Thoughts on tunnel water inrush in changing zones of groundwater level in karst areas

Based on analyses of the water inrush cases in the Jiguanshan tunnel and other examples in karst areas, it is summarized that water inrush in seasonal variation zones of tunnels can be characterized by fast response, large water volume and heavy sediment load. Based on hydrogeological features of tunnels in seasonal variation zones of groundwater, they are divided into two types， general mountain tunnels and tunnels associated with synclinal structural groundwater level variation zones. The mechanisms of water inrush are presented for both types. During the preliminary investigation of tunneling in karst areas, the demarcation of karst groundwater systems and groundwater dynamic zones should be focused on to understand the characteristics of karst development. During construction and operation periods, supplementary investigation should be carried out in combination with karst exposure and water inrush, which includes rainfall monitoring, water inrush and pressure monitoring to predict the maximum water pressure and inflow. Regarding water inrush, two treatment methods, i.e. dredging and blocking, are proposed and explained. The karst caves exposed during construction should not be simply blocked and the original passage should be well maintained. The lining design of the tunnel with possible water inrush should not only consider the surrounding rock structure, but also the external water pressure. Consolidation grouting should be applied where necessary. Due to the poor reliability of the regular annular drainage system and central ditch , in case of the large inflow of water, drainage tunnel should be used to drain water.