Abstract: When leakage disasters occur in shield tunnels within confined aquifers of soft soil regions, significant volumes of water and soil flowed into the tunnel, destabilizing the in-situ stress equilibrium and triggering progressive collapse. To investigate the evolutionary mechanisms of water-sand inrush and the structural response of the tunnel, this study develops an experimental apparatus simulating leakage under varied crack opening speeds, locations, and composite strata conditions. Results demonstrate that narrow cracks induced soil arching, thereby reducing leakage rates and sand loss. Conversely, crack enlargement destabilizes the arch, reactivating sand loss until a new arch formed. The failure surface of the sand loss zone propagates upward along the tunnel periphery, with a critical crack width determining the convergence of the erosion cavity and failure surface—a parameter directly governed by crack location. When the tunnel is positioned at a clay-sand interface, erosion advances rapidly, forming cavities at the crack with failure angles dictated by the soil's internal friction angles.