Hydro-Mechanical Modelling of Anisotropic Deformation and Failure Behaviour of Opalinus Clay Under Saturated and Unsaturated Conditions

Opalinus Clay (OPA) is a key host rock for the geological disposal of high-level radioactive waste in Switzerland and is also under investigation in Germany. Reliable prediction of the long-term performance of deep geological repositories requires constitutive models capable of capturing the coupled hydro-mechanical (HM) behaviour of the host rock, including mechanical anisotropy, strain-dependent stiffness, suction effects, and stress-dependent failure. This study presents a hydro-mechanically coupled constitutive model incorporating anisotropic yield behaviour, hardening/softening, and strain-dependent permeability. The model is calibrated against laboratory triaxial, Brazilian tensile strength (BTS), and uniaxial compressive strength (UCS) tests on OPA, with bedding orientations between 0° and 90°. Implemented in OpenGeoSys (OGS), the model represents bedding-controlled plastic anisotropy using a microstructure tensor approach. The simulations reproduce key experimental trends relevant to repository-induced perturbations, including bedding-dependent strength and stiffness, suction effects on UCS, and the orientation-dependent tensile strength observed in Brazilian tests. Remaining discrepancies under high confining stress indicate the need for improved regularization and dilatancy formulations. Overall, the proposed framework provides a robust building block for HM process modelling and long-term safety assessments of deep geological repositories.