this dissertation adopts the research method combining laboratory test,field test,theoretical analysis,and numerical simulation to study the time-dependent lateral deformation characteristic of soft soil,the time-dependent interaction mechanism between soft soil with lateral movement and pile foundation,the calculation method of active-passive pile response,the passive load acting on pile shaft considering the soil flow around the pile,and the control measures for the deviated pile foundation caused by surcharge loads.The main research work and achievements are as follows:(1)Taking a deviated bridge pile foundation of a high-speed railway in Ningbo as the research background,a series of one-dimensional consolidation-creep tests of the undisturbed soft soil was carried out,and the stress-strain-time variation characteristics of the soft soil were revealed.The Mesri model,the time-hardening model and the one-dimensional Koppejan consolidation-creep model were used for the fitting analysis and parameter identification of test results,respectively.Comparing the strain-time fitting curves,fitting correlation indices,and residual sum of squares of each creep model comprehensively,the one-dimensional Koppejan model is recommended as the best model to describe the consolidation-creep behavior of the soft soil.Then,the Kopepjan model was incrementally extended to the three-dimensional stress space,and the corresponding Abaqus material subroutines were developed based on a tangent-stiffness method.The subroutine was embedded in a finite element(FE)model of surcharge load-pile foundation to enable the numerical application of the extended Koppejan model.(2)Considering the time-dependent characteristic of soft soil lateral deformation,and combining a multiple-case field prototype test of surcharge load-pile foundation,a FE model embedded with the extended Koppejan model subroutine was adopted to investigate the time-dependent interaction process between soft soil with the lateral movement and bridge pile foundation under different surcharge loads.The time-dependent development laws of the horizontal displacement of the foundation,the pile response,the additional lateral pressure acting on the pile shaft,and the soil arching effect between the piles were obtained.The time-dependent weakening mechanism of the lateral deformation of soft soil on the soil arching effect was illustrated,and the time-dependent interaction mechanism between soft soil and pile foundation was revealed.Furthermore,the influences of surcharge load conditions,the thickness of the soft-weak layer,the pile-soil stiffness ratio,and the consolidation coefficient of the extended Koppejan model on the pile response were analyzed.(3)The friction characteristic of the pile-soil interface was characterized by the adhesion c_a and the external friction angleφ_a.Based on the Ito theory,the calculation expression of the passive load considering the friction characteristic was deduced.Considering that the soil around the pile is in a state of elasticity,elastoplasticity,or limited plastic flow,the improved Boussinesq solution,the improved solution of Ito theory,and the theory proposed by Shen Zhujiang were used to calculate the passive load,respectively,and the whole process description method of the passive load was established.Then,the typical clay curve p-y was introduced to describe the nonlinear relationship between soil resistance and pile deformation.Based on the above,a whole process calculation model of active-passive pile response considering the active load on the pile top and the"P-Δ"effect was established,and a calculation program CPAPP based on the finite difference method was compiled to solve the above calculation model.Moreover,the influences of active load at the pile top,surcharge load conditions,pile-soil interface parameters,and p-y curve parameters on the pile response were analyzed.(4)Considering the large deformation characteristic of the soft soil flow around the pile,a modeling method of coupled Euler-Lagrangian(CEL)model with an Euler-Lagrangian coupled contact algorithm for the soft soil-pile foundation interaction was proposed by introducing the Euler boundary conditions.Then,a CEL FE model was used to systematically study the evolution law of the passive load and the characteristic of the soil flow around the pile,and the influence mechanism of the soil flow on the passive load was revealed.By comparing the calculated results of the CEL model with the experimental and theoretical results in the literature,the applicability and superiority of the CEL method in the problems involving large deformation pile-soil interaction were clarified.Furthermore,the influences of the pile spacing,the friction factor of the pile-soil interface,the inflow velocity of the Euler boundary,the internal friction angle of soft soil,and the section shape of the pile on the evolution law of passive load were also analyzed.(5)Based on a typical deviated bridge pile foundation in a soft soil area caused by surcharge loads,considering the time-dependent characteristic of soft soil lateral deformation,a FE model for surcharge load-pile foundation-bridge pier embedded with the extended Koppejan model subroutine was established to investigate the time-dependent deviation process of the pier and pile foundation in soft soil area under surcharge loads.Based on the FE model,the effects of two measures,micro-isolation piles and stress release holes,to prevent the deviation of the pier top were evaluated.Then,a joint rectification measure using unloading+high-pressure rotary jetting pile reinforcement was proposed to correct the deviated bridge pile foundation,and its rectification effect was evaluated by analyzing the pier top displacement and the pile foundation response.Finally,The research results were applied to the actual project,and the field measured results show that the proposed rectification measure has achieved the expected rectification effect.