The construction of the Feldsee project of the Austrian hydropower company KELAG started in 2006 and is scheduled to be completed by the end of 2008. The basic idea is connecting two existing reservoirs, Feldsee at 2200 m a.s.l. and Wurten at 1700 m a.s.l. in a daily pump storage scheme with a peak power output of 70 MW. Since the startup and shutdown procedures of the turbine have to be rapid (due to the chosen operation scheme and capacity of the reservoirs), the dynamic head reaches 820 m at the lowest part of the pressure tunnel. Motivated by the great cost-saving potential, the design of the pressure tunnel lining treated the surrounding rock mass as an integral and primary load bearing element. This approach is justified when having the following facts in mind: the rock mass is composed of massive, unweathered gneiss with widely spaced joints of low persistence and the overburden is high enough to allow the development of tensile cracks induced by the internal pressure loading without endangering the global stability of the slope. Assuming the lateral pressure coefficient of 0,4, the calculations have shown that the crack depth under peak internal load does not exceed 10 meters. This is well within boundaries deduced from the hydropower project Häusling, successfully constructed in similar ground conditions. The final solution features pre-cast concrete elements with a thin steel lining in the lowest part of the tunnel and pre-cast concrete elements with a GFRP (Glass Fiber Reinforced Polymer) lining in the upper part (Figure 1).
Figure 1: The longitudinal section of the Feldsee project(available in full paper)