Water inrush mechanism and the minimum safety thickness of the rock wall of a tunnel crossing a fault fracture zone

Objective With the relocation of major national strategic plans to western China, railway construction has gradually focused on the complex and dangerous mountainous regions of Yunnan, Sichuan, and Xizang Provinces, where the proportion of tunnels along the railway is very high. When a tunnel passes through a water-rich fault fracture zone, the rock mass in front of the palm face is prone to hydraulic fracturing and damage under high osmotic pressure, leading to disasters such as rock collapse and water inrush. Methods The wing crack model is introduced to fully account for the initiation and propagation of secondary wing cracks in water-saturated fractures, as well as the impact of excavation disturbances. The effective tensile stress and rock bridge size between intermittent fractures in the rock are revised. The tensile-shear failure mechanism of the water-insulating rock mass in front of the tunnel face is analyzed, and the critical water pressure for hydraulic fracturing of the water-insulating rock mass is derived. The minimum safety thickness for the tunnel face against water inrush in the proximity of a fault fracture zone is proposed. Results The theoretical formulas indicate that the anti-splitting thickness of the water-insulating rock mass is related to factors such as tunnel section size, fault water pressure, excavation disturbance, in-situ stress, rock mass strength, crack size, and fracture parameters. Through analysis of the sensitivity of the different influencing factors, it is found that the anti-splitting thickness of the rock mass increases with the increase of the tunnel section size, the fault water pressure, and the excavation disturbance factor, but decreases with the increase of the vertical tunnel stress and the rock mass strength. At the same time, the excavation disturbance damage has the most significant impact on the calculated anti-splitting thickness of the rock mass. Conclusion In practical engineering, there are certain empirical judgments and errors in obtaining excavation disturbance factors via rock integrity assessment and rock wave velocity testing. Therefore, this method requires accurate acquisition of the damage conditions of the rock mass in front of the tunnel face. Various assessment methods can be used for comparison and selection, and a conservative approach can be adopted by using a larger value for the excavation disturbance factor. Significance Finally, taking a tunnel in western Sichuan near the Yalahe fault as an example and considering the actual engineering disturbance and fault water pressure, the minimum safety thickness of the rock wall at the tunnel face is calculated to verify the engineering applicability of the proposed method. This research can effectively guide on-site risk prediction and plan formulation; it provides a theoretical basis for the prevention and control of water inrush in tunnels crossing water-rich fault fracture zones.