Understanding time-dependent deformation in Opalinus Clay: insights and implications

Creep and consolidation are two time-dependent, yet fundamentally different deformation phenomena observed in saturated clay-rich soft rocks. Consolidation refers to the volumetric change resulting from fluid drainage, which is primarily controlled by the permeability and the elastic properties of the rock (Terzaghi 1943). In contrast, creep describes the continuous deformation that occurs even under constant effective stress conditions, driven by the rheological properties of the rock skeleton (Taylor & Merchant 1940; Dusseault & Fordham 1993). In the context of nuclear waste repositories in clay shales, both phenomena are crucial for the barrier integrity over time, from the medium-term perspective of the operation to the long-term perspectives of post-closure. Consolidation can favor ground stability around tunnel excavations by dissipation of excess pore pressure and increasing effective stresses. Meanwhile, creep can contribute to a strengthening of the rock structure through self-sealing behavior. However, both phenomena can also facilitate the weakening and destabilization of the ground. Consolidation can facilitate critical effective stress conditions due to dissipation of suction effects, while creep can cause continuous crack propagation under critical stresses, leading to failure. A comprehensive understanding and detailed characterization of these time-dependent phenomena ensures more reliable performance assessments, guaranteeing safety during construction, operation and post-closure. Especially, determining the long-term strength is essential as this threshold differentiates between stabilizing and destabilizing processes over time (Yu et al. 2023; Gotzen et al. 2025a). It is reasonable to assume that creep and consolidation occur simultaneously in clay shales (Chen et al. 2024). However, the extent to which each process contributes to tunnel convergence over different timescales remains poorly understood. Currently, numerical models are not sufficiently distinguishing between them due to a scarcity of laboratory data for calibration of individual input parameters (Leone et al. 2024). Therefore, obtaining more data to quantify creep properties is essential, requiring reliable and time-intensive laboratory tests. For this purpose, we are conducting a laboratory study examining the time-dependent thermo-hydro-mechanical coupled deformation behavior of clay shales, specifically using sample material from the shaly Opalinus Clay (OPA) sampled in the Mont Terri rock laboratory (MTRL). Our investigations focus on both consolidation and creep behavior under fully saturated conditions. The data presented here provides an overview of our latest results and key findings and emphasizes the impact of structural anisotropy on the time-dependency, as well as the missing impact from elevated temperatures up to 60 °C.