Seismic Vulnerability Analysis Of Ultra-large Diameter Shield Tunnels In Local Liquefaction Site

Earthquake-induced liquefaction has caused many significant damage to shield tunnels in past earthquakes,and it also poses a great threat to the safety performance of tunnels.Because of the high cost and the large span of space,shield tunnels are difficult to repair after damaged and it will cause huge loses of economy.Therefore,the accurate assessment of the seismic performance and seismic fragility of shield tunnels in liquefiable soils has become an important issue of concern in tunnel engineering.At present,most of the existing research focuses on the seismic response of tunnel structures located entirely in liquefiable soils.However,only limited studies have been carried out to discuss the seismic response of tunnel structures crossing soft(liquefiable soils)and hard(rock)strata at the same time.On the above,the super large diameter shield tunnel in local liquefaction site was taken as the research object,a refined numerical simulation method based on tunnel-soil dynamic interaction has been established,the basic laws of seismic response of shield tunnels in local liquefaction sites have been studied,a quantitative evaluation method for the optimal ground motion intensity measures has been proposed,and an analysis of the seismic fragility of the tunnel structure was carried out.Main content and results are as follow:(1)In order to study the seismic response characteristics of super large diameter shield tunnels in liquefiable soil deposit with sandwiched non-liquefiable layer,a finite element model of the tunnel-soil dynamic interaction is developed based on the Open Sees finite element calculation platform,taking into account the soil liquefaction effect,the non-linear characteristics of the tube sheet joints and the tube sheet,and the reliability of the model is verified by centrifuge test results and analytical methods.At the same time,the seismic response characteristics of the tunnel structure of the site are analysed in terms of the acceleration response to the site soil,the super-pore water pressure response,the displacement and deformation of the site soil,the internal forces and deformation of the shield tunnel,and the mechanical properties of the tube sheet joints.(2)In order to study the optimal ground motion intensity measures for probabilistic seismic demand analysis of super large diameter shield tunnels in liquefiable soil deposit with sandwiched non-liquefiable layer,this paper obtains non-linear seismic response data onto 60near-site vibrations and 60 far-site vibration records based on the established refined numerical model.The 29 ground motion intensity measures were examined and analysed in terms of efficiency,practicality,proficiency,sufficiency and hazard computability,and the optimal ground motion intensity measures were comprehensively evaluated and selected by combining the fuzzy comprehensive evaluation method.(3)In order to study seismic fragility analysis of super large diameter shield tunnel in liquefiable soil deposit with sandwiched non-liquefiable layer,and considering that the permanent deformation and dislocation of the tunnel joints at the end of the earthquake will directly lead to the risk of structural leakage,this paper selects the external tension of the tunnel tubes as the structural damage index,defines three structural damage state limits of minor damage,moderate damage and severe damage,and derives the probabilistic seismic fragility curves corresponding to the classification of the structural damage state into several classes.The research content and results of this paper further deepen the understanding of seismic response characteristics and seismic fragility analysis of shield tunnels in liquefiable soil deposit,and provide a theoretical basis for future performance-based seismic design of super large diameter shield tunnels,which is of great significance.