Étude de l'effet chimique de fluide interstitiel sur le comportement hydro-mécanique de l'argile de Boom (Couplage C-H-M dans l'argile de Boom)

Deep geological disposal represents a reference solution for the long-term management of radioactive waste by isolating harmful radionuclides from the biosphere through a system of engineered and geological barriers. In Belgium, Boom Clay is considered a reference host formation due to its low permeability, high retention capacity, self-sealing behavior, and favorable mechanical properties. Among the waste forms intended for disposal, Eurobitum waste, produced by bituminization of reprocessing effluents, contains high concentrations of (Na,Ca)NO3. Under repository conditions, the bitumen matrix gradually absorbs water and swells, resulting in the progressive release of hypersaline leachates into the surrounding clay formation. These leachates induce chemical perturbations that can affect the hydro-mechanical behavior of Boom Clay by altering its microstructure, swelling capacity, and strength.This thesis investigates the influence of salinity and sodium occupancy on the hydro-mechanical behavior of Boom Clay by combining a dedicated experimental campaign with the development of a chemo-mechanical constitutive modeling approach. The experimental program includes swelling pressure measurements under constant-volume conditions, salinization and desalinization tests under oedometric conditions, high-pressure oedometric tests, undrained shear strength tests, and microstructural analyses using mercury intrusion porosimetry. The results demonstrate that increasing salinity leads to a reduction in swelling pressure, an increase in shear strength, and modifications of the aggregate structure, especially at high ionic strengths, which also result in increased hydraulic conductivity.To interpret and predict these coupled effects, a chemo-mechanical constitutive model named ACC2-Chem was developed. This model extends an existing elasto-plastic framework (ACC-2) by incorporating a chemo-elastic strain formulation linked to ionic strength and sodium occupancy. The model was implemented in the finite element code LAGAMINE and calibrated using laboratory results. Its validity was verified through the reproduction of salinization/desalinization and triaxial test results at the laboratory scale. Finally, 2D repository-scale simulations were conducted to evaluate the long-term chemo-hydro-mechanical behavior of Boom Clay under high salinity perturbations. The simulations capture the phenomenological evolution of the repository through four sequential phases: (1) an initial excavation phase lasting one day, (2) a 100-year operational phase under drained conditions, (3) a 300-year post-operational equilibrium phase, and (4) a 500-year swelling and/or diffusion phase simulating long-term interactions between Eurobitum waste and Boom Clay.The findings improve understanding of Boom Clay’s behavior in chemically disturbed environments and provide a predictive framework to support safety and performance assessments for the geological disposal of bituminized radioactive waste in Belgium.