The Pumped Storage Scheme Kühtai 2 located between the existing reservoir Finstertal and the currently under construction reservoir Kühtai is part of the expansion of the existing hydro power plant group Sellrain-Silz. The new power station is housed in a cavern at an altitude of 2000 meters above sea level near the village of Kühtai in Sellraintal.
The cavern is located within a gneiss formation (biotite-plagioclase gneiss), which is layered in bands and tectonically faulted.
As the design progressed, a number of variants were investigated, including the simulation of different geological conditions, the variation of distance between the individual caverns (double cavern solution with machine cavern and separate transformer cavern), the arrangement of tunnels surrounding the caverns and the complex construction process, until a solution was developed.
The paper provides an overview to the excavation design process as well as to the geotechnical design analyses, which were adopted, and the experience gained during the excavation of the caverns.
TIWAG-Tiroler Wasserkraft AG is currently expanding the existing Sellrain-Silz power plant group with a second upper stage unit, consisting of the Kühtai reservoir, the Kühtai 2 power plant including headrace and tailrace tunnels as well as a 25 km long water adduction tunnel to the reservoir. The Kühtai 2 power plant is housed in a cavern and will have an installed capacity of 140 MW in turbine mode and 130 MW in pump mode.
The power cavern is situated between the existing reservoir Finstertal and the currently under construction reservoir Kühtai on the orographic right-hand side of Längental. A 1250 m long inclined penstock and a 525 m long tailrace tunnel provide the connection between both reservoirs (figure 1).
The cavern houses the mechanical and electrical plant components for the turbine and pump operation as well as equipment for the monitoring of the power plant.
The size and the shape of the cavern is optimized to meet geotechnical as well as space requirements of the individual components of the power plant.
