The extension of Grimsel 3 hydropower project (600MW) entails the construction of a 2.5 km long tailrace tunnel, a machine and a transformer cavern as well as a new 600m deep vertical shaft connecting the existing conduit with the Räterichsboden reservoir. Generally high in-situ stress conditions are expected for the construction of the extension of Grimsel 3 hydropower project within the massive Grimsel granodiorite and granite. Due to higher ground coverage for the cavern structure, brittle rock mass behaviour is expected influencing the constructionin the course of the relocation for the vertical shaft. The specific intact rock and rock mass conditions as well as the stress conditions were investigated by means of 600 m deep core drillings in order to obtain the geomechanical conditions and parameters at cavern structure level. The investigation included detailed analyses of brittle rock parameters as well as in-situ hydro-fracturing tests. Brittle parameters of rock were utilised for the two and three dimensional finite element analyses to determine the depth of spalling and fracturing expected for the construction. The variances of the two and three dimensional finite element analyses as well as the influence of brittle parameters influencing the analyses are discussed.

1 Introduction

The Grimsel 3 pump storage plant (PSP) will connect the existing pressure tunnels of the Grimsel 2 PSP, built in the late 1970s, with the reservoir of Räterichsboden. The installed capacity of the Grimsel 3 PSP is approximately 660MW with an expected flow rate of 130m3/s. The major construction elements of the pump storage scheme to develop are the approximately 550m deep vertical pressure shaft, the tailrace tunnel with a length of about 2.5 km and the power house complex.

2 Plant Layout

Due to the utilisation of a vertical shaft instead of an inclined shaft as initially foreseen (Wannenmacher 2013), the galleries of Grimsel 3 are situated deep within the ridge of the "Schwarze Nollen". The overburden at the cavern structure is approximately 650 m. The cavern layout foresees a separation of the machine and the transformer cavern, which is situated above the tailrace galleries in-between. The caverns are connected via three small galleries. Three variable speed pumping turbines will be used for electricity production and pumping. The ratio of width to height and length for the machine cavern is 27.6*46.6*92.5mand 18.1*30.9*112.6m for the transformer cavern. The machine cavern also hosts three upstream spherical valves, where else the three butterfly valves are situated in the transformer cavern. All access tunnels to the underground structure are connected to the existing structures of the KWO, guaranteeing access all over the year within this alpine construction area. The access tunnels are further utilised for ventilation, drainage, escape paths and energy transmission.

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