Technical soundness, cost effectiveness and long-term sustainability are the major issues that one should achieve while planning, designing, constructing and operating the hydropower schemes. With the changing world of competitiveness with other sources of renewable energies such as wind and solar, it is important that the hydro-energy still is able to compete in the renewable energy market. One of the possible solutions that helps to achieve the cost effectiveness, technically soundness and timely completion of the hydropower scheme is the use of unlined / shotcrete lined pressure tunnels. Such approach may help replace the traditionally used concrete and steel lined pressure tunnel systems, which are both cost intensive and long construction time taking. Nevertheless, there are several technical, geological and geo-tectonic challenges associated to this solution, which limits the use of this innovative concept. This manuscript will review the history of the use of unlined pressure tunnels / shafts in the world and presents prevailing design approaches in the use with the aid of the use of some very important cases of failures and successes where unlined / shotcrete lined pressure tunnels and shafts were used in the past. The detail evaluation and analysis on the geological and geo-tectonic environment are carried out. As an outcome, the state-of-the-art upgraded design criteria as well as guidelines for the use of unlined / shotcrete lined pressure tunnels are suggested with the belief that the vast hydropower resources still untapped in the Asian region will be even more cost effective, technically sound and long-term sustainable solution.
Unlined pressure tunnels in hydropower schemes are becoming popular worldwide due to the cost effectiveness compared to the tunnels that are lined with concrete and/or steel pipes (Basnet and Panthi, 2018a & 2018b). The unlined tunnels are relatively easy in construction and take much shorter construction time in condition that the topography, geology and geo-tectonics favor. Regarding the history of application, Norwegian unlined high pressure tunnels and shafts are by far the most valuable examples in the world than the one constructed in other countries. The earliest attempts to use unlined pressure tunnels in hydropower projects with the surface powerhouse in Norway was already in 1920s, which is almost 100 years back from now. However, emphasis was given to keep all waterway system and powerhouses inside the mountain after the completion of World War II (Broch, 2013). Today, Norway has over 230 underground powerhouses and over 4300 km unlined tunnels and shafts. Such tunnels and shafts were considered to be possible due to the favorable engineering geological and geo-tectonic conditions that persist in the Scandinavia (Johansen, 1984; Panthi, 2014). Experiences gained in design, construction and operation of such waterway systems has led to the development of different design criteria for unlined tunnels (Broch, 1982).