In Brazil, several hydroelectric projects have been developed in the last 35 years, to meet the energy demand of a growing country. Initially, most, of these power plants were constructed in the South and Southeast regions, which are mainly areas of basaltic rocks from the Parana Basin. However, at present, new projects in the North and west are being developed in a completely different geological environment, including metanorphic, igneous and sedimrentary rocks. The techniques and methodologies used for rock mass characterization and the design of rock foundations, slopes and underground excavations in these hydroelectric projects has also changed and is evolutive to follow newdeveloprents and each project's specific needs. This paper describes the evolution of rock Irechanics in hydroelectric projects in Brazil and also comments on the present design philosophy.
Rock mechanics in Brazil has been progressing hand in hand with engineering geology since the construction of large hydroelectric plants from the fifties on.
There was great expansion in the sixties and seventies when the country started to meet an increasing demand for energy with the construction of 32 large hydroelectric power plants, reaching around 30,000 MW installed, mainly in the South and Southeast regions. The innumerable investigations, "in situ" and laboratory tests carried out, allowed their own methodology and geomechanical criteria to be established, relating mainly to gravity dam foundation studies and designs in basalt rock masses of the Parana basin.
At the end of the seventies and to date, with the slower rhythm in building new hydroelectric plants, and only with the continued building of the Tucurui darn (4,000 MW)to the North of Brazil and Itaipu (12,000 MW)to the South, the Brazilian experts began to devote their studies to the synthesis of the information acquired, and adoption of methodologies suggested by the ISRM. with a view to their entry into the world hydroelectric plant construction market..
The development of rock mechanics in hydroelectric projects in Brazil, the present design philosophy and future developments are discussed in the following sections.
The pioneer signs of rock mechanics in Brazil date from 1951 when Pichler & Campos(1959) carried out "in situ" tests to find elastic constants and "in situ" stresses (stress relief method) of the gneiss rock mass on the site of the power house at the Paulo Afonso hydroelectric plant, by applying hydrostatic water pressure in test galleries. These tests were to support the design of the tunnel linings.
The geologists and engineers at that time, although recognizing the importance of the geological conditioning factors in the behaviour of dams, still had no basis for quantifying their observations. For exanple, they knew that the presence of interlaid clay beds in the sandstones could prejudice their shear strength, but they did not know howto characterize such a feature; they＿ understood that schistosity and gneissification gave anisotropy to the rock mass in terms of resistance and emphasized its weathering, but they did not know how to analyse it; they were afraid that open joints would totally endanger the imperviousness of dams