A Regularised Anisotropic Elastoplastic Damage and Viscoplastic Model and Its Hydromechanical Application to a Meuse/Haute-Marne URL Drift

Clay formations are one of the options currently being considered for the storage of radioactive wastes worldwide. In France, the National Radioactive Waste Management Agency (Andra) operates the Meuse/Haute-Marne Underground Research Laboratory in the Callovo-Oxfordian (COx) clay formation to assess the feasibility and safety of an industrial radioactive waste repository. A good level of understanding of the thermo-hydromechanical behaviour of the host rock is paramount for the safety assessment. A new anisotropic elastoplastic damage and viscoplastic model is proposed to describe the hydromechanical behaviour of the COx claystone based on a large dataset of experimental evidence. The model is based on the Hoek and Brown criterion and considers recent findings of the COx hydromechanical behaviour. The key mechanisms considered are: plastic strain hardening prior to reaching the peak strength, a post-peak behaviour characterised by strain softening in the frame of continuum damage mechanics and a residual stage represented by a perfectly plastic behaviour. Time-dependent deformations are also included based on a creep model, which in this work is coupled with damage. The proposed model was implemented with a regularisation scheme based on the non-local implicit gradient in Comsol Multiphysics (R) with the purpose of performing THM modelling (1D, 2D and 3D) in the framework of the Cigeo project. The numerical implementation is first validated based on several simulations of creep tests at different deviatoric stress and triaxial compression tests at different confining pressures and angles between the loading direction and the bedding, alpha. Then, the GCS drift hydromechanical behaviour is simulated considering transverse isotropic conditions. It is shown that the model is capable of reproducing the measured peak of pore pressure in sensors near the GCS wall as well as the drift convergence. Importantly, the predicted extent of damaged zones around the drift is consistent with the in situ observations. The impact of damage on the time-dependent behaviour and the permeability was investigated numerically. It was found that the magnitude and anisotropy of drift convergence and the pore pressure drop are sensitive to this coupling and the best agreement was obtained when this effect was taken into account. Finally, the performance of the regularisation scheme is demonstrated with a set of simulations of the drift with different mesh refinements. It is concluded that the proposed model captures the key features of the hydromechanical behaviour of the COx claystone. An advanced constitutive model that takes into account the recent findings on the key mechanisms of deformation and failure of COx claystone is proposed.Strain hardening/softening, anisotropic elasticity and plasticity, damage described in the framework of CDM and time-dependent behaviour of COx claystone are considered.The effect of damage and shear fracturation on the transport and viscous properties are also addressed in the model.The model with a regularisation scheme is implemented in Comsol Multiphysics (R), which allows several physics to be considered, then to perform 1D, 2D and 3D THMC modellings.The operational nature of the proposed model is successfully evidenced on the GCS drift of the MHM URL, for which the in situ observations are the most challenging to reproduce.