From process to system understanding with multi-disciplinary investigation methods: set-up and first results of the CD-A experiment (Mont Terri rock laboratory)

Abstract

Abstract. A potential repository site for high-level radioactive waste should ensure the highest possible safety level over a period of one million years. In addition to design issues, demonstrating the integrity of the barrier is essential as it ensures the long-term containment of radioactive waste. Therefore, a multi-disciplinary approach is necessary for the characterization of the surrounding rock and for the understanding of the occurring physical processes. For site selection, however, the understanding of the respective system is essential as well: Do fault zones exist in the relevant area? Are these active and relevant for interpreting system behavior? What is the role of the existing heterogeneities of the claystone and how do these site-dependent conditions influence the physical effects? To answer these questions, the site-selection procedure requires underground exploration, which includes geophysical and geological investigations on milli- to decameter scales. Their results serve as the basis for numerical modelling. This combined, multi-disciplinary interpretation requires extensive knowledge of the various methods, their capabilities, limitations, and areas of application. In the cyclic deformation (CD-A) experiment in the Mont Terri rock laboratory, the hydraulic–mechanical effects due to excavation and the climatic conditions within the rock laboratory are investigated in two niches in the Opalinus Clay. The twin niches differ mainly with regard to the relative humidity inside them, but are also characterized by different boundary conditions such as existing fault zones, the technical construction of the neighboring gallery, etc. In order to gain insights into the relevance of the individual influences, comparative studies are being carried out on both niches. The presented results provide a first insight into the initial experimental years of the CD-A long-term experiment and illustrate the benefits of multi-disciplinary investigations in terms of system understanding and the scale dependency of physical effects. Amongst other effects, the assessment of the impact of heterogeneities on the deformation behavior and the evolution of pore water pressure is very complex and benefits from geological interpretation and measurements of for example deformation, water content, and pore pressure. The numerical modeling allows statements about the interaction of different processes and thus enables an interpretation of the overall system, taking into account the knowledge gained by the multi-disciplinary investigation.

Prototype of a Virtual Experiment Information System for the Mont Terri Underground Research Laboratory

“…Besides the previously described data, another important data set in the scope of the Cyclic Deformation Experiment are the results of a coupled hydro-mechanical simulation with OGS (see Ziefle et al (2021; for details). The model consists of 33,552 nodes and 180,161 tetrahedra, covering a volume of 47 × 27.65 × 18 m 3 .…”

“…Up to now, all this data has mostly been presented in form of 2D diagrams or non-interactive 3D visualisations with a focus on single experiments and is often only usable for experts from the field with additional prior knowledge. There are, for example, investigations based on the manual extraction and processing of the measured and simulated data for a single experiment like the work by Ziefle et al (2021Ziefle et al ( , 2022 concerning the Cyclic Deformation Experiment. They present multi-disciplinary research results for one experiment within the Mont Terri URL and provide images of 3D visualisations for the measured and simulated data.…”

Modeling Desiccation Cracks in Opalinus Clay at Field Scale with the Phase-Field Approach

“…However, in the latter, the temporal and spatial scales are much larger than those in the laboratory. The Cyclic-Deformation (CD-A) experiment (82,83,85) carried out in the Rock Laboratory Mont Terri (7) in an Opalinus Clay formation in Switzerland (Fig. 1a) focuses on understanding coupled hydro-mechanical effects by comparing the response of two identical (twin) niches to the excavation and ventilation conditions.…”