Hydromechanical and mass transfer phenomena in argillaceous rocks mass are currently studied in order to predict the perturbations around ventilated galleries of a nuclear waste storage. This paper presents the results of drying experiments performed on argillite samples bored at 500 m depth at Bure (France) where an underground research laboratory will be built. Relative humidity and mass evolution of thin plates and hollow cylinders samples are continuously measured in tight box in which relative humidity is imposed by saline solutions. The mass transfer phenomenon is characterized by the moisture diffusivity coefficient computed by comparing the measured mass evolution and the linearized analytical solutions for each drying step. The linearized water moisture diffusivity increases exponentially according to the relative humidity from about 0.5x10-10 m2/s-1/ for the 44 % to 32% relative humidity step to 1.2x10-10/ m2/s-1/ for the 97% to 90% relative humidity step. A theoretical moisture transfer model, accounting for the vapour water transfer and liquid water transfer is then proposed. The confrontation with the experimental data leads to the identification of permeability of the unsaturated rock. The model assumes an intrinsic permeability value of 10-22/ m2/, which is a lower bound of measured permeability in the saturated state for this argillite.
The French Radioactive Waste Management Agency (ANDRA) has selected an argillaceous site in the east of France (Bure, Haute-Marne) as potential nuclear waste storage host. An underground research laboratory at a 500 m depth is currently under construction. The digging of the galleries is expected to create a damaged zone around the galleries, increasing the permeability by several orders of magnitude. Additional damage could be induced by the desaturation due to the ventilation of the galleries. Such hydric induced cracks have already been observed in the argillaceous Tournemire tunnel . A complete experimental investigation is currently conducted at the LMS (Laboratoire de Mécanique des Solides) in order to characterize, on one hand, the unsaturated hydromechanical behaviour of the rock at moisture equilibrium and on the other hand, the mass transfer process during desaturation steps.
The argillite under investigation belongs to the Callovo-Oxfordian layer at a depth of 400-500 m. The tested samples are bored from cores at a depth of about 480 m. The mineral content is: 40% of clay materials, 25 to 30% of quartz and 20 to 30% of carbonates. Microscopic observations have shown that the argillaceous phase is continuous whereas the quartz and carbonates phases are discontinuous . The observed quasi-linear relationship between the water content and the logarithm of suction is typical of clay materials. Water permeability in saturated state has been measured by transient method and ranges from 10-22 m2 to 10-20 m2 ,  for the undamaged material. The hydromechanical behaviour of Bure argillite has been extensively studied in the saturated state; however in the unsaturated state, few data are available, especially when the mass transfer process is concerned. We present in this paper the results of drying experiments leading to the experimental characterisation of the mass transfer process in term of moisture diffusivity coefficient as a function of air relative humidity.