Assessment on the in-situ rock stress condition along an unlined pressure tunnel/shaft of a Norwegian Hydropower Project using numerical modeling

Confinement is a pre-requisite in the “Norwegian state-of-art design principles” for unlined pressure tunnels/shafts where the minimum principal stress must exceed internal water pressure. Reliable estimation of in-situ stress state is crucial in implementing unlined pressure tunnels/shafts so that hydraulic jacking/fracturing in the rock mass is avoided. The Norwegian thumb rule may not offer a complete solution in all unlined pressure tunnels/shafts since these are based on the 2D geometry of terrain and thus do not fully represent the in-situ stress environment in 3D. This article evaluates the minimum principal stress state of the unlined headrace system of Bjørnstokk powerplant using 3D numerical modeling. The 3D model is developed with the integration of geological, topographic, and geotectonic features. The model is calibrated using measured in-situ stress data of fixed locations along the waterway system. Discussions are made on how in-situ rock stress differs upon changes in topography and geological setting.