Rock mechanical aspects for cavern stability

The extension of Grimsel 3 hydropower project (600 MW) entails the construction of a 2.5 km long tailrace tunnel, a machine and a transformer cavern as well as a new 600 m deep vertical shaft connecting the existing conduit with the Raterichsboden 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.