Desaturation and structure relationships around drifts excavated in the well-compacted Tournemire's argillite (Aveyron, France)

Abstract

This study aimed to explore the relationships between the extension of rock desaturation and the Excavation Damaged Zone (EDZ) subsequent to the excavation of a century-old tunnel and of recent drifts (1996 and 2003) at the Tournemire Underground Research Laboratory (URL) located in the Aveyron county (South of the Massif Central, France). The other objective of this work was to assess the impact of desaturation on the hydraulic head profile measured around the tunnel. One section was selected per drift. Two boreholes were drilled for each section: one parallel and one inclined (45°) with respect to the bedding. For each borehole, we performed on-site drill core mapping, petrophysical measurements, pneumatic and hydraulic tests by means of a Modular Mini-Packer System (MMPS). Results indicate that the 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 the drifts of 1996 and 2003 and is 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 seasonal variations of 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 an Excavation disturbed Zone (EdZ) of about 40 m around the tunnel. This EdZ is likely due to the existence of sub-atmospheric water pressures clearly seen in the first meter around the tunnel. We have tried to quantify the impact of the tunnel since its excavation on saturation degree and on hydraulic heads. The simulation was performed by considering, as a first approach, the absence of fracturing in the EDZ. A constant suction of − 3300 m, 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 overestimated in the horizontal direction and is in the domain of experimental values in the vertical direction, but the lack of intermediate data cannot enable us to conclude on this consistency. This study demonstrated the role played by fracturing 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.