基于两种拱盖法修建的青岛地区上软下硬岩质地层大跨隧道结构稳定性研究

Study on Structural Stability of Mined Large-Span Tunnels Constructed by Two Arch-Cover Methods in Upper-Soft and Lower-Hard Rock Stratum of Qingdao Area

拱盖法是基于纵深刚度差异较大的“上软下硬”地层条件,在充分吸取盖挖法、双侧壁导坑法等工法的成功经验或者不足之处的基础上发展起来的一种暗挖大跨隧道修建技术,拱盖法主要有双层初期支护拱盖法和二次衬砌拱盖法。文章以青岛地铁某暗挖大跨车站为例,结合站址范围内上软下硬复合地层的特征,通过数值方法对两种拱盖法修筑的隧道结构稳定性进行分析研究。结果表明:1)二次衬砌拱盖法采用纵向9 m拆撑安全步距施工时,外层初期支护棚护作用的“空间刚度效应”不足以弥补其拱盖的“拱”效应,双层初期支护拱盖法修筑技术更有利于隧道变形控制;2)隧道下部岩体开挖时拱盖结构的承载、保护作用显著,但在相同等地质环境条件下,二次衬砌拱盖法施工对拱脚岩体的塑性破坏作用更明显,换言之即二次衬砌拱脚对岩基的强度和刚度要求更高;3)初期支护拱盖法结构受力控制部位为拱肩及拱脚,二次衬砌拱盖法结构受力控制部位为拱脚,后者拱脚控制内力约为前者的1.56倍,前者的“梁”效应比较明显,后者的“脚”效应比较明显;4)为地铁百年服务工程安全计,建议优先考虑采用双层初期支护拱盖法施工。

As for the upper-soft and lower-hard rock stratum with larger stiffness differences in vertical depth,the arch-cover method is developed based on the cover-excavation method and double side heading method.The archcover method includes the arch-cover method with double-layer primary support and the arch-cover method with secondly lining.Taking a mined large-span station of Qingdao metro as an example,a numerical analysis on struc tural stability of the tunnel constructed by two arch-over methods is conducted in light of the characteristics of the upper-soft and lower-hard rock stratum.The results show that:(1)as for the arch-cover method with secondary lin ing,when tunnel is constructed at a safe strut dismantling distance of 9 m in longitudinal direction,the"spatial stiff ness effect"of shed protection of outer primary support can′t make up for the"arch"effect of arch-cover,the con struction technology of arch-cover method with double-layer primary support is more conducive to tunnel deforma tion control;(2)the arch-cover structure has significant bearing and protection functions during excavating the lower part of the tunnel,however,the plastic damage of rock mass at arch springing is more obvious during the construc tion by arch-cover method with secondary lining under the same geological conditions,in other words,the arch springing of secondary lining requires higher strength and rigidity of the rock foundation;(3)the internal force con trol positions of structure by arch-cover method with double-layer primary support are spandrel and arch springing,and the control position is the arch springing for the structure by arch-cover method with secondary lining,the con trol internal force of arch springing of the latter is about 1.56 times of that of the former,the"beam"effect of the for mer is more obvious while the"foot"effect of the latter is more obvious;(4)from the perspective of safety,it is sug gested that the arch-cover method with double-layer primary support should be adopted.