A Parametric Study Illustrating the Effects of Moderately Anisotropic Rock Strength on the Stability of Large Slopes Using Limit Equilibrium Analysis Available to Purchase

Accounting for moderately anisotropic conditions where one or more dominant structure sets affect slope stability analysis is challenging. Most analysis methods and software assume isotropic behaviour that better suits randomly jointed rock masses. Some methods for assessing anisotropic formations (bedding, schistosity, etc.) exist, but the computationally expensive Synthetic Rock Mass approach is the only method to capture moderately anisotropic behaviour. Therefore, guidance is needed to identify when a rock mass is moderately anisotropic. The limit equilibrium method software Slide2 was used to perform a parametric study on the Generalized Anisotropic strength model for a hypothetical rock slope. The key contributors to determining if a rock mass is moderately anisotropic were the shear strength (cohesion only) of the weak plane was a quarter or less of the intact strength, the angular distance between two joint sets was approximately 30°, and the angle of anisotropy was within 15° of the slope face.

When multiple structure sets influence anisotropic rock mass behaviour instead of a single plane of weakness, accounting for the combined reduction in rock mass strength can be challenging. The orientation of single or multiple geological structures (e.g., bedding, schistosity, etc.) can drastically reduce the overall rock mass strength from the intact strength. Figure 1b illustrates how the orientation of joints to the axial load (α_j) and the joint friction angle (ϕ_j) both play a significant role in whether rock mass strength is driven by the intact rock failure or joint slip (Jaeger & Cook 1979). Numerical methods have been developed to estimate rock mass strength, which can be incorporated into a slope stability analysis. However, most tools for assessing rock mass strength are best suited for randomly jointed rock masses under isotropic conditions. Some methods exist for evaluating fully anisotropic (FA) rock masses where anisotropy exists at the grain scale (foliation) and the block scale (bedding), as seen in Figure 1c. Still, when multiple dominant structure sets at the block scale are present (Figure 1d), rock mass behaviour can be classified as moderately anisotropic (MA), and slope stability can be associated with a complex failure surface instead of a single weak plane.