Study on the Water Inrush Failure Characteristics and Disaster-Inducing Threshold of Tunnels Overlying Karst Caves under Stress-Strain-Seepage Coupling

Tunnels traversing overlying water-filled karst caves face high risks of water inrush disasters. This study develops a stress-strain-seepage coupling model for karst tunnel surrounding rock, centered on a four-stage linear strain hardening-softening model derived from triaxial tests on intact lime-stone. A corresponding parameter inversion method applicable to different rock mass grades is proposed. Using the Shuangbei Tunnel as a case study, FLAC3D simulations analyze disaster char-acteristics—deformation, plastic zone, permeability, and water inflow—under varying karst cave water pressures, identifying the disaster-inducing threshold. Model reliability is confirmed via physical model tests. The impacts of key geological factors—cave spacing, diameter, tunnel depth, and lateral pressure coefficient—on water inrush features and critical threshold are examined. Re-sults show karst cave water pressure is the primary control factor. Once the pressure reaches the threshold (1.4 MPa under studied conditions), displacement surges and overall instability occur in the intervening rock, triggering inrush. Other factors jointly influence the disaster threshold and channel geometry, constraining the potential instability zone and complexity of the water inrush channel. This study provides a theoretical basis for stability analysis and disaster prevention in tunnels with overlying karst caves.