The Callovo–Oxfordian (COx) claystone has been selected, subject to authorisation, as the potential host formation for the deep geological disposal of high-level and intermediate-level long-lived radioactive waste in France. In the repository, the COx will be exposed to thermal loads reaching up to 90 \(^{\circ }\) C due to the decay heat of radionuclides. This study investigates the transversely isotropic poro-elasto-viscoplastic behaviour of COx claystone and evaluates the potential impact of in situ thermal loading. Rock cores previously subjected to an in situ heating experiment conducted by Andra at the Meuse/Haute–Marne Underground Research Laboratory were extracted and tested in the laboratory. Triaxial compression tests were performed under various stress paths to failure on re-saturated specimens cored either parallel or perpendicular to bedding. Cyclic loading allowed for the separation of elastic and inelastic strain components, from which stress-dependent elastic parameters were determined. The inelastic response was used to identify and calibrate a non-associated elasto-plastic constitutive model, incorporating transverse isotropy via a non-uniform scaling approach. Pore pressure dissipation following pseudo-undrained unloading enabled the estimation of intrinsic permeability. Time-dependent strain measurements confirmed the viscoplastic behaviour of COx, which was implemented in the constitutive model using an overstress formulation. The resulting poro-elasto-viscoplastic model was implemented with MFront and integrated into the finite-element code FEniCSx, enabling reproduction of the experimental results and paving the way for repository-scale simulations. Comparison between heated and non-heated samples indicates that COx claystone preserves its key properties as a suitable host rock after in situ thermal loading.
