Time-dependent deformation behavior of Opalinus Clay: A triaxial multi-step creep study under fully drained conditions

Ensuring the long-term integrity of deep geological repositories for nuclear waste, i.e., safety and sustainability, remains a critical concern for disposal solutions. Reliable predictions on long-term rock mass behavior require a precise characterization and understanding of time-dependent phenomena such as creep and consolidation. While consolidation involves changes in effective stress, creep is characterized by continuous deformation even under minimal to zero effective stress changes. Parameters which describe the drained creep behavior under fully-saturated rock mass conditions and the long-term strength boundaries (i.e., creep failure) of low-permeable clay shales are limited mainly due to the significant amount of time required for laboratory tests. Creep mechanisms may lead to tunnel convergences and delayed failure, but may also favor self-sealing, even under stress conditions below the short-term peak strength. Consequently, a comprehensive understanding of creep mechanisms is necessary for long-term safety analyses and associated precautionary measures.

In this context, we performed hydro-mechanically coupled triaxial creep experiments using samples of the shaly Opalinus Clay obtained from the Mont Terri Underground Research Laboratory in Switzerland. These tests were conducted on fully saturated and consolidated specimens with bedding orientations parallel and perpendicular to the axial loading direction. We applied step-wise, strain-controlled increases in differential stress under drained conditions, succeeded by creep stages at constant effective stresses. Two different multi-stage stress paths were used. The procedure allows observing deformations related to creep mechanisms for different sample geometries and quantifying the influence of the stress path on the creep behavior.

The study findings reveal the occurrence of both primary and secondary creep even at low differential stresses, along with a distinct anisotropic creep behavior related to the bedding orientation. Increased differential stresses generally result in accelerated secondary creep rates, ultimately leading to creep failure below the short-term strength. The strain-rate data allow a subdivision into a stress-insensitive and a stress-sensitive creep behavior depending on the stress conditions, indicating a difference in the dominating creep mechanisms. Our study also shows that the long-term strength and the creep rates depend on the multi-step stress paths. For smaller incremental increases in differential stress, we find a decreased long-term strength and higher creep rates for the same differential stress.

Our results provide insights into the creep behavior of clay shales and yield crucial parameters for incorporating drained creep as a distinct, time-dependent phenomenon into a constitutive model for Opalinus Clay.