Advanced thermo-hydro-mechanical modelling of Callovo-Oxfordian claystone: Temperature effects and multi-scale applications for geological disposal safety

To support the feasibility of the Cigéo deep geological repository, Andra has carried out extensive thermo-hydro-mechanical (THM) investigations on Callovo-Oxfordian (COx) claystone. These investigations combine in-situ experiments conducted at the Meuse/Haute-Marne Underground Research Laboratory (M-HM URL) with detailed laboratory-scale characterisations. This study introduces an enhanced rheological model that integrates temperature-dependent mechanical characteristics derived from recent experimental data. Implemented in COMSOL Multiphysics®, the model is validated against triaxial THM tests at different temperature levels (20°, 40°, 60° and 80 °C), accurately capturing short-term strength degradation and volumetric behaviour transitions under increasing temperature. Long-term behaviour simulations, including simulation of triaxial creep tests at 40° and 60 °C, show excellent agreement with the analytical results, with deviations remaining below 0.1 %. The proposed model was first applied at the underground structures scale to the GCS drift, which serves as a reference case for validating the constitutive models of the COx claystone. The simulation covered a 20-year period, and the results were successfully compared with the convergence measurements recorded since the gallery's excavation. The model is further applied to the HITEC near-field benchmark to assess thermal impacts on the Excavation-induced Fracture Zone (EFZ) surrounding a heat-emitting waste cell. The results confirm the robustness and applicability of the proposed THM framework for large-scale repository design under thermal loading conditions.