The Anisotropic Behavior of a Clay Shale: Strength, Hydro-Mechanical Couplings and Failure Processes

Many rocks exhibit a structural composition, which leads to an anisotropic behavior of different properties. A proper understanding of the directional dependency of these properties is required to analyze and predict the failure behavior of the rock mass upon stress changes during many geo-engineering applications. This study investigates the selected host rock for nuclear waste disposal in Switzerland, Opalinus Clay, for its anisotropic unconfined compressive and tensile strength, poromechanical response, and effective shear strength in an extensive laboratory testing campaign. The results show the lowest unconfined compressive strength at angles of 30 degrees-45 degrees between the bedding plane and the compressive load direction, whereby the lowest tensile strength is found to be normal to the bedding orientation. Triaxial consolidated-undrained compression tests reveal an anisotropic poromechanical behavior as well as peak and residual effective strength values, which are largely controlled by the orientation of the bedding plane with respect to the maximum principal stress. The magnitude of excess pore water pressures and dilation are both functions of loading configuration. The comparison of peak strength values for different loading angles indicates that the lowest effective shear strength can be expected at a loading configuration of approximately 45 degrees between bedding orientation and the loading axis. The variation in the hydro-mechanical response is associated with the microstructure controlling the poroelasticity and the failure processes. The results provide a deeper understanding of failure in anisotropic rocks contributing to the development of constitutive models for predicting the rock mass response.

Automated summary

This study investigates the anisotropic behavior of Opalinus Clay, a selected host rock for nuclear waste disposal in Switzerland, by examining its unconfined compressive and tensile strength, poromechanical response, and effective shear strength. The results reveal that the rock exhibits a directional dependency in its strength properties, with the lowest values observed at specific angles between the bedding orientation and the load direction. The study also shows that the orientation of the bedding plane with respect to the maximum principal stress significantly influences the poromechanical behavior and the effective strength of the rock.