Catastrophe mechanism and early warning indicators of seepage erosion-induced water inrush in karst cavities

Seepage-erosion-induced water inrush in karst cavities is a typical form of water-inrush disaster in karst tunnels. It is governed not only by the spatial distribution of karst cavities and hydraulic recharge conditions, but also by the particle-size gradation and composition of the cavity fill. During seepage erosion, fill particles are progressively transported by flowing water, which may trigger a sudden water-inrush catastrophe. To reveal the catastrophe mechanism and establish early-warning indicators, this study employs the Smoothed Particle Hydrodynamics (SPH) method to simulate the evolution of seepage-erosion-induced water inrush under different particle-size gradations, cavity confining stresses, and seepage velocities. The inflection point of the cumulative particle loss rate is used as an indicator of catastrophic transition. The relationships among particle-size gradation, confining stress, seepage velocity, and particle loss rate at the transition point are then analyzed to determine early-warning thresholds. The results show that fill-particle loss is positively correlated with both seepage velocity and confining stress. When the content of fine particles, such as rock cuttings and fine sand, exceeds 60%, the inflection point corresponds to a seepage velocity of 1.6 m/s and a confining stress of 2.6 MPa, with an early-warning particle-loss range of 8%-15%. When the content of coarse particles, such as coarse sand and gravel, exceeds 40%, the inflection point corresponds to a seepage velocity of 3.0 m/s and a confining stress of 4.5 MPa, with an early-warning particle-loss threshold of approximately 45%.