Upward propagation of ground disturbance induced by water–sand inrush into a defective tunnel in a dual-stratum geological condition

Water-sand inrush is one of the most destructive disasters for underground structures buried in saturated erodible soils. The related upward propagation of ground disturbance has not been well understood so far. In this study, a series of numerical simulations were performed to investigate such disturbance propagation behavior. Typical dual-stratum geological condition was modeled since this was the most common condition where water–sand inrush disasters had been widely reported previously. The numerical simulations applied discrete particles to model the lower sand layer, continuum mechanics to model the overlying clay layer, and Darcy’s law to model the underground water flow, with solid–fluid coupling considered. The simulation results were reported and were compared with available data from the literature. It was found that the sand and water loss rates were highly dependent on location of the inrush opening, which was attributed to the different local porosity near the opening, a result of the particle-scale fluid–solid coupling. A funnel-type sand flowing pattern can be identified, and the upward propagation of ground disturbance can be described by evolution of a loosening ellipse that outlines the extent of ground experiencing displacement. As the water–sand inrush proceeded, the tendency of vertical alignment of the loosening ellipse suggests an increasing role of gravity relative to the fluid–solid coupling force in driving particle motion. The ground disturbance propagation was accompanied with localization in strain rate, dissipative work, and particle spin rate. The non-uniform ground displacement was accompanied with re-distribution of stress in the form of continuously evolving soil arches. The cross-stratum soil arch can lead to unloading process at the stratum interface and consequently cause upper payer instability.