Stability assesment of the headrace tunnel system at Brattset Hydropower Project

Unlined high pressure tunnels and shafts are regarded as a Norwegian specialty in the hydropower industry. During the last 60 years, more than 4000 km of hydropower tunnels have been excavated, where most of the tunnels have only 2-4 \% of concrete or shotcrete lining. The development of the hydropower industry in Norway has been possible due to cost saving solutions such as the unlined high pressure tunnels and shafts, and air cushion chambers. In the coming years, the Norwegian hydropower system will be an important resource for balancing the demand and production in the European electricity market. This will require a change in the production pattern, from supply driven to demand driven, which involve higher frequencies of start-and-stop sequences of the turbines in the hydropower plant. This production pattern is in use at Brattset Hydropower Project, where there has been an increase in rock falls and other stability problems in the recent years. Stability assessments of the different instability issues experienced at Brattset have been carried out. Both empirical and analytical methods are applied, as well as numerical modelling. Input parameters to the analyzes are based on laboratory work, field observations and evaluation of rock mass parameters found in literature. The pressure amplitudes of hydraulic transients at Brattset are also estimated. It is suggested that due to its long period, the mass oscillations that arise from fluctuating production will have the potential to affect the stability. The effects are mainly concentrated to discontinuities and would influence on the long-term stability of unlined tunnels. The degree of jointing is an important factor in relation to the proposed effects. The water pressure fluctuations due to mass oscillations would therefore be most critical for hydropower schemes situated in jointed rock mass. In order to fully understand the consequences of the new production regime, further research is required. This could involve continuous monitoring of the pressure response within discontinuities during fluctuating operation of the hydropower plants