Karst hydrogeological problems in the route selection of Yuanguping tunnel

Avoiding high-risk areas of gushing water and mud is the basic principal for karst tunnel selection. The Yuanguping tunnel is a typical long and deep karst tunnel,which passes through the area with strong karst development. There are some important factors affecting the safty of tunnel construction,such as karst underground river, syncline water storage structure, reservoirs on both sides, and the large height difference between the groundwater level and the roof,,which are also key hydrological issues to be considered in route selection. To better compare and select the three design lines of the east, middle and west of the tunnel, a 1：10,000 special survey of karst hydrogeology was carried out in the tunnel working area.The karst development and hydrogeological characteristics of the working area were found out, and the karst groundwater system was divided, so as to compare the horizontal and vertical zoning of the three tunnel routes. The spatial relationship between tunnel and underground rivers and karst springs was analyzed, and the influence of syncline water storage structure was analyzed combined with geophysical data. The results show that: the east route tunnel passes through the surface watershed zone and is located in recharge area of the karst hydrodynamic system and the seasonal variation zone and shallow saturated zone of the vertical distribution zone. It basically does not intersect with underground river pipelines and is far away from karst springs with large flows, the rainfall replenishment area is the smallest, therefore, the overall risk of water gushing is the smallest, which is the best tunnel route. The west route runs through the middle of the groundwater system Ⅲ in the upper reaches of Chixi river passes through the runoff and drainage area,and is located in a shallow saturated zone with karst hydrodynamic vertical zoning. It intersects with multiple karst pipelines,and is close to the west side of Gaojiaxi reservior,with the greatest risk of water gushing. The northern section of the east route tunnel near Huangyuxi reservoir and the southern section passing through the Guojiajie syncline water storage structure are all high-risk areas for water gushing. Therefore,it is necessary to expand the supplementary survey of karst hydrogeology, and strengthen the research of advanced geological forecast during construction so as to avoid karst geological disasters to the most extent.