This paper presents a study of the deformational behaviour of the rock mass during the excavation of the underground powerhouse cavern of the Salamonde II re-powering scheme, located in a mountainous region. The rock mass parameters considered in the three-dimensional numerical model and the initial state of stress resulted from previous studies, which included stress measurements and a global analysis of the results. The calculated and measured displacements along the complete excavation sequence were compared and, by using a back analysis process, the elastic moduli of the rock mass that lead to the best approximation were identified. Considerations regarding the results obtained, namely its dispersion, are presented.
For the design of large underground caverns, once the geological investigation helped defining their location, the most important tools used for their geometric definition and for the design of the support system are in situ and laboratory tests, which provide information on the rock mass strength and deformation and on the stress field, as well as the numerical models that make use of this information.
The Salamonde II hydroelectric scheme, promoted by Energies of Portugal (EDP), is currently under construction and corresponds to the re-powering of the 50 years old Salamonde scheme, built in a mountainous zone in the North of Portugal. It includes a concrete dam, a hydraulic conduit and a powerhouse cavern. Salamonde II has a new, 2 km long, underground hydraulic conduit and a much larger powerhouse cavern, 66m long, 26.5m wide and 56m high at the turbine hall, located 150mbelowthe ground surface (COBA 2009). The rock mass is a coarse grained granite, mostly of good quality, with several fault zones intersecting the hydraulic conduit. Figure 1 shows the location of the new and old powerhouses, the Cávado river where the dam is located and the Mau river, a tributary.A perspective of the powerhouse cavern and of the excavations in its vicinity is also represented in Figure 1.
For the design of the Salamonde II hydroelectric scheme a complete site characterisation programme was carried out. As part of it, LNEC carried out an in situ stress measurements programme and obtained the most probable state of stress in the rock mass (LNEC 2012).
