Engineering Geological Investigation and Design of Transition Zones in Unlined Pressure Tunnels

The transition zone, i.e. where the pressurized water enters the steel lined section of the waterway, is a key component of the entire tunnel system in any underground hydropower project utilizing unlined pressure tunnels. Not only is it important for conveying the pressurized water into the turbine, its positioning also defines the location, length and layout of several other tunnels. The key for deciding on positioning of the transition zone is to identify a place with sufficient in-situ rock mass stress to withstand the internal water pressure generated from the hydraulic head of the pressure tunnel. To prepare complete tender documents, preliminary positioning of the transition zone must be defined before sufficient stress data for final positioning are available. In Norway, such early assessments have often been made on empirical basis with little or no testing. Even though this approach normally is necessary in the preliminary phase of a project development, the final decision on transition zone positioning, must be based on in-situ testing. Recognizing that stress testing is required for safe design of the transition zone, flexibility in both layout and construction schedule should be incorporated in the project. To ensure this flexibility, the tender documents must include a specific plan that describes what ground investigations should be done, where they should be done, what the acceptable criteria for such tests are, and how the results should be adopted in the design. In this paper, a proposal on the outline of such a plan will be presented based on Norwegian experience and practice in investigation, design and construction of the transition zone area for selected projects. The work presented is part of the hydropower research being performed at HydroCen, based at NTNU in Trondheim, Norway.