The proximity of a tunnel face to a high-pressure filled karst cavity easily triggers water and mud inrush disasters, endangering tunnel construction safety. To mitigate construction risks, it is essential to clarify the drainage pressure relief mechanism and post-drainage excavation response when a tunnel face approaches a filled karst cavity during construction. In this study, a three-dimensional drainage pressure relief test system was built to reveal the drainage mechanism under different water heads and filling types, and to determine the optimal drainage scheme. On this basis, step-by-step excavation tests were carried out to investigate the response of the anti-outburst rock mass and karst cavity fillings under excavation disturbance. The results show that both the water discharge and pressure relief gradient under the slurry filling condition are lower than those under the pure water condition. At a 110 cm water head, the peak discharge of the middle-upper pipe under the slurry condition reaches 588 ml/10 s, lower than 680 ml/10 s under the pure water condition. Affected by solid-phase migration in fillings, the earth pressure at the rear of the karst cavity first decreases and then increases under the slurry condition. The slurry condition features progressive deterioration of the anti-outburst rock mass and self-stabilization of fillings, while the pure water condition presents a rapid seepage failure mode with faster instability. The maximum tunnel face deformation is 3.0 mm under the slurry condition, versus only 0.45 mm under the pure water condition. With the multi-pipe diversion effect, the middle-upper pipe combination has larger discharge than the single middle pipe, avoids single-pipe clogging risks, and costs less than the three-pipe scheme, making it the optimal solution.
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An et al. (2026) studied this question.