Abstract: To analyze the influence of hydraulic fracturing in hidden karst structures on the safe thickness and fracture evolution of a waterresistant rock mass, a highperformance GDEMDAS based on the continuumdiscontinuum element method is used to conduct threedimensional numerical simulation. The critical thickness of the rock mass is determined on the basis of the displacement evolution law of the tunnel face, and the fracture degree, a dimensionless index, is introduced to quantitatively describe the fracture evolution law of the rock mass during the excavation process. The results show that the changing curves of displacement and fracture degree are obtained at water pressures of 1, 2, and 3 MPa. The safe thicknesses of the rock mass under different water pressures are almost the same, at 7 m. In addition, the curves of displacement and fracture degree show inflection points of fracture degree 6 m in front of the inflection point of displacement, showing that displacement mutation will occur when the internal damage of the rock mass reaches a catastrophic level. Moreover, under the same simulation conditions, the inflection points of displacement and fracture degree under different water pressures in the karst cave are consistent, indicating that the water pressure is not the primary factor affecting the stability of the waterresistant rock mass.

Key words: karst tunnel, waterresistant rock mass, fracture degree, safe thickness, GDEM, numerical simulation