Study On The Seismic Response Law And Design Method Of Subway Station Structure In Liquefiable Site Conditions

With the large-scale development of urban rail transit construction in China,more and more subway projects inevitably cross the liquefiable stratum.However,the current research in seismic resistance of underground structures(especially practical seismic design methods and seismic performance of underground structures in complex environments,etc.)significantly lags behind engineering practice.Therefore,it is of great academic signigicance and engineering application value to study the seismic performance of subway underground structures in liquefiable sites and their practical seismic design methods.Firstly,the seismic response law of a typical two-story three-span box frame subway station structure and the factors affecting the effect of the seismic response law were studied by numerical simulation in this thesis.Secondly,the key problems encountered in the seismic design of underground structures using traditional response displacement method were analyzed,and combined with the basic principles of the traditional response displacement method,an improved response displacement method for seismic design of subway station structures in liquefiable sites was proposed.The main work and achievements of this paper are as follows:(1)Based on the research background of an actual engineering of a typical two-story,three-span subway station in Taiyuan passing through a certain thickness of saturated silt sand layer,a numerical model of soil-underground structure dynamic interaction was established by using the finite difference software FLAC3 D.And taking the corresponding free-field simulation results as comparison conditions,the site liquefaction distribution characteristics,structural stress and floating characteristics,surrounding site displacement settlement and vector characteristics and other indicators were analyzed.The research shows that the soil on both sides of the underground structure will produce continuous circular displacement flow during the floating process of underground structures.And the existence of underground structures during an earthquake increases the deformation of the liquefiable soil,but the seismic response of a certain area of soil above the structure will be significantly reduced.Moreover,the seismic response of the subway station structure will be somewhat reduced when the liquefiable soil layer is penetrated compared with the general site soil conditions,that is,the lower liquefiable soil layer has a certain seismic isolation effect.(2)The influence of the position of the liquefiable soil layer on the seismic response of the subway station structure was studied.Six typical working conditions of liquefiable soil layer were designed,in which there were the liquefiable soil layer on the left and right sides of the upper and lower part of the subway station structure,the liquefiable soil layer directly above,and the upper and lower left sides of the subway station structure,lower left side and upper right side at the same time.The location of large liquefaction deformation,distribution of liquefaction zone,variation and difference of excess pore water pressure ratio,displacement of station structure and the law of differential settlement were summarized.The results show that the subway station structure has an obvious effect of restraining the liquefaction of the foundation soil.And the influence of the liquefiable soil layer on the pore water pressure is greater when it is located in the lower part of the subway station structure than in the upper part In the liquefiable site,the movement direction of the soil is not only horizontal,but also upward and downward.According to the displacement of soil,it can be concluded that the severity of liquefaction zone is in the order of left lower part,left upper part,and right upper part.(3)A numerical analysis model of dynamic interaction between the liquefiable site-PBA(vault roof)and box-frame(flat roof)subway station structures was established,and the characteristics of pore water pressure,acceleration characteristics,stress response of underground structures and floating up were analyzed.Studies have shown that,due to liquefaction of the soil at the bottom of the structure,the foundation will lose its stability and the structure will rise up and incline,which indicates that it is not appropriate to directly use the traditional response displacement method in seismic design of subway station structure in the liquefiable site.The lower the buried depth of the subway station structure,the greater the displacement of the structure,and shear deformation mode of the whole station structure is presented.In the liquefied soil,the station structure is in the state of three-dimensional force,and there is a difference between the top and the bottom,the left and the right side walls.Under the condition of flat top,the vertical displacement of the subway station structure is obviously larger than that of dome top.(4)This paper systematically analyzes the key problems of the traditional response displacement method in the seismic design of subway underground structure,and an improved response displacement method for seismic design of underground structures under liquefiable site conditions was proposed,combining the characteristics of the traditional response displacement method and the problems existing in its application.Then the applicability of the method was verified by changing the overlying soil thickness,structural rigidity,and ground motion strength compared with the dynamic time history method.The comparison results show that the accuracy of the internal force and displacement calculation of underground structures using the improved response displacement method can meet engineering requirements.