Monitoring and Modeling Groundwater Flow Near the Vemork and Såheim Hydraulic Power Plants, Norway

Abstract

Most of unlined pressure tunnels are successfully operated in Norway without significant instability problems. Even though all unlined pressure tunnels follow the same established design criteria and principles, there are still cases of failure. Reasons for failures of some unlined pressure tunnels, a few years after their development, is a well-studied issue. There is still a limited knowledge on interactions between the groundwater aquifer and the pressure tunnels, and the instabilities of long-term operating pressure tunnels still need to be understood. The present master project is a first step to develop a better knowledge on how groundwater interacts with pressure tunnels and pipes from case studies of two hydropower plants in Norway. The main purpose of the thesis is to numerically model the equilibrium groundwater flow in order to achieve a better understanding of the groundwater flow and pressure near the tunnels feeding the hydraulic power plants. Two cases of numerical modeling of groundwater flow (longitudinal and transverse), applied to two hydropower plants, are carried out in this thesis by using a finite element program package and groundwater numerical modeling code FEFLOW. An expected result is that the groundwater pressure distribution around the lined pressure pipe is not dependent on the boundary condition set at the pipe perimeter, i.e., the drainage of lined pressure pipe or the load change do not influence the hydraulic head distribution around the pipe. Contrary, both the drainage of unlined pressure tunnel and the load change influence the hydraulic head distribution and the amount of water leaking into or out of the tunnel. The study also suggests that for the numerical solution of groundwater pressure distribution and calculation of inflows into unlined pressure tunnel, the model domain extent must be carefully selected and large enough with respect to the extent of the tunnel to provide accurate results.