An Elastoplastic Constitutive Model for Anisotropic Hydro-Mechanical Response of Opalinus Clay

Opalinus Clay is currently proposed as the host rock for a deep geological repository (DGR) for radioactive waste in Switzerland. Gas is expected to be generated within the repository due to anoxic corrosion of metallic components and the degradation of organic material, and understanding gas migration is therefore critical, as pressure build-up may induce fracturing and potentially compromise the long-term integrity of the repository. One relevant gas transport mechanism is dilatancy-controlled flow, in which localized dilatant pathways can lead to transient increases in permeability, facilitating gas migration and potentially affecting the barrier performance of the host rock. To investigate and simulate such processes, a constitutive model is required that captures hydro-mechanical (HM) behavior, including the influence of bedding-induced anisotropy, yield behavior, and strain-dependent hardening and softening. In this study, consolidated undrained (CU) triaxial tests performed on Opalinus Clay specimens with varying bedding orientations (in increments of 15∘) are used to calibrate and evaluate the columnar-basalt (COMBA) constitutive model implemented in FLAC3D. The model allows for the explicit representation of bedding through oriented joint sets, enabling orientation-dependent failure criteria, anisotropic stiffness, and strain-dependent evolution of strength parameters. The constitutive model is subsequently applied to a coupled hydro-mechanical simulation of a gas injection experiment conducted at the Mont Terri Rock Laboratory. Comparison with a previously applied ubiquitous-joint formulation shows that the COMBA constitutive model reproduces the observed hydraulic response and deformation localization, while providing a more accurate prediction of the timing of deformation measured by distributed fiber-optic sensing, although the accumulated deformation is overpredicted at later times. The improved representation of bedding anisotropy further provides a clearer insight into the gas migration mechanism, with the close correspondence between plastic shear strain and gas breakthrough supporting shear–slip dilation along the bedding planes as a likely mechanism for pathway formation.