Dynamic Response And Vulnerability Analysis Of Saturated Soft Soil Tunnels

With the promotion of the western development strategy,a large number of tunnels and other underground major projects have been built one after another.The southwest region belongs to high intensity area with frequent earthquakes,and concrete structures in this region are prone to cracking and collapse under earthquake action.As one of the main structures of lifeline projects,the earthquake damage of long and large tunnel structures should be given great attention.In this paper,a tunnel under construction in Yunnan is used as a research object to establish a finite element model of tunnel-soil interaction based on the site conditions of saturated soft soil.The analysis of the dynamic response of the tunnel under unanimous excitation and multi-point excitation,considering the effect of the spatial effect of ground shaking;Based on the incremental dynamic analysis method and the theory of seismic vulnerability analysis,a study on the seismic vulnerability of tunnels was carried out.The main work of this paper is as follows.1.The basic process of synthesizing multi point seismic motion is systematically described.A detailed introduction was given to the methods,models,and basic parameters of earthquake synthesis.A program was developed using MATLAB for fast synthesis of acceleration time courses for multipoint seismic motions.The program generates multipoint ground shaking applicable to different site types for seismic input of long and large structures at different site types.It is verified by arithmetic examples that the multi-point seismic motions generated by the program are reasonable.2.The seismic response of the tunnel structure under unanimous excitation and multi-point excitation is compared and the relative displacement time curves of the vault and arch bottom of the tunnel section under different excitation conditions are given.The maximum relative displacement of the structure under multi-point excitation is generally larger than the relative displacement of the same part under unanimous excitation,which is due to the different time courses of seismic acceleration at different locations of the cross-section,and therefore there is a certain difference in the displacement time courses at each location.When the difference of displacement time range between adjacent parts is large,the relative displacement will be larger.The acceleration time curves of the liner section vault at different positions in the longitudinal direction are given.The results show that the acceleration response at the vault of the tunnel under multi-point excitation gradually increases with the increase of propagation distance.The acceleration peak at the lining vault under multi-point excitation gradually exceeds the acceleration peak at the same location under uniform excitation.3.According to the site conditions of the structure,12 sets of seismic waves were selected,and the incremental dynamic analysis of the tunnel structure was carried out after amplitude modulation processing of the seismic waves.The fitting effects of linear regression equations with different intensity parameters are compared,and PGA,which has a lower degree of dispersion,is used as the ground vibration intensity indicator.According to the lining cross-section shape of the horseshoe tunnel,the relative deformation rate and the moment carrying capacity ratio are used as damage indicators to measure the damage level of the tunnel,respectively.Three seismic damage classes were defined according to the damage state of the tunnel.The parameters of the seismic vulnerability function were obtained by linear regression analysis,and the seismic vulnerability curve based on the relative deformation rate and bending moment bearing capacity ratio was given.4.The seismic vulnerability curves based on the joint index are given by combining the probability of exceeding different performance levels under two damage indicators.The seismic vulnerability curves under the joint index predicted the probability of occurrence of various levels of seismic damage in the tunnel under different earthquake intensities.The vulnerability assessment based on this joint index is on the conservative side since the effects of both damage indicators on tunnel vulnerability are considered simultaneously.The vulnerability curves under the joint index can provide a reference for pre-earthquake hazard prediction and post-earthquake damage assessment of the tunnel.5.The effect of different site conditions on the seismic performance of tunnels is studied.The results show that the degree of site softness has a great influence on the seismic performance of the structure,and the tunnel structure in softer sites is more dangerous.Therefore,the seismic performance of tunnels at soft sites should be considered in the structural design to ensure structural safety.