4.6 Strength anisotropy

Most rocks have anisotropic strength properties, thus, strength depends on the loading direction. The plot in Fig. 4.18 shows anisotropy of shear strength. Consider a rock with well defined planes of weakness in one particular direction. These plane of weakness may be constituted by fractures, weakly bonded layers, or weak rock layers. For example, there are some rocks with mica foliation that have almost no shear strength whatsoever in the foliation planes. The plots on the right of Fig. 4.18 show that the shear strength depends on the loading orientation. The sample is the weakest when the orientation of weak planes coincides with the plane that meets the shear failure line, that is $\pi/4 + \varphi/2$. The sample is the strongest when the orientation of weak planes is perpendicular to the expected shear failure plane. The videos in this playlist https://www.youtube.com/playlist?list=PLv0npDbE5HXvEdptgajRDG3x-lwmtGDbr show how bedding interfaces affect failures processes under deviatoric stress. A similar phenomenon applies to tensile strength. Planes of weakness can greatly reduce tensile strength for stresses applied in direction perpendicular to those planes of weakness.

Figure 4.18: Shear strength anisotropy example in which bedding planes are weaker than the rock layers. [add real data of shales]