This is a geological study of the type of rock mass at the foundation of the future Puyiaurent dam (Massif Central français) which was carried out considering that the knowledge of the regional structural organisation is fundamental for the interpretation of the geotechnical characteristics of the rock mass. For this reason, the different tectonic phases which have transformed the region have been analysed . Thus, the creation, geometry and the resulting interplay of the faults have been established. A precise structural study of the metamorphic rock which rnakes up the foundation of the dam have made it possible ta establish, in addition to the geometrical strains that it under* went a hierarchical and chronological relation with the regional faults. The existence of the eventual anisotropy of the actual stresses have been examined using earthquake mecanisms, in-situ stress measurements and seismic velocity measurements. These measurements have been successful in making clear the actual stress direction N-S to NNE-SSW. The comparison of the geological data and the results of the reconnaissance work (borings, Lugeon tests, exploratory drifts) shows some cases where a high permeability of the foundation in linked to the actual stress orientation.
