Study On Seismic Behavior Of Pier With High Pile Cap Considering Pile-soil Interaction

High pile cap foundation is widely used in bridge engineering with deep water,long span and special geological conditions(such as permafrost regions in the west).The failure mechanism of high pile cap foundation pier in earthquake is significantly different from that of low pile cap foundation pier,and the exposed part of pile foundation is also the weak seismic area in the whole bridge structure.In this paper,the pier with high pile cap foundation used in railway bridges in western China is taken as the research object,and the seismic failure mechanism and main influencing factors of seismic performance of the pier with high pile cap foundation considering pile-soil interaction are systematically studied by using the method of pseudo-static model test and numerical simulation.The main research contents are as follows:(1)Based on the prototype bridge,a 1/8 scale model pile foundation pier is designed and manufactured.The failure mechanism,hysteretic curve,skeleton curve,energy dissipation capacity and stress distribution law of soil layer around the pile foundation of railway high pile cap pier under horizontal reciprocating load are studied by pseudo-static model test.The results show that in the pile-soil-pier system,the nonlinearity mainly comes from the pile foundation and soil,the soil cracks before the pile foundation,and the crack development area of the pile body concentrates at 1.5~2 times the pile diameter under the pile-soil interface.During loading,the separation of pile-soil interface leads to the phenomenon of "pinching" in hysteretic curve.The maximum stress of pile body is concentrated at 2 times the pile diameter under the pile-soil interface,and the stress at 5 times the pile diameter and below at the interface is basically zero.(2)Several common constitutive models in geotechnical engineering are introduced,including linear elastic model,porous medium elastic model,Mohr-Coulomb model,extended Drucker-Prager model,modified Drucker-Prager cap model and Cambridge model.By comparing and analyzing the basic theories of typical soil constitutive models,the Mohr-Coulomb model is selected in the numerical simulation analysis of this paper.(3)The finite element analysis model of high pile cap foundation pier is established by ABAQUS software,and the correctness of the finite element model is verified by comparing with the pseudo-static test results.Through Pushover analysis method,the stress and displacement distribution law of pile is further analyzed.(4)Considering the particularity of seismic performance of high pile cap foundation piers in frozen soil area,the effects of negative temperature,frozen soil thickness and freeze-thaw cycles on seismic performance of high pile cap foundation piers are analyzed by finite element method.The results show that the increase of frozen soil thickness and the decrease of temperature will lead to the increase of the bearing capacity of the structure system,and the deformation ability of the structure will increase with the increase of frozen soil thickness and temperature.The number of freeze-thaw cycles has a great influence on the mechanical properties of soil in the early stage,and the influence decreases with the increase of the number of cycles.