Experimental investigation of ground collapse induced by Soil-Water leakage in local failed tunnels

Soil-water leakage in local failed tunnels poses significant risks of ground collapse, particularly in water-rich strata. Despite the acknowledged threats, there is a lack of clarity regarding the underlying mechanisms and assessment criteria for the collapse process. This study conducted a series of model tests that visually reproduce the ground collapse above the local failed tunnel using the digital image correlation (DIC) technique. The soil-water leakage variation and strata deformation were monitored during the test under different tunnel opening sizes, buried depths, hydraulic gradients, and sand particle sizes. Based on the test results, a rapid evaluation method is proposed for ground subsidence induced by local tunnel leakage. The research findings reveal that surface subsidence curves align with the Gaussian normal distribution, while the soil loss rate eta predominantly falls within 60% to 70% under hydraulic scouring. The inclination of the final failure surface closely approximates the internal friction angle of the strata. Moreover, the tunnel opening diameter exerts the most significant impact on the strata failure rate, followed by the buried depth, sand particle size and hydraulic head, while only the buried depth affects the final leakage mass of the sand-water mixture. The progressive nature of strata failure is evident, and two novel indicators are proposed to assess the failure mode and stage of strata: the sand content ratio beta of leakage mixture to saturated strata and the opening degree zeta of the strata loosening contour. The critical value for beta, determining whether there will be a stable arch or continuous collapse of strata, is approximately 50%. Finally, a rapid assessment method of strata progressive failure is established based on the indicators, effectively categorizing the progressive failure into distinct stages of strata loosening, surface subsidence, and re-stabilization and proposing reinforcement suggestions for practical engineering. In summary, this study not only sheds light on the influence of tunnel local failure and strata characteristics on the strata progressive failure but also provides a rapid assessment method for ground collapse accidents induced by soil-water leakage in locally failed tunnels.