The impact of extreme rainfall and drainage system failure on rock tunnels: A case study of deep-buried karst tunnel

In the light of climate change, extreme rainfall weather events have resulted in severe defects, for example, cracks, water leakage, concrete spalling, in many underground tunnels around the world, particularly for those with inadequate drainage system capacity and crystalline blockage issues. This paper presents a case study of a deeply buried karst tunnel subject to extreme rainfall. Field observations show a clear relationship between tunnel defects with the blockage of the tunnel drainage system and categorized the defects into three main types: non-seepage cracks, cracks with seepage, and seepage at construction joints. Subsequently, hydromechanical coupled finite element analysis was conducted to evaluate the impact of varying degree of tunnel drainage system blockage (DB) on the tunnel lining. The integration of DB into relative ground-lining permeability (RP) defines the Modified RP, which is able to consider the change of ground-lining permeability due to the impact of tunnel drainage system blockage. Furthermore, finite element results indicate that with the increase of the degree of the drainage system blockage (increasing DB), the maximum pore water pressure behind the lining builds up due to heavy precipitation following three stages: slow increase, rapid rise, and level off. In particularly, extreme rainfall leads to: (1) severe clogging of the drainage system (ΔDB) and therefore greater rise in pore pressure (ΔNPP); (2) extreme precipitation itself induces an additional increase in pore pressure (ΔNPP’).