Aspects of hydraulic-mechanical coupling in the sandy facies of Opalinus Clay — CD-A experiment in the Mont Terri rock laboratory

Since 2019, the CD-A experiment is located in the Mont Terri rock laboratory in the sandy facies of Opalinus Clay and visualized in Graebling et al. (2024). It is characterized by the twin niches and aims at understanding of coupled hydraulic-mechanical (HM) effects due to excavation and desaturation. While the Open Twin Niche is characterized by the ongoing convergence and desaturation of the host rock due to the seasonally varying climatic conditions in the niche, the Closed Twin Niche (generally color coded in red) is characterized solely by the ongoing convergence behavior, while the surrounding host rock remains almost saturated due to the constant climatic conditions in the niche. A comparison of the two niches primarily highlights two distinct influencing factors: the seasonality of moisture and the general effect of desaturation. Fundamentally, the experiment provides insights into the following key questions: What is the extent of the affected zone? Which parameters are influenced - and how? By analyzing the temporal and spatial evolution of parameters such as intrinsic permeability, water content, and mechanical deformation, the study reveals how moisture fluctuations and desaturation affect the mechanical and hydraulic behavior of the host rock. These observations enable an accurate characterization of near-field processes and support the enhancement of constitutive modeling approaches.