Study On The Safety Of The Straight Wall Tunnel Under The Condition Of Void Behind The Lining

In the early days in China,most of the tunnels were built in the form of straight wall tunnel.With the extension of the tunnel operation life,a large number of lining damages appeared in the straight wall tunnel.As a common quality defect,the void behind the lining is one of the main causes of the tunnel damages,which seriously threatens the operation safety of the straight wall tunnel.In order to analyze the mechanical properties and safety of the straight wall tunnel structure under the condition of the voids,this paper establishes a three-dimensional numerical model to analyze the safety of the lining structure under the influence of the voids,and analyzes the cracking mechanism and evolution law of the straight wall tunnel lining under the condition of void by using the XFEM,The main research results are as follows:(1)Combined with the inspection data of 21 straight wall tunnel lining damages,the statistical results show that: the lining leakage is mainly located in the arch,and the leakage water is mainly in the form of infiltration;the cracks are mainly distributed in the arch,and the longitudinal cracks >circumferential cracks > oblique cracks;the voids behind the lining are mainly distributed in the arch,the average longitudinal length of the voids is about 10 m,and the average vertical height is about19.4cm.(2)Based on the three-dimensional finite element model,the internal force distribution rule and safety of the lining structure under the condition of the existence of the void are distributed.The main conclusions are as follows: the change of the circumferential range of the void has a significant impact on the internal force and safety of the lining structure,and the overall safety of the structure decreases with the increase of the circumferential range.The vertical deformation of the lining decreases with the distance from the void,and the deformation rate is the largest at the boundary of the void.The increase of the longitudinal length of the tunnel has little effect on the safety of the lining cross section,but with the increase of the longitudinal length of the tunnel,the proportion of the dangerous section of the lining increases,further reducing the bearing capacity of the tunnel lining structure.The change of radial height has little effect on the safety of tunnel structure.(3)Based on the analysis of the cracking mechanism and evolution rule of the lining structure of the straight wall tunnel under the condition of the existence of the void by the extended finite element method(XFEM),it is found that when the circumferential range of the void behind the arch lining is small,only two cracks appear at the arch foot on both sides of the lining structure,just like the case without the void.When the circumferential range of the void increases to a certain extent,the arch structure reaches the limit value of tensile stress and cracks.From the process of crack propagation of the arch crown,the crack propagation in the early stage of loading needs a larger load gradient.With the increase of crack propagation rate,the load gradient required for crack propagation decreases gradually.The increase of the circumferential range of the void behind the arch worsens the mechanical characteristics of the lining structure,and then changes the cracking mechanism of the lining structure,resulting in the decrease of the bearing capacity of the lining structure.(4)Comprehensively consider the indicators such as the void behind the lining,and establish a safety evaluation model for the lining structure of the straight wall tunnel under the influence of the void.Taking the void behind the lining as the core index,a multi-level comprehensive evaluation under the influence of the void behind the straight wall lining is established.system.