Failure analysis and countermeasures of a tunnel constructed in loose granular stratum by shallow tunnelling method

Abstract

To solve the failure problems encountered in the tunnels constructed by shallow tunnelling method in loose granular stratum, taking a tunnel construction project on the Sichuan-Tibet 318 Highway Line in China as an example. Failure phenomena, characteristics, and modes were systematically analyzed during tunnel construction. Then, the evolution at different failure modes was analyzed through DEM calculation and the prevention mechanism was discussed. Finally, corresponding construction countermeasures were put forward from the aspects of pre-support, excavation, bolt driving, and collapse prevention. The results show that: (1) The conventionally strong support system cannot play an active role in the tunnel. The core problems in construction are how to reduce the disturbance of excavation to surrounding rock and realize the restraint of the support system to the surrounding rock in the arch-foot area. (2) According to the loss of the surrounding rock, the failure modes can be divided into two types: Ⅰ-the rapid development failure from vault collapse and Ⅱ-the lag failure of ground cavity moving up. Construction optimization aims at avoiding the mode-Ⅰ and alleviating the mode-Ⅱ. (3) For the mode-Ⅰ, maintaining the stability of the initial ground-arch is the practical measure; For the mode-Ⅱ, when the cavity area is more significant than 5 m2, it must be filled in time. (4) The field deformation data of surrounding rock after treatment shows that the effect of the proposed countermeasures is remarkable, which can provide valuable references for the tunnel design, construction, and disaster control in a similar area.