Effects of confined aquifer on water-sand gushing disasters in soft soil

Water-sand gushing (WSG) disasters in confined aquifers pose significant challenges to the utilization of deep underground spaces in soft soil areas. Since few studies have considered the impact of confined aquifer thickness and confined water pressure on WSG disasters, a novel visual model test system was developed to investigate the influencing characteristics and mechanisms of the two aforementioned factors. The test results showed that the WSG process in clay aquiclude-confined aquifer composite strata exhibits two prominent stages. First, the sand loss zone expands vertically in an ellipsoid shape. Then, it expands horizontally once the ellipsoid reaches the boundary of the clay layer. The sand loss continues until the overlying clay sinks to the bottom to clog the gushing crack, creating a large sinkhole at the surface. Increasing the confined aquifer thickness can increase the vertical expansion of the ellipsoid and delay the clay-clogging effects, thereby considerably increasing the severity of sand loss, stratum deformation, and surface settlement. An increase in the confined water pressure markedly increases the hydraulic gradient along the seepage path, which contributes to increasing the gushing rates of water and sand. As a result, substantial sand loss occurs before the clay clogs the gushing crack, inducing more cracks and deeper sinkholes at the surface. All the aforementioned results provide insights into the effects of confined aquifer on WSG disasters in clay aquiclude-confined aquifer composite strata.