Cause of the ＂8.5＂ water burst incident at Yesanguan tunnel along the Yi-Wan railway

Yesanguan Tunnel in Yichang-Wanzhou railway had a large-scale karst water inrush accident on August 5, 2007. Based on karst hydro geologic investigation, two tracing experiments with industry salt were carried out in the two main karst depressions in the study area. The tracing experiments prove that the Shuidongping karst depression is the main water source of the ＂5th, August＂ inrush accident, and the water runs to the tunnel along fault No. 18 which cut off the Zhoujiabao underground river developed in the P1m limestone layer. And the experiments also prove that the water coming from Daoziping seeps and pours into the tunnel from the west part of the inrush incident. Besides, the flow speed of the groundwater lies between the two karst stratum P1m and T1d is calculated too. The flow speed of the groundwater in karst stratum P1m is 50 m/h based on the experiment in Shuidongping. The flow speed of the groundwater in karst stratum P1m is 45.5 m/h based on the experiment in Daoziping, and the flow speed of the groundwater is 10. 8 m/h in karst stratum T1d. All the above provide scientific basis for the later restoration and reconstruction as well as the anti-infiltration engineer of the Yesanguan tunnel. This paper also gives reference for design and construction of deep buried tunnel in karst area.