Characteristics and mechanisms of water-sand gushing disaster in clay aquiclude-confined aquifer composite strata: An experimental study

A primary challenge confronting urban deep underground space development is the water-sand gushing disaster occurred in the confined aquifer in soft soil areas, and its evolution is influenced by the overlying clay layer. Therefore, a novel visual model test system was developed to conduct gushing tests in clay aquiclude-confined aquifer composite strata. The mass of gushing water and sand, stratum deformation, surface settlement, water pressure, and earth pressure were acquired without strata disturbance during the test. The test results show that the overlying clay layer has a remarkable barrier effect on disaster transmission caused by sand loss, leading to the Peck's shape settlement at the surface; the clay layer can also clog the gushing crack at the final state. Thereafter, the beam-bearing mechanism was proposed to interpret the effect of the overlying clay layer, which is evidenced by the evolution of earth pressure at the bottom. The severity of water-sand loss, stratum deformation, and surface settlement deteriorate sharply with the increase of gushing crack width, and a critical crack width exists. When the crack width is smaller than the critical value, the gushing disaster can be stabilized without causing significant surface settlement. Moreover, three gushing critical modes and corresponding convergence modes were summarized based on the ratio of crack width to skeleton particle size of confined aquifer. Finally, the implications for engineering applications were discussed.