Technical Papers and Presentations

Clayey soils and rocks are often used to secure contaminants, dissolved in the interstitial water, or fluids immiscible with water, because of their hydraulic properties (very low permeability and high water retention capabilities, see e.g. McCartney et al., 2016). However, their hydro-mechanical behaviour is largely influenced by the physico-chemical interaction with the wetting pore fluid. This interaction is caused by their mineralogical structure, and from the presence of minerals which might dissolve/precipitate in response to chemical changes (e.g. Musso et al. 2024). Robust integration of multi-physics modeling and experiments allows characterizing the complex interaction between pore fluids and clayey soils. Thus, an advanced oedometer for testing the hydro-mechanical response of clayey soils and rocks exposed to acid solutions and non-aqueous fluids was designed. It is an enhancement of an existing cell allowing for the tomographic reconstruction of the electrical conductivity and measurement of the elastic (small strain) properties of soils (Comina et al., 2008).

Electrical tomography requires a large number of measurements, where two electrodes on the sample surface inject the current and the other two measure the potential drop. Different combinations of quadruples are used to ensure a single reconstructed map of the sample conductivity. When the sample is non-homogeneous, as expected during the transient hydro-geochemo processes, the quality of the reconstruction largely depends on combinations of the used quadruples. To design the most appropriate experimental protocol, numerical analyses were performed in COMSOL®. Simulations of flushing tests, where distilled water is forced into an initially brine-saturated sample, were run accounting for water Darcian flow and advection/diffusion of chemical species using the “Darcy’s Law(dl)” and “Transport of Diluted Species (tds)” physics. This allowed us to reproduce the evolution of the pore fluid composition expected to take place in real tests. Maps of the electrical conductivity of the soil at different times were predicted on the basis of the electrolyte concentration using Archie’s law. The “Electric Currents (ec)” physics was then used to investigate the electrical potential fields associated to 314 different electrodes quadruples. The position and combination of some quadruples were such that the measuring electrodes lay almost on the same potential line and were excluded from the inversion data. For example, when the current is injected into the electrodes which are opposite to each other and the measuring electrodes which are also opposite to each other will fall on the same potential line. These allowed us to determine which of them can be used, in real tests, for reliable reconstructions and which would provide less meaningful information.