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In the framework of a deep geological radioactive waste disposal in France,the hydromechanical properties of the designated host rock, theCallovo-Oxfordian claystone (COx), are investigated in laboratory tests.Experiments presented in this study are carried out to determine severalcoefficients required within a transversely isotropic material model. Theyinclude isotropic compression tests, pore pressure tests, and deviatoricloading tests parallel and perpendicular to the bedding plane. We emphasize theadapted experimental devices and testing procedures, necessary to detect smallstrains under high pressures, on a material, which is sensitive to water andhas a very low permeability. In particular, we discovered a significantdecrease of elastic stiffness with decreasing effective stress, which wasobserved to be reversible. In both isotropic and deviatoric tests, a notableanisotropic strain response was found. The Young modulus parallel to beddingwas about 1.8 times higher than the one perpendicular to the bedding plane. Anotably low Poisson ratio perpendicular to the bedding plane with valuesbetween 0.1 and 0.2 was evidenced. While the anisotropy of the back-calculatedBiot coefficient was found to be low, a significant anisotropy of the Skemptoncoefficient was computed. The performed experiments provide an overdeterminedset of material parameters at different stress levels. Using all determinedparameters in a least square error regression scheme, seven independent elasticcoefficients and their effective stress dependency are characterized.Parameters measured under isotropic loading are well represented by this set ofcoefficients, while the poroelastic framework with isotropic stress dependencyis not sufficient to describe laboratory findings from triaxial loading.

Transversely isotropic poroelastic behaviour of the Callovo-Oxfordian claystone: A set of stress-dependent parameters