Karst Waterburst Mechanism And Prevention Countermeasures In Yuanliangshan Tunnel

Yuanliangshan tunnel, with a total length of 11.068km, is the longest tunnel on Chongqing-Huaihua railway now under construction. The tunnel goes through complex geological conditions in Maoba syncline, Tongmaling anticline and Lengshuihe shallow-buried section. Due to the influence of geological structures, there exist two layers of artesian water. The deep-hole drilling exploration shows that the pressure of the artesian water reaches 4.42～4.6MPa. During the construction of the tunnel, 5 deep-buried karst caves of filling type are discovered, with each karst cave having different shape and different filling materials such as silty fine sand layers, silty clay and clay. Several water burst and mud inflow accidents occurred due to the influence of high pressure, rich water and karst geology. This also lies the technical difficulty of the project. On basis of the construction of Yuanliangshan tunnel, as well as on the investigation on the engineering geological conditions, the process, pattern and features of the karst water burst, as well as the prediction and control technology for the waterburst, are studied by means of laboratory test, numerical modeling, theoretical analysis and in-situ tests, which gives primarily the mechanism of karst water burst in Yuanliangshan tunnel. On basis of the spatial relation between the locations of the karsts and the tunnel, the karst water burst is divided into different geological modes, under which the mechanism of the karst water burst is the same. The scope of the destruction of the karst caves resulted from the influence of the filling materials and the water pressure, and the interconnection of the plasticized zones around the tunnel are direct causes for the karst water burst. The karst water burst has a beginning and developing process. The karst water burst in Yuanliangshan tunnel has such features as sudden, high pressure and rich water. When the tunnel passes closely by the karst structures, keeping the completeness and stability of the rock pillars is of key importance to avoid water burst and to minimize the relevant loss; when the tunnel goes directly through the karst structures, the reliability of the advance reinforcement schemes and the relevant measures are the most important technology. Therefore, detailed technical measures are planned for different karst structural types and different geological modes. With these measures taken, satisfactory effects are achieved and the construction safety is guaranteed. Thus, breakthrough has been achieved in the construction technology for tunnels under complex karst geological conditions, which has important guiding significance for future tunnels under similar geological conditions.