Multi-disciplinary investigation of water content and displacement evolution around the Mont Terri twin niches in Opalinus Clay

Abstract

Coupled hydro-mechanical effects in Opalinus Clay (OPA) are of significant interest regarding the  stability and integrity of a potential storage facility for high-level radioactive waste (HLW). In the Mont Terri Rock Laboratory, the CD-A twin niches differ only in their ventilation and enable the investigation of near field effects like desaturation and development of the excavation disturbed zone (EDZ). Amongst others, the evolution of water content and deformation are observed and provide a comprehensive data set as basis for a pragmatic approach to capture effects in the EDZ in safety assessment. The measuring campaign includes electric resistivity tomography (ERT), providing daily measurements of the electric resistivity distribution up to a few meters into the rock formation, as well as the nuclear magnetic resonance method (NMR) aiming on the evolution of water content near the surface of the niches. Due to the corresponding effort in handling the relatively new single-sided application, the NMR measurements are only conducted once per season. Nevertheless, the combined interpretation of NMR and ERT provides valuable data of water content and hydraulic conductivity evolution enabling a meaningful optimization of numerical modelling approaches. The interpretation of the water content evolution in combination with the geologic characterization demonstrates the high heterogeneity of the sandy facies of OPA, a significant impact of the ventilation, and a correlation of high resistivity regions with open fractures, indicating potential preferential flow paths that are relevant for transport processes. As a matter of fact, the results illustrate a need to discuss the set-up of numerical modelling, especially concerning the boundary conditions, the heterogeneities, and the parametrization and set-up of the EDZ. Further measuring focus on the displacements around the niches. While laser scans represent the convergence behavior near the surface, extensometers generate information about the displacements a few meters around the twin niches. Summarizing these data, the convergence behavior near the surface as well as a few meters around the twin niches depends significantly on the ventilation of the niches, indicating seasonal effects and differences in the long-term convergence behavior. Future work aims on an increased system understanding considering long-term effects due to climatic conditions, geotechnical boundary conditions, heterogeneities amongst others and their realistic representation in safety assessment.