title: Forecasting and prevention of water inrush during the excavation process of a diversion tunnel at the Jinping II Hydropower Station, China.
abstract: BACKGROUND
  Estimating groundwater inflow into a tunnel before and during the excavation process is an important task to ensure the safety and schedule during the underground construction process.
  
  
  METHODS
  Here we report a case of the forecasting and prevention of water inrush at the Jinping II Hydropower Station diversion tunnel groups during the excavation process. The diversion tunnel groups are located in mountains and valleys, and with high water pressure head. Three forecasting methods are used to predict the total water inflow of the #2 diversion tunnel. Furthermore, based on the accurate estimation of the water inrush around the tunnel working area, a theoretical method is presented to forecast the water inflow at the working area during the excavation process.
  
  
  RESULTS
  The simulated results show that the total water flow is 1586.9, 1309.4 and 2070.2 m(3)/h using the Qshima method, Kostyakov method and Ochiai method, respectively. The Qshima method is the best one because it most closely matches the monitoring result. According to the huge water inflow into the #2 diversion tunnel, reasonable drainage measures are arranged to prevent the potential disaster of water inrush. The groundwater pressure head can be determined using the water flow velocity from the advancing holes; then, the groundwater pressure head can be used to predict the possible water inflow. The simulated results show that the groundwater pressure head and water inflow re stable and relatively small around the region of the intact rock mass, but there is a sudden change around the fault region with a large water inflow and groundwater pressure head. Different countermeasures are adopted to prevent water inrush disasters during the tunnel excavation process.
  
  
  CONCLUSIONS
  Reasonable forecasting the characteristic parameters of water inrush is very useful for the formation of prevention and mitigation schemes during the tunnel excavation process.
authors.affiliation: State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu, 610065 Sichuan China.
authors.forename: Tian-Xing
authors.initials: TX
authors.lastname: Hou
authors.affiliation: College of Water Resources and Hydropower, Sichuan University, Chengdu, 610065 Sichuan China.
authors.forename: Xing-Guo
authors.initials: XG
authors.lastname: Yang
authors.affiliation: College of Architecture and Environment, Sichuan University, Chengdu, 610065 Sichuan China.
authors.forename: Hui-Ge
authors.initials: HG
authors.lastname: Xing
authors.affiliation: College of Water Resources and Hydropower, Sichuan University, Chengdu, 610065 Sichuan China.
authors.forename: Kang-Xin
authors.initials: KX
authors.lastname: Huang
authors.affiliation: State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu, 610065 Sichuan China.
authors.forename: Jia-Wen
authors.initials: JW
authors.lastname: Zhou
keywords: Diversion tunnel; Forecasting; Geological condition; Pressure head; Water inflow; Water inrush