Deformational behaviour of a clay-rich shale with variable water saturation under true triaxial stress conditions

This study has investigated the impact of true triaxial deformation and variable water saturation on the static mechanical properties of Pierre Shale. The impact of increasing the stress anisotropy (the difference between the intermediate principal stress, σ2, and the minimum principal stress, σ3) during axial loading was systematically investigated. Low to moderate stress anisotropy resulted in increasing strength and stiffness at any given level of σ3, which then peaked or decreased at the highest stress anisotropy. Systematic decreases in water saturation were also noted to increase strength and stiffness. The increased strength resulting from variations of both these parameters changed the macroscopic fracture patterns observed in the shales. Microscopically, however, dilatancy was suppressed by increasing the intermediate principal stress, often to levels in excess of 90% of peak strength. The experimental results were interpreted in the framework of Wiebols and Cook models as well as Mogi type models for true triaxial behaviour of rock materials and were found to have good fits to these models for all levels of water saturation.