Study On The Influence Of Karst Cavity On The Stability Of Large Section Highway Tunnel

High mountains,deep valleys,and ravines karst dissolution landform in China's western mountainous areas have brought the construction problems of building city fast road network.Large and extra-large cross-section tunnel through karst stratum are often the control engineering.Tunnel construction problems in karst stratum become the whole line's control problems.In karst stratum,karst growth formed the karst cavity,which caused the distribution of surrounding rock stress field greatly different with ordinary formation.There is a strong stress concentration area in the vicinity of the karst cavity,and the heterogeneity of the stress distribution will be intensified when the tunnel is excavated.Because of the complexity of karst formation(especially when the tunnel is close to the backside karst cavity)and the influence of seasonal,periodic and paroxysmal precipitation,the stress of lining in tunnel operation period is very complex.Therefore,it is necessary to study the influence of karst cavity on the stability of tunnel construction and operation period.This paper is based on an important part of the fast road network "three line" in Chongqing--the Shuangbei tunnel.The numerical analysis methods is adapted to study the influence of karst cavity on the stability of tunnel construction and operation period.The stability of tunnel excavation during tunnel construction and the characteristics and safety of the secondary lining during operation are simulated.The main research contents and conclusions are as follows.1)By using plane strain model and considering different working conditions such as the cavity's size,position,and distance from tunnel,simulating the tunnel excavation,monitoring the displacement around the tunnel and initial support force during tunnel excavation and evaluating the influence of karst cavity on stability of tunnel surrounding rock and support structure when the hidden karst cavity is behind the tunnel2)By using three-dimensional finite element model,simulating of karst cavity three-dimensional space structure and the complete tunnel excavation process,the construction cycle along the longitudinal excavation.Selecting three typical section of karst cavity along the longitudinal space size change,monitoring and analyzing the displacement around the tunnel,displacement field of surrounding rock,initial support force,and the plastic zone.The influence of karst cavity on the stability of tunnel longitudinal excavation is studied.3)By using seepage structure model,monitoring secondary lining external water pressure and the force of the secondary lining structure.Studying the influence of the existence of karst cavities near tunnel on the external water pressure behind the tunnel.In the design of the drainage system,the force distribution characteristics of the secondary lining structure and the safety of the structure are obtained.