A high-speed railway running 350 km·h⁻¹ is being built between Guiyang in Guizhou Province and Nanning in Guangxi. Chaoyang tunnel is a double-track single-hole tunnel located in southeastern Guizhou. The tunnel clearance is 100 m² with the total length of 12,734 m (including more than 7-km-long soluble rock), and its maximum burial depth is 386 m. The railroad track elevation of the tunnel is 782-468 m, and it gradually decreases from the entrance to the exit, with a slope of −22.7‰ to −25‰. An auxiliary tunnel scheme of "2 transverse tunnels +2 parallel pilot tunnels +1 inclined shaft" has been adopted, with the parallel pilot tunnel on the left side of the main tunnel. The midline of the parallel pilot tunnel is 30 m away from the main tunnel, and 3 m lower than the main hole. The inner headroom size is 5.0 m (width) ×6.0 m (height).

Construction of the tunnel began in early 2016 and will be completed in 2022. On June 10, 2018, karst water and mud inrush, lasting about 1 hour, occurred at the exit of parallel pilot tunnel of Chaoyang tunnel. The total amount of water and mud inrush was about 1.6×10⁶ m³. In order to complete the construction tunneling scheme and drainage scheme, it is necessary to analyze the causes of water and mud inrush, evaluate its risk in the subsequent construction, and calculate the water inflow of the tunnel. In this study area, the exit section of the tunnel is in the sloping medium-and-low mountainous area, belonging to the karst peak cluster landform, and the terrain fluctuates greatly. The tunnel passes th exit section of the tunnel is in the sloping medium and low mountainous area, belongs to the karst peak cluster landform with relatively sharp topographic relief. The tunnel passes through a regionally wide and gentle symmetrical anticline, and the exit of the tunnel is located on the southeastern wing of the anticline widely distributed by thick-to-extremely-thick bedded limestone with pure rock texture. Extreme development of surface karst funnels, dissolution basins, karst drop holes, karst caves and other phenomena provides favorable conditions for the collection, infiltration and runoff of surface water, but there is no long-term water system on the surface. After rainfall, the catchment of the non-soluble rock segment is mainly infiltrated through the contact zone between the soluble rock and the non-soluble rock, but the water catchment of the soluble rock section penetrates directly into the underground karst water system through the surface negative topography. Controlled by topography, geomorphology and transverse faults, except for the direction of groundwater seepage along the tectonic line in some sections, the infiltration groundwater is mainly discharged from the Zhangjiang river, which is basically perpendicular to the axis of the anticline and to the southeast wing (the exit of the tunnel). As a result, a series of transverse gullies and underground rivers cutting through the main structural lines were formed.

The present study is mainly focused on the following aspects: analyzing the topography, engineering geology and hydrogeology of the tunnel, introducing the implementation of advance geological prediction on the face of the parallel pilot tunnel, and describing the process of water and mud inrush. In this study, the engineering geology and hydrogeology conditions in the area affected by the disaster were investigated and a one-year dynamic observation of the relationship between water inflow and rainfall was also completed.

The research finding shows that a huge cavity and pipeline system with a water head of up to 84 m in front of the tunnel face caused the water and mud inrush. After excavation and exposing the bottom of the cavity, the pressurized water flow filling in the whole karst water system carried mud and sand rapidly into the tunnel face and rushed out of the parallel pilot tunnel with a large kinetic energy, resulting in the water and mud inrush event on June 10. In the process of this event, the surrounding karst water system was supplied in time, and the amount of water and mud inrush was much larger than the volume of large dissolution cavity near the tunnel face. The process of "water gushing-water shutoff-water gushing-water shutoff after dredging" occurred many times, indicating that the deposition in the lower part of the cavity is very serious and the drainage is not unobstructed. If the groundwater in the saturated cavity is not discharged effectively, the blasting excavation will cause serious water and mud inrush disaster. Long-term rainfall and water inflow monitoring shows that the karst water system at the exit of the tunnel receives rainfall infiltration recharge and the runoff is unobstructed. The catchment area corresponding to the water inrush to the tunnel is 6.423 km², and the maximum water inrush in the tunnel after the extreme rainstorm is 5×10⁴ m³·h⁻¹. After the occurrence of water and mud inrush, the static reserves in the mountain body have been fully released, and the groundwater level has dropped to the elevation of the tunnel floor. On the premise of ensuring the unobstructed drainage during the tunneling process, the risk of encountering water and mud inrush in the subsequent construction is low.