CD-A twin niches in the Mont Terri Rock Laboratory: Characterization and interpretation of hydraulic parameters with regard to safety aspects

In the Mont Terri Rock Laboratory (Switzerland), a comprehensive interdisciplinary exploration program is carried out to increase knowledge about long -term coupled hydraulic-mechanical effects in Opalinus Clay. As these effects are of significant interest regarding the stability and integrity of a potential storage facility for high radioactive waste, the "Influence of humidity on cyclic and longterm deformations"(CD-A) experiment aims to characterize the Opalinus Clay, especially its heterogeneous sandy facies, and to investigate the evolution of the water content, pressure and deformation in the formation. This article provides an overview of measurement examinations and geologic investigations carried out in the context of the CD -A experiment. The twin niches of the CD -A experiment are located in the upper sandy facies of the Opalinus Clay which is characterized by the existence of different subfacies types and fault zones. This contribution focuses on the detailed characterization of the claystone, while the evolution of the water content. Combined geophysical and geological investigations during the first two years after the excavation of the niches have been carried out. They indicate that the subfacies type and pre-existing fault zones do not affect the distribution of water content severely, which could be an essential finding in the context of detection procedures for potential high level waste (HLW) repositories. Furthermore, miniseismic as well as pulse tests indicate clearly, that the extent of the excavation damaged zone (EDZ) strongly depends on the climatic conditions in the underground galleries: After 1.5 years the EDZ and the increase of permeability is much more pronounced in the open than in the closed twin. Future work aims on an increased process understanding based on a verified constitutive model for Opalinus Clay and a comprehensive system understanding considering long -term effects due to climatic conditions as well as the impact of geotechnical boundary conditions and heteregeneities.