Combining magnetic resonance imaging and X-ray microtomography to analyse water transfer in unsaturated expansive clay

Water imbibition column experiments have been extensively used over the last 25 years to investigate unidimensional water transfer in unsaturated expansive clays under confinement. These studies typically focus on determining soil properties such as water diffusivity and unsaturated hydraulic conductivity, which are crucial for designing long-term sealing systems in deep geological repositories constituted of bentonite-based materials. This study introduces a novel experimental procedure combining non-destructive, high-spatial-resolution techniques: Magnetic Resonance Imaging (MRI) and X-ray Computed Microtomography (μCT). These methods allowed a detailed analysis of a 290-day imbibition column experiment conducted on a bentonite-sand mixture. MRI measurements were used to determine water mass profiles, while μCT was employed to monitor water absorption and dry density changes. Notably, μCT provided insights into the evolution of technological voids initially present in the sample, enabling improved dry density estimations. The final dry density profiles aligned well with previous experiments and numerical simulations. Furthermore, unsaturated hydraulic conductivity was derived using the instantaneous profile method. The experimental results highlighted a transition in apparent unsaturated hydraulic conductivity from a vapour diffusion-dominated regime to a capillary-driven regime as the degree of saturation increased. Near saturation, the measured hydraulic conductivity values were consistent with other bentonite-based materials of similar dry densities. The original experimental procedure reduces the required sample height and the duration of imbibition tests, making it a promising tool for laboratory studies. The findings provide valuable insights into the hydration behaviour of bentonite-sand mixtures, with applications in modelling hydration processes in such systems.