Release of the in situ state of stress is often the most relevant action during the excavation of underground structures in rock masses, such as powerhouse caverns for hydroelectric schemes, which are often excavated at great depth and have large dimensions. Design of the supports, namely using numerical models, requires an estimate of the magnitude and direction of the principal stresses. However, a large number of factors influence the in situ stress field and its characterisation is a difficult task. Besides, the number of tests carried out for determination of the in situ stresses in rock masses is usually small, due to cost and time constraints, and all the testing methods have limitations inherent to their nature.
The paper presents methodologies for analysis of the stress field obtained from the results of in situ stress measurements using three methods: overcoring, hydraulic fracturing and flat jacks. These methodologies integrate all stress measurements and use numerical models of the rock mass that represent the ground topography, the lithology, and the geometry of the underground openings, so that the most likely stress field in the zone of interest for the design is obtained. Instances of application to three underground powerhouses are presented.
Powerhouse caverns are underground structures of relevance in the design of hydroelectric projects. In power schemes with high water head in mountainous regions, the power conduit is often built underground. Thesual reasons for this are the better quality of the rock mass at depth and the higher in situ stresses, which enable the construction of unlined pressure tunnels and shafts, with economic advantages. The increase in the number of reversible, pump-storage projects, where the hydraulic power units (pump turbines) must be located considerably below the downstream water level, also calls for the construction of large underground powerhouses at great depth, which often exceeds 500 m.