6. Summary and outlook

6.1 Importance of geological structures

For any underground facility, the geological site is a major factor in the project, including the nature of the rock, the structure of the rock massifs and the hydrogeology. We have seen that there are storage facilities in both chalk and granite. As far as underground power plants are concerned, EDF studied a STEP at Venteuil-Tincourt, near Épernay, in the chalk. Although deemed acceptable in terms of rock mechanics, it was abandoned due to the hostility of Champagne winegrowers. In fractured rock massifs, we know that the orientation of elongated structures is a means of optimizing the response to instabilities: we prefer to place the elongation of the cavity at right angles to the direction of the most developed fractures, which is rarely...