High pressure unlined tunnels: Exemplified with Nye Tyin Hydropower Project
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Topographic condition in Norway is favorable for the development of hydropower and more than 99% of annual production of energy in Norway is generated from hydropower. Unlined tunnels and underground powerhouses have been constructed starting from the end of World War I in 1919. From the valuable experience gained through the design, construction and operation of tunnels in the past 100 years in Norway, at the present the high head unlined pressure tunnels have been developed up to reach 1046m hydrostatic head for Nye Tyin Hydropower Project. Nye Tyin Hydropower Project is a world record with its ultra-high-unlined tunnels/shafts and the project exploits fall between Torolmen and Årdal. Two turbines are installed with a capacity of 375 MW each and discharge 2 x 20m3/s that is giving an annual production of 1398 GWh. The project geology is rather complex and predominantly overthrusted homogeneous dark gneisses, quartzite and mica schist, followed by a large unit of meta-arkose and conglomerate. The power station area and the lower part of the pressure tunnel are located in rocks consisting mainly of dark gneiss, gabbro, amphibolite and pyroxene-granulite. The main objective of this thesis is to carry out the possibility of the feasibility to develop ultra-high-unlined tunnel and shaft with even higher static heads for the exemplified Nye-Tyin HEP project. In this regards, different design principle and approaches has been used based on empirical, deterministic, numerical analyses and using other sources of design procedures and principles. The highest water pressure in the project was 10.5 MPa and different trial static heads has been used by increasing the existing static head to different level up to water pressure of 18MPa for the evaluation of confinement criterion requirements for hydraulic fracturing and uplift. The minimum principal stress along the alignment of the unlined tunnel was calculated and compared with the water pressure intended inside the tunnel and finally factor of safety along the unlined pressure shaft has been evaluated with regards to hydraulic splitting and failure due to the increased experimental static heads. The ?First design criteria?, Norwegian rule of thumb, Snowy mountain criterion, design charts method based on FEM and finally verification using simulation of a model by two dimensional phase2 software has been used to ensure safety against hydraulic fracturing and uplift. Based on the outcomes of these varies design approaches, the feasibility to development of ultra-high-unlined tunnel and shaft have been evaluated for higher static heads in general considering the engineering geology conditions of the rock masses, which should be sufficiently durable and sound to satisfy long term requirements. Finally, conclusion has been made on the possibility to develop ultra-high-unlined tunnels in general and particularly for the exemplified project for higher hydrostatic head based on the outcome of the empirical, deterministic and numerical analysis and recommendations have been proposed for further conclusions on this thesis.