Effects of desaturation–resaturation on mudstone

Hydromechanical behavior of argillaceous rocks is currently studied in order to predict the perturbations around ventilated galleries of a nuclear waste storage. This paper presents the results of desaturation–resaturation experiments performed on mudstone samples bored at 500 m depth at Bure (France) where an underground research laboratory is under construction. We characterize the mass transfer process in the Bure mudstone, in terms of moisture diffusivity as a function of the relative humidity. A diffusivity identification method based on the measured mass evolution during transfer is proposed for the cases of uniaxial and radial diffusions. A significant non-linearity of moisture diffusivity has been observed. The retention curve, the diffusivity evolution, the ultrasonic velocity and the hydric strains have been investigated during the desaturation–resaturation process without mechanical loading. Hydric strains are anisotropic and non-linear with respect to relative humidity. A hysteresis has been observed on the ultrasonic velocity evolution, on the hydric strain variation and on the retention curve. Moisture diffusivity evolution is not the same along the desaturation path than along the resaturation path; this could be explained by the difference in moisture capacity on these two paths. The effect of desaturation–resaturation on mechanical behavior of the Bure mudstone is analysed by carrying out uniaxial compressive tests on samples with different water contents. Elastic parameters and compressive strength depend clearly on the suction. The mechanical behavior seems to be sensitive to the hydric path but further testing is needed to confirm these preliminary results.