In-Situ Stress Evaluation in an Unlined Pressure Tunnel of a Hydropower in South-Western Part of Norway

Abstract In-situ stress is a key parameter in applied rock mechanics and a governing factor in design of unlined pressure tunnels. In Norway and some other countries, Norwegian thumb rules are commonly applied for the design of unlined pressure tunnels, intended to ensure that internal water pressure remains below the minimum in-situ principal stress. In-situ stress can also be evaluated using geo-mechanical numerical models. However, constructing such models presents challenges, especially in defining representative boundary conditions that best-fit all measurement points. To address this, the authors present an automated two-step optimization approach for obtaining best-fit in-situ stress model. The method is first used in a synthetic model and then applied to Løkjelsvatn Hydropower Project, where in-situ stress measurements are available. Results show that best-fit in-situ stress model can be achieved by integrating two-step optimization approach. Analysis suggests that weakness zones traversing the tunnels hold potential of attenuating stress in the vicinity, requiring careful consideration while orienting unlined pressure tunnel. In current geological setting, stiffness contrast between host rock and weakness zones is about two, but a higher contrast could significantly increase risk of leakages due to hydraulic fracturing. The study recommends using thumb rules mainly for early planning purposes. The manuscript also emphasizes the need for a systematic study of the stress state around weakness zones, so that geological heterogeneity could be technically represented in the geo-mechanical numerical models. The devised method can potentially be applied in locations where hydropower projects are envisaged.