Research on environmental geological problems caused by water and mud inrush in tunnels and long-term drainage: taking Chaoyang Tunnel of Guinan High-Speed Railway as an example

Karst environments are vulnerable to water and mud inrushes for tunnel engineering, and prolonged drainage in tunnels leads to environmental geological issues such as karst collapse and soil erosion in the surrounding area. This article examines these environmental geological issues that arose during the construction and operation of the Chaoyang Tunnel of Guinan High-Speed Railway in southeastern Guizhou Province, China. The tunnel intercepted strong karst strata, a shallow saturated zone within the karst hydrodynamic vertical zone, and karst structural planes. These factors together with the abundant rainfall are responsible for the water and mud inrushes in the Chaoyang Tunnel. The formation mechanism of karst collapse has changed from the rapid decline of karst groundwater level caused by the water and mud inrushes during the construction phase to the sudden change of water (gas) pressure in the karst system caused by the rainstorm and long-term drainage during the tunnel operation phase. The mud and sand from the tunnel drainage caused soil erosion up to 89,432 kg/h during the rainstorm and primarily originated from the exposed cavities and the fillings in the karst conduits and the overlying soil in the collapse pits. Long-term drainage in tunnels can facilitate hydraulic connections of the karst conduits, increasing the likelihood of karst collapses. Groundwater depletion resulting from prolonged drainage in tunnels can be categorized into two modes: a direct mode in which the water is within the tunnel's depression cone, and an indirect mode in which the tunnel intercepts the recharge sources.