Structural Failure Characteristics of Operational Pipeline Tunnel Adjacent to Pressurized Karst Cave

Internal pressure surges in karst caves from rainfall and groundwater fluctuations critically affect adjacent tunnel stability. This study first analyzes water inrush mechanisms from pressurized karst caves and lining-defect/karst-cave distributions. Then it examines contact pressure, maximum principal stress, mechanical responses, and deformation-failure characteristics as cave pressure increases at three positions (vault, left arch waist, left sidewall) using numerical simulation and model tests. Results show that cave pressurization markedly increases nearby contact pressure, with location effects ordered: vault > left arch waist > left sidewall. For vault or left-arch-waist positions, axial-force distribution nonuniformity increases with cave pressure. Conversely, left-sidewall locations show fluctuating uniformity in axial-force distribution. Under all three working conditions, the inner side of the lining adjacent to the karst cave is subjected to significant positive bending moments, while both sides experience negative bending moments. The structural deformation patterns under the three working conditions are generally consistent, with the deformation rate being the fastest at locations adjacent to the karst cave, where abrupt changes occur before structural instability. For vault or left arch waist positions, tensile cracks tend to develop on the inner surface of the lining adjacent to the cave. For left-sidewall positions, the critical water pressure for sidewall and invert failure is the lowest because of the combined action of three factors: the sidewall simultaneously bearing compression and tension, stress concentration at the sharp corner transition, and the invert being in an eccentric compression state.