Deformation and failure of a tunnel in the restraining bend of a strike–slip fault zone: an example from Hengshan Mountain, Shanxi Province, China

The Yanmenguan Tunnel runs through Hengshan Mountain in Shanxi Province, China. The mountain is mainly composed of metamorphic rock cut by a well-developed strike–slip fault system. As a result of the local geological setting, significant deformation and mud bursting occurred during excavation of the tunnel. In this paper, a representative 170-m-long section of the tunnel (from Sta. 118 K + 30.0 to Sta. 118 K + 200.0), which is located in the restraining bend area of the strike-slip fault system, is chosen as a case study to investigate the influence of such a fault on tunnel stability. The characteristics of strike-slip faults and asymmetrical pressure are then investigated, in conjunction with monitoring stress and displacement in the tunnel. The results of the analysis suggest that the strike-slip fault complicates the geological condition and decreases the quality of the rock masses. Different rock mass classifications for the surrounding rocks are the dominant factor causing asymmetrical pressure in the tunnel, which in turn significantly influences deformation in the tunnel. Shear and crushing failure are the mechanisms of failure in the surrounding rocks in the surveyed section, where significant deformation and mud bursting occurred. It indicates that stress and the classifications of surrounding rock masses should be carefully investigated and analyzed during the design and construction of underground engineering works in such a setting.