In this paper, we are presenting on the one hand the main results of the geomechanical characterisation obtained on the clayed rocks studied by ANDRA, on the other hand, an overview of the in-situ experimentations carried out by ANDRA.
ANDRAs research work on the mechanical behaviour of deep clayed formations began with the 1983 inventory of areas that could be appropriate for the storage of high activity, long-lived wastes in geological formations. In fact, the clayed formations cover large areas, due to France's extensive sedimentary basins. The CASTAING Commission (October 1983- October 1984) requested that these clayed formations be among the four geological materials selected for studies on deep storage. The Commission furthermore insisted that the mechanical properties of these formations must be known, in order to define the potential for building underground structures, understand possible fissuring phenomena, and predict the evolution of the storage site after the excavations are plugged. From 1984 to 1987, ANDRA carried out studies on the mechanical properties of various clays (Tables 1 and 2), using samples taken during drilling performed for oil exploration, scientific research on French geology, etc. ANDRA was also able to participate in the CEN/SCK experiments on digging a small well and a recess in the Boom clay, without prior freezing. Beginning in 1988, ANDRA was able to carry out drilling survey work in the Callovo-Oxfordian and Toarcian Jurassic clayed formations in the north-east part of the Paris basin. This allowed notable progress to be made in logging interpretation, sample conservation, and the testing methodology used to obtain mechanical parameters (that describe the behaviour of the rock during drilling in order to estimate certain geomechanical parameters). The geotechnical investigations carried out by ANDRA involve very different time scales: a hundred years, for the maintenance of underground structures, or thousands or tens of thousands of years, for the evolution of the geological medium after storage. To attain these very specific objectives, major research programs were established in order to understand the behaviour of both macroscopic and microstructural phenomena, and in order to establish predictive behavioural models, even for time scales for which direct observation is not possible. At the same time, ANDRA carried out several experiments at the MoI underground laboratory, as part of European Community research programs. These experiments concerned ways of supporting galleries in the clay, and the effect of temperature and ventilation on hydromechanical behaviour. The December 30, 1991 law concerning research on the management of radioactive wastes was a new stage of this program. The discussions between elected officials and local populations regarding potential sites for research laboratories led to the selection of two sites in clayed formations, among those possible. One site is in the eastern part of the Paris basin, in Callovo-Oxfordian marlstone, and the other is in southern France in Cretaceous clayey siltstone. Prior to requesting authorisation to install and operate underground laboratories, survey work in these areas began in 1994 and has just been completed.