Opalinus Clay is a key geological formation in Switzerland, as potential host rock for the storage of Swiss nuclear wastes and as a caprock material of interest for deep geological storage of CO₂. The Opalinus Clay formation has been studied in the facility of the Mont Terri rock laboratory (MTRL) for 30 years thanks to the international Mont Terri Consortium. Based on existing transportation tunnels, the development of the underground laboratory facility has led to the excavation of research drifts to access and study this geological formation. Such excavations trigger a series of processes that can alter physical properties of the surrounding rock mass. These changes include modifications to the rock’s mechanical behavior, driven both by the excavation itself and by desiccation, creating an Excavation Damaged Zone (EDZ) around the underground galleries. The MTRL allows the in-situ study of EDZ developments at different stages after excavation. It allows rare access to this material and evolution, enabling groundbreaking research of the behavior of clay-rich rocks submitted to desiccation. However, the development of an EDZ is strongly dependent on the rock heterogeneities (sub-facies) and bedding orientation (Kruschwitz & Yaramanci 2004). To identify or monitor the extent of the EDZ, researchers often rely on non-intrusive approaches like the one proposed by geophysical methods. Among the several existing methods, electrical and acoustic (or seismic) methods are often chosen for their sensitivity to the rock water content and physical integrity (i.e., fracture density). Nevertheless, these methods are only indirect methods that provide proxies for these properties. Petrophysical and rock physical relationships are required to translate measurable geophysical signals into properties of interest. In our contribution, we present the results of a series of electrical and acoustic laboratory measurements conducted on several Opalinus Clay samples subjected to desiccation and compared to existing physical models to identify the most suitable one and study the importance of various parameters such as the considered sub-facies and orientation toward the bedding.
Jougnot et al. (Thu,) studied this question.