Analysis of the mechanism of ground collapse induced by water and sand inflow during tunnel construction: A case study of xi’an Metro Line 8

To investigate the mechanism and development process of ground collapse induced by water and sand inflow during tunnel construction, this study is based on a collapse incident that occurred on June 30, 2024, during the construction of a cross passage of Xi’an Metro Line 8. The incident involved sudden inrush of water and sand, leading to large-scale surface subsidence and municipal pipeline damage. A comprehensive analysis was conducted, including emergency response measures, post-incident geotechnical investigation, and numerical simulations. The incident response included emergency backfilling, pipeline reinforcement, and composite ground stabilization using Metro Jet System (MJS) and secant pile walls. Two post-incident geological surveys revealed significant stratigraphic disturbance, especially in the medium sand and loess layers, which were key to the collapse. A DEM-FDM fluid–solid coupling numerical simulation method is employed to discuss the development process of the geological disaster after water and sand inflow occurs in the tunnel, and to summarize its macro phenomena and micro mechanisms. The results indicate that: (1) After water and sand inflow in the tunnel, significant surface settlement occurs centered around the cross passage, resulting in a noticeable collapse pit. (2) The process of water and sand inflow leads to soil seepage erosion, causing a redistribution of stress within the strata and promoting the formation, development, and instability of soil arches. (3) the disaster development can be divided into three stages: initial soil loosening and arch formation, arch failure and ground settlement, and final surface breakthrough with rapid subsidence. The findings of this study provide valuable references for the prediction and prevention of tunnel-related water and sand inflow disasters.