Extension of the desaturated zone around structures excavated in the Tournemire argillite by means of petrophysical measurements coupled to gas tests

Titre du congrès :Clays in natural and engineered barriers for radioactive waste confinement Ville du congrès :Tours Date du congrès :14/03/2005

Studies carried out on URL constructed in compacted argillaceous formations like in Mont Terri and Tournemire have demonstrated the occurrence of a rock de-saturation around tunnels or drifts as a consequence of excavation works and drift ventilation. The Tournemire URL owns structures realized at three distinct periods: a century-old tunnel and drifts excavated in 1996 and 2003. It is also characterized by the appearance of desiccation fractures in the drifts. This behavior has conducted IRSN to record since 1999 the properties (Relative humidity and temperature) of the air ventilating the different structures. Time series have shown seasonal cycles for the Relative Humidity and temperature that are negatively correlated with a mean annual RH and temperature values of ca 70% and 10°C, respectively. These variations produced hydration/dehydration cycles at the structure walls since their excavation. These cycles explain the shrinkage/swelling behavior of the rock and the subsequent opening and closure of inter-layers spaces (Valès et al., 2004). However, the too short periods of saturated-air conditions did not allow a complete re-saturation of the rock at the structure wall that progressively de-saturates. This study aims to assess the extension of this de-saturation with time and location by means of densitometric petrophysical measurements coupled to pneumatic tests. This abstract presents the main results of the first 2m borehole dedicated to this study and that was drilled parallel to bedding in the most recent drift excavated in 2003