Effect of pressure fluctuations in long-term stability of unlined pressure shaft at Svandalsflona Hydropower project, Norway

The main design principle of unlined pressure tunnels/shafts in a hydropower scheme is to provide sufficient confinement (minor principal stress) to withstand the internal hydrostatic water pressure. Rock support is only provided in areas where very weak rockmass is encountered. The principle uses maximum hydrostatic water head as a design parameter and thus implies that occasional pressure transients will not have significant impact on the stability of a pressure tunnel. Minor rock falls and resulting headloss are accepted as a compromise against higher cost of supporting the whole tunnel. However, because of changing operational regime of power plants in recent years, start and stop sequences have become more frequent in Norway. This has resulted in more frequent water hammer and mass oscillations in pressure tunnels compared to the time when these tunnels were designed/constructed. Consequently, collapses have occurred in some unlined pressure tunnels/shafts, which have been in long-term operation. This article aims to assess the influence of pressure fluctuation in the long-term stability of unlined inclined pressure shaft of Svandalsflona Hydropower Project located in southern Norway. The main issue is to review and analyze the collapse that was identified in 2008 at the inclined shaft of the headrace system of the project. The article reviews the engineering geological condition of the collapse area, evaluates mechanical properties of the rockmass at the failure location, tunnel hydraulics over the period of its operation and hydraulic impact on the rockmass/installed rock support. As the outcome, the mechanics of failure resulting due to pressure variations and conditions that may have led to the instability in this inclined shaft subjected to frequent start and stop sequences during operation are discussed. The manuscript is a part of large research initiative in Norway in the field of renewable energy called HydroCen (Norwegian Research Centre for Hydropower Technology).