Water inrush mechanism and minimum safe thickness of rock wall of karst tunnel face under blast excavation

The hydraulic fracturing of rock mass is one of the main factors for water inrush in karst tunnels. From the perspective of fracture mechanics, the impact of explosive stress wave on extension of water-carrying fracture is studied under drilling and blast construction. The results show that the expansion pattern of fracture is influenced by the pore water pressure produced by the explosive stress wave. By establishing the computational model for fractured rock mass, the critical water pressure (P_c) under the explosive stress wave is deduced, and it is verified by cases. Based on the theoretical analysis, numerical experiments and engineering cases, the hysteresis effect of water inrush caused by extended compression-shear failure of rock mass is obtained. According to the hysteresis effect, constructors can terminate construction and leave immediately before the occurrence of water inrush so as to ensure the safety of personnel and mechanical equipments. By analyzing the minimum safe thickness between the fractured rocks of karst tunnel face and high-pressure aquifer, “Two-band theory” is put forward. Furthermore, the formula for the minimum safe thickness is deduced, which can reflect the excavation and water pressure reasonably and is verified by engineering cases. It may provide a reference for the effective measures for water inrush in high-risk karst areas.