Desaturation and structures relationships around drifts excavated in the well-compacted Tournemire's argillite and their impact on the hydraulic head profiles

Jean-Michel Matray (1) , Sébastien Savoye (1) , Justo Cabrera (1)

1 IRSN/DEI/SARG/LETS - Laboratoire d'étude des transferts dans les sols et le sous-sol

This study aimed to explore the relationships between the rock desaturation and the EDZ extension subsequent to the excavation of a century-old tunnel and recent drifts (1996 and 2003) at the Tournemire Underground Research Laboratory. The other objective of this work was to assess the impact of this desaturation on the hydraulic head profile measured around the tunnel. One section was selected per drift. Two boreholes were realized for each section: parallel and inclined (45°) with respect to the bedding. For each borehole, we performed on-site drill core mapping, petrophysical measurements and pneumatic and hydraulic tests by means of a Modular Mini-Packer System (MMPS) device. Results indicate that EDZ around drifts is mainly a combination of unloading joints, mimicking the drift shape, and of desaturation cracks, parallel to the bedding. The EDZ extension around the tunnel is twice to three times that of drifts 1996 and 2003 and essentially composed of unloading joints resulting from the mechanical response of the rock. The masonery covering the tunnel walls is assumed to have protected the rock from the seasonal variations of the air humidity, thus limiting (without excluding) the formation of desaturation cracks. The EDZ extension deduced from core mapping is in agreement with that deduced from pneumatic tests with permeabilities several orders of magnitude greater than in the undisturbed zone. Degrees of saturation for the three sections range between 0.9 and 1 in the EDZ area and reach 1 in the undamaged zone. The head profile deduced from measurements recorded since 2002 indicates the occurrence of sub-atmospheric water pressures with an extension of ca 40m around the tunnel. We have searched to quantify the impact of the tunnel since its excavation on the degrees of saturation and the hydraulic heads. The simulation was performed by considering, as a first approach, the absence of fracturation in the EDZ area. A constant suction of -3300m, deduced from the mean annual values of relative humidity and temperature measured in the tunnel atmosphere since 2002, was applied at the tunnel wall. The degrees of saturation simulated around the tunnel are underestimated in the EDZ area and consistent to experimental data in the unfractured zone. The modelling of hydraulic heads is quite consistent to experimental values in the vertical direction and overestimated in the horizontal direction. This study has demonstrated the role played by fracturation on the distribution of petrophysical parameters and of heads around drifts and the century-old tunnel. It has also demonstrated the necessity of coupling mechanic and hydraulic calculations by considering capillary forces.

Mots clés: Tournemire; argillite; permeability; EDZ; desaturation

Domaines: Milieux fluides et réactifs; Géologie appliquée; Hydrologie; Minéralogie

Jean-Michel Matray, Sébastien Savoye, Justo Cabrera. Desaturation and structures relationships around drifts excavated in the well-compacted Tournemire's argillite and their impact on the hydraulic head profiles. Engineering Geology, 2007, 90, pp.1-16. ⟨10.1016/j.enggeo.2006.09.021⟩. ⟨irsn-00175833⟩