For analyzing the influence of structural anisotropy on the hydro-mechanical behavior of a clay shale, we performed three consolidated, undrained triaxial compression tests with different geometric specimen configurations. Opalinus Clay specimens were tested with bedding plane orientations of 30°, 60°, and 90° with respect to the horizontal. Results indicated different peak strengths at failure with highest and lowest values for the 90° and 30°-specimens, respectively. Failure occurred at different mean effective stresses with different magnitudes of pore water pressure built up. The 30°-specimen showed a decreasing effective mean stress up to and beyond failure compared to the initial effective consolidation stress of 10 MPa, while the 90°-specimen increased in effective mean stress during undrained loading. Dilation was found to be highest in the 30°-specimen and lowest in the 60°-specimen, demonstrated by both the effective stress path and the post-experimental microstructural analysis of the shear zones. The macroscopic shear band formed parallel to the bedding plane orientation for the specimen loaded in 60°-orientation. Here, only minor microstructural fabric changes such as increased porosity or deformed grain structures were observed, which verifies the minor volume changes inferred from the effective stress path.
Anisotropic rocks play an important role for engineering applications in the subsurface. Transversal isotropy is commonly associated with layered sedimentary or metamorphic rock such as sand- and siltstones, mud- or clay rocks, shales, slates and schists. Clay-rich rocks are currently investigated for their application as geological barriers in nuclear waste repositories in many countries (e.g. Belgium, Canada, France, Germany, Japan, Switzerland, United Kingdom and the United States). The hydro-mechanical response around a repository tunnel during and after the excavation is influenced by the anisotropy of the rock. The anisotropy ratio, i.e., the ratio between rock properties parallel and normal to the plane of transversal isotropy, has been investigated in a variety of experimental studies (e.g. Wild and Amann, 2018; Minardi et al., 2021). However, these two endmember orientations, i.e., 0° and 90° between the loading direction and the plane of anisotropy, cover only a minor portion of geometrical constellations around the full tunnel circumference (Fig. 1.). For the majority of geometric constellations the plane of structural anisotropy, i.e., the bedding or foliation, is oblique to the tangential stress orientation. Favorable boundary conditions for a nuclear waste repository include tectonically-inactive sites, where the bedding is oriented (sub-)horizontally and the major principle stresses are oriented vertically and horizontally. Fig. 1. shows the anticipated geometric constellation in a sub-horizontal layered clay shale at large depth. Although stress rotation may take place during excavation, this simplified sketch suggests that the two endmember constellations, where maximum load is oriented parallel (P-configuration) and perpendicular (S-configuration) to the plane of anisotropy, are limited at the roof/top and the side walls of the tunnel, respectively. All other constellations represent an oblique orientation of the tangential stress in respect to the bedding plane orientation (further referred to as Z-configuration).