Application of groundwater multi-element tracing tests to water hazard prediction of karst tunnels: An example of the Lanhua tunnel on the Zhangjiajie-Jishou-Huaihua high-speed railway

Tunnel water inrush is a common geological hazard during the tunnel construction in karst areas. Thus, it is of great significance to clarify the spatial relationship between the tunnel and karst groundwater system, especially the spatial relationship with the underground river course, which is the key to prevention and control of water hazard in the karst tunnel. This paper presents an example on this issue, the Lanhua tunnel on the Zhangjiajie-Jishou-Huaihua high-speed railway. On the basis of karst hydrogeological investigation and rainfall-spring discharge dynamic monitoring, groundwater multi-element tracing tests were conducted at the concentrated recharge points of groundwater in the area. The spatial distribution of underground river courses and its relationship with the Lanhua tunnel were clarified, the location of water damage in the tunnel was determined and the maximum water inflow was predicted, which provides a hydrogeological basis for the prevention and control of the tunnel water hazard. The results show that,(1)The Lanhua tunnel and adjacent areas host exposed Cambrian carbonate rocks, which are characterized by peak clusters and depression landforms, with highly developed surface and underground karst. (2) The concentration curves of four groups of groundwater multi-element tracing tests are all single-peak symmetrical forms, the tracer recovery rate is more than 68%, and the largest groundwater flow rate is 387 m·h-1, indicating that the pipeline development in the tunnel site area is unobstructed. (3) There are two independent underground river systems, namely the Lanhua cave system and the Daiye cave system. The three karst water sub-underground river systems of No.1, 2 and 3 belong to the Lanhua cave system, while the No.4 karst water sub-underground river system belongs to the Daiye cave system. (4) The No.4 karst groundwater system will not pose a threat of tunnel inrush water, because it does not intersect with the Lanhua tunnel in plane and section. (5) The Lanhua cave underground water system can be divided into two sections, the upstream section and downstream section, with the karst window in the middle of the Lanhua underground river as the boundary. The No.1 and 2 karst water systems belong to the upstream section, and the No.3 karst water system belongs to the downstream section. The upstream section of the Lanhua cave system does not intersect with the Lanhua tunnel in plane and section, and it will not pose a threat to the inrush water of the Lanhua tunnel.The No.3 karst water system intersects with the tunnel in plane (the intersection mileage is DK60 + 100), which may create a risk of water inrush in the tunnel. (6) Based on the high resolution rainfall-hydrological dynamic monitoring data, the rainfall infiltration coefficient method is used to predict that the maximum water inflow of No.3 karst water system pipeline of the tunnel is 70,800 m3·d-1under extremely heavy rainstorm conditions.