Study On Seismic Response Characteristics And Failure Mode Of Subway Station In Soft And Hard Uneven Stratum

In the development and use of urban underground space,a large number of subway stations are inevitably built in complex stratum sites.When the site has large uneven soft and hard stratum,the force form of subway stations is complicated,however,in the current research,although there are some studies on the seismic response of subway stations in non-uniform strata,the understanding of the influence of such non-uniformity on the internal forces,deformation,especially the failure of the station is relatively insufficient.This paper discusses the seismic response characteristics and failure modes of subway stations in soft and hard heterogeneous formations through numerical simulation analysis.The main work and results of this paper are as follows:(1)Aiming at uneven sites with different stratum combinations,12 kinds of uniform and non-uniform stratum conditions have been established with reference to actual engineering,and the seismic response characteristics of this complex site characteristics to the structure have been explored,the results show that the existence of soft and hard uneven strata in the site has a significant impact on the internal force and deformation of the structure: the station components near the station roof and the stratum interface would have a fairly obvious stress concentration effect,which increases not only with the decrease of the overall site stiffness,but also with the increase of the degree of nonuniformity;The interlayer displacement of the structure mainly depends on the characteristics of the stratum in the depth range,and is directly affected by the position of the stratum interface.The harder the stratum is,the smaller the displacement is.(2)Based on the time history analysis results,the applicability of two quasi-static analysis methods for underground structures in soft and hard uneven ground is studied.The results show that the two response displacement methods,which use different equations to calculate the foundation spring stiffness,cannot reflect the stress concentration effect in soft and hard uneven ground well,and may produce dangerous calculation results,therefore,this method is not suitable for the stations in the uneven sites.By comparison,the seismic response characteristics of the structure calculated by the response acceleration method are identical with the time history analysis results,which can well reflect the stress concentration phenomenon of the structure near the stratum interface,and has good accuracy.The calculation results are also safe,hence the method has good applicability.(3)Three typical stratum conditions are selected and the failure mode of three story three span station structure is discussed by using dynamic elastoplastic time history analysis method,the results show that although the failure mode of the station in uniform and non-uniform stratum is basically similar,the plastic zone appears at the end of the middle plate and develops continuously on the components,then the bottoms of the walls on both sides all reach the bearing capacity limit,causing the bottom and the top of the middle column to yield in turn,and finally reach the elastoplastic limit.However,the difference of the interface location of the soft and hard stratum results in various site stiffness and redistribution of internal forces,which makes the middle plate which first yield is different,and the development degrees of the plastic zones of the middle plates in the limit state are diverse,the displacement experienced by the structure from plastic yielding to the elastic-plastic limit decreases with the increase of stratum stiffness.For the limit value of the displacement angle between layers used for testing the seismic performance of the structure,the limit value of the layer displacement angle of seismic performance index is only acceptable in the homogeneous weak stratum,when there is a non-uniform rock stratum with large rigidity in the site,the station reaches the limit state when the deformation is less than the limit value of the specification.Therefore,the limit on elastic and elastoplastic displacement angle given in the code cannot be directly applied as the seismic performance index of the underground structure.