Laboratory insights into land subsidence caused by water-sand mixing inrush in viscous sand layers

Water-sand inrush has emerged as a significant geological hazard during underground construction in water-rich sand layers, posing severe risks to engineering safety. This study investigates the land subsidence caused by water-sand inrush using a custom-built, three-dimensional (3D) transparent model testing system. Experiments were conducted under four water pressures (0, 0.05, 0.1, and 0.15 MPa) and three clay contents (1%, 5%, and 10%). The findings indicate that both the rate and shape of land subsidence caused by water-sand inrush are significantly influenced by the clay content and water pressure. Primarily, the initial rate of water-sand inrush was determined by water pressure, whereas its effect on the final subsidence configuration is minimal. In contrast, clay significantly affects both the rate of sand inrush reduction and the final subsidence morphology. Increasing clay content from 1 to 10% reduced the maximum subsidence depth from 57.5 cm to 42 cm and extended the stabilization time from approximately 20-70 min. At clay concentrations below 5%, the subsidence trough has a two-stage characteristic, transitioning from a steep to a gentle slope. However, at higher clay contents (above 10%), the subsidence trough maintains a consistent gentle slope. No distinct steep section is observed. Based on these findings, a quantitative model was developed to describe the evolution of subsidence troughs resulting from water-sand intrusion. This model was validated against an actual engineering case in Yulin City, where the predicted maximum subsidence depth (4.04 m) and the subsidence radius (65.15 m) was accurately captured. The research results might provide guidance for predicting land subsidence due to water-sand mixing inrush hazard during the construction of tunnels or mines in viscous water-rich areas.