Poromechanical Behavior of a Deep Shale Core for Geological Radioactive Waste Storage

In the context of deep geological radioactive waste disposal in Switzerland, this study investigates the hydromechanical behavior of Opalinus Clay from a borehole in Northern Switzerland at around 800 m depth. Laboratory experiments include the determination of petrophysical and water retention properties, and hydromechanical tests in a high-pressure isotropic cell. Various loading sequences were conducted on samples saturated under conditions approximating in situ stress. The cyclic drained loading and unloading revealed linear deformation above initial stress and nonlinear volume increases below it, with both reversible (elastic) and inelastic strains exhibiting anisotropy. Inelastic swelling, potentially indicative of damage, was gradual and nonlinear. Main poroelastic properties, drained bulk modulus, pore pressure modulus, and undrained bulk modulus, each with anisotropic strain components were identified. Positive correlations with effective stress were observed for the drained bulk modulus and the pore pressure modulus. The anisotropy ratio of strains perpendicular and parallel to bedding did not show significant effective stress dependency. Very low permeability was observed, at least one order of magnitude lower than measured at other sites, with negligible effective stress dependency. This study enhances the Opalinus Clay database concerning various geomechanical properties and provides direct input properties for a poroelasticity constitutive model, in particular the stress-dependent Biot tangent tensor components bij which are compatible with the Voigt–Reuss–Hill average of the mineral compressibilities, and the unjacketed pore modulus Kϕ.