Damage Mechanisms of Soft Rock Tunnels in the Western China: A Case Study on the Dujiashan Tunnel

In the Western China, soft rocks, in particular, phyllites are very widespread. With the increase of soft rock tunnels in recent years, the secondary lining cracking of newly-built tunnels have frequently occurred. In this work, a case study on the Dujiashan Tunnel in the Western China was conducted in order to get insight into the damage mechanisms of such soft rock tunnels in the strong earthquake zone. Furthermore, for the engineering background, the cracking characteristics, on-site monitoring data, influencing factors and damage causes for the the second lining of Dujiashan Tunnel were also investigated. It was demonstrated that with the concentration of the bending moment and shear stress, the reduction in axial pressure appeared on the lining structure at the cavities due to local collapses. Moreover, the aftershocks of the Wenchuan earthquakes could further weaken the strength characteristics. It was revealed that the secondary lining measures were useful for the reinforcement of the Dujiashan Tunnel, and the treatment measures could achieve mainly the backfilling collapse cavities, leading to reinforcing the surrounding rocks, preventing the cracking behavior, strengthening lining, and smoothing drainage. It is therefore suggested that for the newly-built soft-rock tunnels, especially those located in strong earthquake zones, the use of the secondary lining should be at the appropriate time and backfilling the cavities due to local collapses timely, and all the factors are important for the protection and reinforcements of the surrounding rocks to ensure the safety of the tunnel structure.