Insights into the anisotropic, hydro-mechanical behavior of Opalinus Clay through experimental and microstructural investigations

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.