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.
As water conduits for energy production, rock tunnels constitute an integral part of underground hydropower projects. In Norway, the use of unlined pressure tunnels has become the norm for any underground hydropower project, and the concept has achieved worldwide recognition (Rancourt, 2010). The main motivation for adopting the unlined concept is the potential for very large cost reductions when replacing a steel or concrete lined tunnel with an unlined tunnel. There are, however, several reports of unlined pressure tunnels not performing as expected due to hydraulic failure of the tunnel caused by the internal water pressure, as documented by Broch (1982), Merritt (1999) and Palmstrom and Broch (2017) amongst others. To reduce the risk associated with hydraulic failure of unlined pressure tunnels, knowledge about rock stresses in the area of interest is required. Obtaining such information in due time for preparation of tender documents can be difficult due to the inherent limitations of the stress measuring methods, combined with the often difficult physical access to the investigation point. Therefore, project owners and engineers often have to make their tender stage tunnel design without direct stress measurements, and postpone rock stress measurements to the construction phase. This implies that changes to the tender design must be allowed for, in case insufficient stress levels are identified during construction stage stress measurements (Halvorsen and Roti, 2013).
A specific plan describing the testing- and design methodology for the siting of the transition zone is a useful common basis for both Contractor and Client when deciding the final design. The outline of such a plan is presented herein, based on Norwegian experience and practice in investigation, design and construction of the transition zone area in hard rock conditions. The plan is specially emphasizing the rock stress measurements, though it is recognized that knowledge about the overall rock mass conditions is essential in any underground design.
A key requirement for using unlined pressure tunnels is that, at any point along the pressurized waterway, the rock stress surrounding the tunnel must be larger than the internal water pressure. If not, the water pressure can lift, or jack, the rock mass so that hydraulic failure and associated large leakages occur. Excessive leakages from the pressure tunnel can cause catastrophic events such as flooding of nearby underground structures and even landslides as reported by Brekke and Ripley (1987), Benson (1989) and Palmstrom and Broch (2017). To avoid such incidents it is absolutely required to ensure that any section of unlined pressure tunnel has sufficiently high rock stresses to accommodate the internal water pressure.
