| Metro tunnel with asymmetric section with small spacing is a special tunnel structure between separated tunnel and multi-arch tunnel,it has the difference of geometric asymmetry of double-line section.At present,there are no written norms and standards for calculating the surrounding rock pressure of small spacing asymmetric section Metro tunnels,it is still a reference method for calculating the surrounding rock pressure of small spacing symmetric section tunnels,however,there is obvious bias effect in the construction of small spacing asymmetric cross-section Metro tunnels,so there are obvious errors in analysis and engineering practice of small spacing symmetric section tunnels.This paper takes the construction of subway tunnel between Pingchuan Road and Jiujiawan Tunnel of Xinjiang Metro Line 2 as engineering background combines theoretical derivation,numerical simulation and monitoring data analysis to study the surrounding rock pressure calculation and construction optimization of small spacing asymmetric cross-section Metro tunnels under shallow burial conditions in order to provide a reference for the construction of small spacing symmetric section tunnels.The main contents of this paper are as follows:(1)On the basis of summarizing and analyzing the theory of surrounding rock pressure commonly used in tunnels at present,considering fully the influence of geometric asymmetry and excavation sequence of small distance tunnel on surrounding rock load,defining the loading mode of surrounding rock of shallow-buried asymmetrical small clear-distance tunnel,putting forward the corresponding calculation method of surrounding rock pressure and discussing the distribution law and characteristics of surrounding rock pressure.(2)Combined with the Pingchuan Road Station-Jiujiawan Station section of Xinjiang Urumqi Rail Transit Line 2,the construction sequence,construction method,spacing between left and right tunnel face and net distance of shallow-buried small net distance asymmetric cross-section metro tunnel are numerically simulated by using finite element software MIDAS/GTS NX and field measured data.,the variation law of displacement and stress of tunnel surrounding rock under different construction techniques and the optimal construction scheme under different working conditions are analyzed.(3)The thickness of small spacing asymmetric section Metro tunnels is much smaller than that of the separated tunnel,so there will be many disturbances during the construction of the middle rock pillar and this lead to the superposition of the stress and displacement fields of the middle rock pillar is prominent,however the stability of the middle rock pillar plays a decisive role in the construction safety of small spacing asymmetric section Metro tunnels.Based on the study of the deformation of surrounding rock and the plastic zone state of small spacing asymmetric cross-section Metro tunnels under different reinforcement schemes with middle rock,the optimum reinforcement scheme of intermediate rock pillars is obtained for the tunnel of Section II excavated in the Pingchuan Road to Jiujiawan Section. |
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