Response of Callovo-Oxfordian claystone during drying tests: unsaturated hydromechanical behavior

The French National Agency for Nuclear Waste Management is currently studying the Callovo-Oxfordian claystone as a potential repository for nuclear waste. This study focuses on the hydromechanical (HM) unsaturated response of the rock subjected to hydraulic loading. Static atmosphere drying tests highlighted the HM coupling and mechanical anisotropy of the samples. A coupled poroelastic model was proposed, involving highly nonlinear equations. Numerical simulations were performed using the finite element code Code_Aster. Through a sensitivity analysis, the most suitable parameters for estimation were found to be the Biot coefficient and 3 of the 4 independent parameters of the Van Genuchten?CMualem relations. The parameters were estimated according to an inverse procedure that minimizes a cost functional. The estimates could not outline any hysteretic behavior, but they appeared to be divided into two groups (depending on their fitted values). The convergence and stability of the inverse algorithm were studied, and the results showed that direct computations must be run prior to the inversion to ensure its convergence. No plasticity occurred in the samples (according to the model), whereas irreversible phenomena do take place in situ.