A 1.6 km long headrace tunnel has been constructed for the Tauernmoos hydro power plant. For the dimensioning of the tunnel lining, a minimum elastic modulus requirement of 15 GPa for the surrounding rock mass was formulated. The objective for the geological documentation was therefore the identification of zones with an elastic modulus below the required minimum. This was achieved by detailed mapping of the embrasure as well as by correlating the UCS, determined by Schmidt hammer, with the elastic modulus. For parts of the headrace tunnel, additional dynamic elastic moduli were established using refraction seismic surveying. That way, critical areas in terms of deformational behavior along the headrace tunnel could be determined. A consolidation grouting campaign with primary and secondary injection rings was carried out to improve the elastic moduli. The success of this measures was confirmed with a post-grouting seismic survey.
Currently, the Austrian Federal Railways (ÖBB) are constructing the Tauernmoos pumped-storage hydro power plant in the Stubach valley (Salzburg, Austria). The difference in altitude between the existing upper (Weißsee) and lower reservoir (Tauernmoossee) equals 230 m. A 1.6 km long headrace tunnel connects the upper reservoir to the power cavern (Figure 1).
The bottom 216 m of the pressure tunnel will be equipped with an embedded steel penstock with 4 m inner diameter. The remaining part of headrace tunnel with an inner diameter of 5.2 m will be supplied with a PVC-membrane and an in-situ concrete lining without reinforcement. This section is designed as prestressed concrete lining. The design is based on the method defined by Seeber (1999), considering that the internal water pressure, causing extension and tensile stresses in the concrete lining, will be taken partly by the surrounding rock mass and partly by the concrete lining. In the concrete lining, tensile forces could cause cracks and seepage of water out of the tunnel. To prevent such a seepage that could have a destabilizing effect on the rock mass, high-pressure annual gap grouting is carried out. The bearing capacity of the surrounding rock mass is defined by the rock mass overburden and rock mass stiffness characteristics. If the minimum primary stress along the headrace tunnel is lower than the internal water pressure, hydrofracturing of the surrounding rock mass by water seeping out of the tunnel cannot be excluded. As an additional safety against water seeping out of the penstock, a 3 mm PVC membrane is installed between the concrete lining and the surrounding rock mass. The membrane must bridge possible tensile cracks in the rock mass and the system requires a deformation modulus for the latter of at least 15 GPa.
