The Mont Terri underground rock laboratory (Switzerland) and results of hydrogeological studies of an argillaceous formation

In many countries, argillaceous formations are being considered as potential host rocks for repositories of radioactive waste. Therefore, in 1995 several organisations decided to start an international research project in the reconnaissance gallery of the Mont Terri motorway tunnel (NW of Switzerland), in a Mesozoic shale formation, the Opalinus Clay (Aalenian). This project is under the patronage of the Swiss National Hydrological and Geological Survey and the following organisations are partners in the project: ANDRA and IPSN (France), BGR (Germany), ENRESA (Spain), NAGRA (Switzerland), JNC and Obayashi (Japan), SCK/CEN (Belgium). The general aims of this research program carried out from this underground laboratory are to analyse the hydrogeological, geochemical and rock mechanical properties of an argillaceous formation together with the changes of these properties induced by underground works (drillings, excavation of galleries), and to evaluate and improve appropriate investigation techniques. In January, 1996, eight niches were excavated in order to carry out fifteen experiments from boreholes of 30 metres maximum in length. In the winter of 1997/1998 a new research gallery was excavated to host further experiments. Different excavation, drilling and investigation techniques have been tested and improved. Significant results of the different experiments are already available (Thury and Bossart, 1999). Some of those dealing with hydrogeology and hydro-chemistry are reported in this paper. Results provide important data in order to estimate feasibility and safety for a final radioactive waste disposal. (authors)