In this study, the impact of salinity on the water retention properties and microstructure of the Opalinus Clay from the lower sandy facies (LSF) of Mont Terri site was investigated by exposing a series of unsaturated Opalinus Clay samples to two sodium nitrate \({NaNO}_{3}\) solutions of controlled osmotic suctions ( \(\pi =15\) and \(34 MPa\) ) and to a synthetic water (same salinity as the natural pore water) \(\pi =1 MPa)\) at different water contents. The water retention curves were determined along wetting path for the three osmotic suctions, and microstructural analyses were conducted at different saturation states. While the total suction increased with salinity, the porosity decreased, suggesting the decrease in the swelling potential. Exposing the Opalinus Clay to a highly saline solution at low water contents increased the specific surface area and the peak of the dominant mesopore population. This is due to the dispersion of the Na-clay fraction and to the swelling of the initially unsaturated clay fraction. At higher water content, the water transfer from the meso to the macropores prevails and a decrease in specific surface area, peak and density of the mesopores were observed. The higher the osmotic suction, the lower the water content at which the shrinkage of the mesopores is initiated.
