一种无中导洞连拱隧道衬砌开裂原因分析及结构优化

Cracking Analysis and Structural Optimization of Lining of Multi-arch Tunnel without Middle Drift

为解决传统连拱隧道中导洞法施工工序繁琐、结构受力转换复杂等问题,云南某隧道采用了一种无中导洞连拱隧道,其后行洞钢拱架焊接于先行洞钢拱架,使初期支护相互搭接形成了连拱隧道中墙,从而避免了中导洞开挖,但在隧道施工中,先行洞二次衬砌左侧起拱线至左拱腰出现了大量纵向与斜向裂缝。结合隧道实际地质和施工状况,运用Flac3d建立了地层-隧道结构数值模型,研究分析了初期支护各自独立封闭成环和初期支护搭接处界面特性对无中导洞连拱隧道衬砌开裂的影响。结果表明:后行洞初期支护独立闭合成环时,即使初支结构之间产生了一定的滑移,隧道衬砌结构仍处于安全状态;后行洞初期支护未独立闭合成环时,搭接处会产生较大的相对滑移,初期支护承载力不能充分发挥,难以有效控制围岩变形和塑性区发展,先行洞二次衬砌承受了较大的围岩荷载,其左拱腰内侧边缘拉应力远超过了衬砌混凝土的抗拉强度,由此造成了左拱腰处衬砌开裂。因此,为避免无中导洞连拱隧道衬砌出现裂缝,建议在设计中应使连拱隧道初期支护各自独立闭合成环,合理加强中墙位置初期支护结构,施工中对关键施工环节、关键受力部位采取有效控制措施,保证支护结构的整体承载力。

To solve the problems of complicated construction process andcomplicated structural force transformation in the middle drift methodused in traditional multi-arch tunnel,a multi-arch tunnel without middledrift has been applied in a tunnel in Yunnan Province.Its following holesteel arch frame is welded to the front hole steel arch frame,so thatthe primary supports can overlap each other to form the middle wall ofthe tunnel,avoiding the excavation of middle drift.However,there are alarge number of longitudinal and oblique cracks ranging from springingline to left hance in the left side of secondary lining of front holeduring construction.According to the actual geology and constructionsituation of the tunnel,the numerical model of formation-tunnelstructure is established by using Flac3 d.The influences of the primarysupport with freestanding closed loop and the interface characteristicsat lap joint of primary support on lining cracking in multi-arch tunnelwithout middle drift are analysed.The result shows that(1) the lining isstill in a safe state when the primary support of the following holewith independently freestanding closed loop,even if a certain slipoccurred between the primary support structures;(2) when the primarysupport of the following hole without non-freestanding closed loop,alarge relative slip may occur at the connection position.Thereby,theprimary support cannot fully exert the bearing capacity and effectivelycontrol the deformation of the surrounding rock and the development ofplastic zone.Subjected to heavy surrounding rock loads, the secondarylining of the front hole cracked because the tensile stress at the inneredge of the left hance far exceeded the tensile strength of concrete.Inorder to prevent the lining cracks in the tunnel,it is suggested todesign the freestanding close loop primary support and reasonablystrengthen the primary support structure of the middle wall.Moreover,totake effective control measures in key construction links and key stressparts can ensure the overall bearing capacity of the support structure.