Geological analysis of the hydro-mechanical responses of Opalinus claystone to excavation-induced damage via engineering

Abstract

In the realm of engineering geology, specifically concerning the containment of high-level radioactive waste, this study examines the excavation-induced damage zone (EDZ) in underground drifts and its evolution, which is critical for the effectiveness of underground repositories. The focus is on the self-sealing capability of the EDZ in Opalinus claystone (OPA) under mechanical compression and hydration induced by a bentonite core sealing component post-nuclear waste emplacement. This core applies a swelling pressure of 5 MPa to the drift walls during resaturation, affecting repository integrity. Using numerical modeling, this study pioneers the simulation of real-time self-sealing mechanisms in OPA, a crucial geological barrier in nuclear waste containment. The approach integrates a plastic damage model with an auxiliary deformation model and Biot’s model to accurately depict moisture-dependent OPA deformation. This modeling is essential for understanding claystone geomechanics in nuclear waste repositories. Detailed post-analysis of crack features and unsaturated fractured OPA permeability, following fracture energy regularization principles and the cubic law, provides insights into the geotechnical properties of the EDZ, enhancing the accuracy of geological forecasts in nuclear waste management. The model’s reliability is confirmed through various numerical simulations, including triaxial compression tests, observations of swelling deformations due to moisture variations, and comprehensive water permeability tests assessing the self-sealing efficiency of fractured OPA during water injection. In situ experiments at the Mont Terri Rock Laboratory support these findings, particularly regarding EDZ self-sealing under combined mechanical and hydration conditions. Comparative analysis of numerical results and experimental data validates the model’s ability to replicate fractured OPA self-sealing behavior, crucial for assessing nuclear waste repository safety and effectiveness. Effective self-sealing is underscored by reduced water permeability in the EDZ during resaturation, affirming the model’s relevance in evaluating nuclear waste containment strategies. Moreover, insights from this model extend to assessing CO2 migration, given claystones’ role as caprocks in CO2 geological sequestration schemes, thereby enhancing its importance in long-term environmental safety evaluations for both nuclear waste and CO2 storage scenarios.