Investigating the evolution of debris flow disaster in tunnels: model testing and numerical simulations

Abstract

Water-mud inrush disasters in tunnels are prone to occur under high-pressure water-rich conditions in karst, granite alteration zone, jointed fracture dense zone. The debris flow strata have the characteristics of water-sensitive fractured geological structure, which easily lead to hydraulic connectivity, seepage failure and chain instability subjected to tunnelling disturbance. Accordingly, some tunnels have suffered from water-mud inrush disasters during construction in debris flow strata, even under general water level conditions. This research reproduces the evolutionary process of debris flow disaster from seepage failure to catastrophe, and reveals the characteristics of seeping, eroding and piping on the working face by model tests. Furthermore, PFC-CFD coupled numerical simulation in microscopic scale is applied to analyze the partial to overall instability behavior of the working face and the governing influencing factors. Considering the tunnelling disturbance, theoretical formulas for identification of minimum safe thickness of the rock mass in debris flow disaster are established. The results show that: (1) Prior to the occurrence of debris flow disasters, the seepage channel of the rock mass is gradually formed, and seepage holes appear on the working face. With seepage-erosion intensified, the seepage holes expanded, causing partial failure. Following a brief period of stability, the overall failure of the rock mass in front of the working face resulted in the occurrence of debris flow disasters. (2) As the debris flow disaster progresses, there are two mutations in the characteristic values, including those of water flow and kinetic energy. The evolutionary process is divided into three stages: pregnant, latent and occurrence. (3) Material composition and water pressure are the governing influencing factors for the start-up potential of debris flow disasters. When the equivalent friction angle of debris flow strata is reduced from 30° to 26°, the maximum kinetic energy is increased by about 26.7 %. The critical seepage pressure for debris flow disasters is about 0.5 MPa. (4) Subjected to the influence of tunnelling disturbance, the minimum safe thickness will exceed 5.3 m, with a maximum of up to 8.1 m. Qilian Mountain Tunnel has definitively established and implemented the minimum safe thickness of 9 m.