The pumped storage plant Obervermuntwerk II with an underground power house cavern is located in the western part of Austria. Since May 2014, the works on the pre-cuts and excavations of the tunnels and the machinery and transformer cavern are in progress. In the pre-designing phase 2D-FE analysis for the power house cavern were performed and during the main excavation of the cavern 3D finite element calculations have been conducted. The high in-situ stress situation caused shotcrete spalling in the right top heading. Therefore renovation had to be done after understanding the mechanism of the spalling. In this paper an overview of the performed numerical calculation analyses is given and the renovation will be discussed in detail.

1.1 Project area

The Vorarlberger Illwerke AG, Vorarlberg's state-owned energy supplier, is constructing the 360MW Obervermuntwerk II pumped storage power. The project area is located in the rear Montafon in the municipality of Gaschurn, Austria. Upon completion, it will be the second biggest pumped storage power plant of the Vorarlberger Illwerke AG.

The construction of the approx. 500 million € project began in May 2014 and is expected to start operating in 2018.

1.2 Technical description

The new pumped storage power plant (PSPP) is located parallel to the existing Obervermuntwerk I which started operating in 1943. The average construction altitude in the power house cavern is 1700m above sea level.

The Obervermuntwerk II, which is about 300 m below the Silvretta reservoir, will utilize the existing water capacities of the Vermunt and Silvretta reservoir to generate peak and balancing energy. The water from the Silvretta reservoir feeds the underground power house cavern through a 170 m inlet tunnel with a diameter of 6.8 m and the 2800 m long Silvretta high pressure tunnel. The maximum overburden is roughly 1100 m. Both the inlet tunnel and the Silvretta high pressure tunnel are lined with pre-casted segmental linings in the invert and an in-situ concrete shell. The last 600 m of the Silvretta tunnel are first lined with a thin steel shell and an interior concrete core ring at a length of 255 m and a diameter of 4.9 m followed by a thick-walled steel lining which is designed to withstand the external pressure. The thick-walled steel lining tracks from the high head pressure tunnel onto the penstock and gets divided by the bifurcated pipe at the base of the penstock where the internal diameter reduces from 4.9 m to 3.1 m. The distributing conduit to the power house cavern follows with the same diameter. The steel lining in the disturbing conduit will be prestressed by grouting.

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