Analysis Of Buckling Failure Characteristics Of Bridge Pile Foundations In Liquefiable Ground Under Strong Earthquakes

Earthquake-induced liquefaction has caused many significant damage to pilesupported structures in past earthquakes.Pile foundations in liquefiable soils subjected to seismic shaking may fail due to bending,settlement and buckling.Many attempts have been made to study the failure mechanisms of bending and settlement of a pile in liquefiable soils,these mechanisms are well understood and the codes of practice use them to set design guidelines.However,only limited studies have been carried out to discuss the buckling failure of pile in liquefiable soils,and all these studies are focus on the influence of axial load on the pile's buckling instability,ignoring the effect of the combined action of lateral load and axial load on pile's buckling instability in liquefied soil.Meanwhile,there is no design guidelines has consider the failure mechanisms of pile buckling in liquefying ground.In this work,two shake-table tests considering the separating of load about the seismic response of socketed pile in liquefiable soils are conducted.It reveals the reason on buckling failure of pile.Then,a new method to calculate the critical load for buckling failure of pile was proposed,a ground motion parameter was identified as the best parameter in characterizing the behavior of the soil-pile system in liquefying ground,and a discriminant method for buckling failure of pile was proposed.Main content,method,and results are as follow:1.Two shake-table tests considering the separating of load about the seismic response of socketed pile in liquefiable soils are conducted.In these tests,the failure mechanisms of pile under different loading has been studied.It is found that the exist method to calculate the buckling critical load of pile can not keep the pile safe,and the buckling failure of pile in liquefiable soils is effected by the combined action of lateral load and axial load.2.Based on the Beam on Non-linear Winkler Foundation method,a new method to calculate the critical load for buckling failure of pile considering the soil liquefaction,geometric imperfections,nonlinearity of pile was proposed.In this method,the soil liquefaction effect was modeled using strain-hardening p-y curves,the geometric imperfections were simulated by preset the pile nodes,the pile was simulated with nonlinear force-based beam-column elements,the Corotational transformation which is based on the finite displacement theory was used to model the geometric nonlinearity of pile.Then,the axial load-displacement curve could be obtained by loading in displacement-control mode,and the peak point could be regarded as the critical load of pile based on the nonlinear buckling analysis theory.Finally,this method was validated by the centrifuge tests and the finite element method.3.The effects of soil relative density,initial geometric imperfections of pile,flexural rigidity of pile,unsupported pile length,and slenderness ratio of pile are examined based on the proposed buckling critical load calculation method.Then,based on the results of the parametric study,the discriminant methods for buckling failure of pile was proposed for different soil relative density by putting into the Euler's buckling critical load formula.Finally,this method was validated by the centrifuge tests and the shaking table tests.4.Finite element analysis was conducted to simulate a centrifuge experiment on the nonlinear behavior of a pile founded in liquefiable soil.The result of the finite element analysis was found to be in reasonable agreement with the experimental data.As such,the calibrated finite element model was used to investigate the seismic performance of pile under both near-fault and far-fault ground motions,and the influence of ground motion parameters on the pile-soil-structure response.It was found that peak ground velocity(PGV)is an appropriate ground motion parameter to characterize the response of the soil-pile system in liquefying ground.Near-fault ground motions could result in more severe damage to the pile compared to far-fault grounds motions.Moreover,the formulas based on the parameter PGV was proposed to estimate the maximum bending moment and displacement of pile.5.A new buckling critical load calculation method and a discriminant method for buckling failure of pile considering the effect of inertial load were proposed by combining the finite element analysis method and the proposed buckling critical load calculation method.Based on the cases of pile buckling failure with a different mass of superstructure under different ground motions,and the proposed pile buckling discriminant method,a simplified discriminant method for buckling failure of pile considering the effect of inertial load was proposed using the parameter PGV.Then,this method was validated by the shaking table tests.This study provided a new insight on the buckling failure mechanisms of pile in liquefiable soils.The finished shaking table tests,the proposed buckling critical load calculation method and the discriminant method for buckling failure of pile in this study would greatly help improve understanding of failure mechanism and accurately predicting the ultimate bearing capacity of pile,which is significant in improving the design guidelines for pile under strong earthquakes,and it is significant in engineering practice as well as in theory research.