The mountain railway No. 42 Tunnel once collapsed during construction when only the primary shotcrete lining was in place. According to geological analysis and excavation records, the major cause of disaster was presumably due to time-dependent behavior of rock. In order to elucidate the impact of creep-induced displacement during excavation, a simulation-based methodology, i.e. a viscoplastic model combining the two-component power model and the ubiquitous-joint model, is proposed. The time dependent parameters were calibrated using the back analysis from the convergence measurement data. The modelling results have illustrated that the excavation induced displacement can be dissociated into two parts, the instantaneous deformation due to stress relaxation, and the time dependent deformation due to creep. The overall study in terms of current findings has shown that our proposed model is a useful tool for investigating long term stability of tunnels surrounded by creep potential rock material.
On September 28, 2015, the mountain railway No. 42 tunnel was affected by Typhoon Dujuan, and the traffic service was thus interrupted. The reconstruction plan of the tunnel was to move the original alignment toward the mountain side. During reconstruction, the side wall close to the 59K+160 mileage experienced circumferential cracks. Various structural support techniques were utilized to restrain rock deformation. Despite these efforts, the shotcrete continued to be squeezed and cracked, which eventually led to the collapse of the tunnel.
During excavation stage of mountain tunnels, the rock mass surrounding the tunnel will deform due to stress relaxation. It is necessary to implement appropriate support methods according to factors such as geological conditions, in-situ stress, rock mass strength, and construction methods to maintain the stability of the tunnel (Hsiao 2010). In addition to the above-mentioned reasons, the time-dependent deformation behavior of the rock mass has an impact on the long-term stability of the tunnel. For example, the Zhegu Mountain 317 Highway Tunnel located in Sichuan continues to exhibit significant surface cracking on its lining structure, even after more than a decade of operation. Creep behavior of the surrounding metamorphic rock mass was presumed to be the cause of this phenomenon (Meng et al. 2013). Numerical simulations were extensively deployed to reveal tunnel mechanical behavior (Galli et al. 2003, Lorig & Varona 2013 and Barla & Barla 2000). This study attempts to use geological records and deformation monitoring data obtained from actual tunnel excavation to identify potential causes of this disaster. A numerical conceptual model based on finite difference FLAC3D model (Itasca 2014) is built-up to investigate the mechanism of tunnel close to the section where the collapse had occurred. The impact of time-dependent deformation of the surrounding rock on the long-term stability of the tunnel is also discussed in the present work.
