Appraisal of the rock mass interested by underground excavations for the extension of a hydroelectric power plant

this paper illustrates the geognostic investigation campaign carried out in the rock mass which will be encountered during the excavation works for the extension of the Premadio (Italy) hydroelectric power plant. These works envisage, in particular, the implementation of an underground powerhouse. The cavern, approx 57x26x38 m, shall be excavated a few meters from the nearby powerhouse, in operation since 1956. The excavation will be carried out in metamorphic rocks characterized by an anisotropic behaviour due to the highly schistose texture and the presence of important dislocation structures with cataclastic rock fillings. The investigation campaign performed on the site where the powerhouse will be excavated has been carried out from a suitably excavated adit and includes: geostructural investigations, geomechanics investigations, both at site and in the laboratory, and geophysic investigations. These investigations have made it possible to define the geostructural aspect of the site, as well as the rock deformability features and the natural stress condition. The performed investigations, besides providing the geomechanical parameters necessary for the work design, have also made it possible to acquire data on the vibrations induced by excavation blastings in the adit on structures and equipment of the nearby operating powerhouse. These data will allow the future blastings for the second powerhouse to be sized correctly.