Directional shear strength models in 2D and 3D limit equilibrium analyses to assess the stability of anisotropic rock slopes in the Pilbara Region of Western Australia

Neil Bar and Geoffrey Weekes

The bedded iron ore and gold deposits in the Pilbara Region of Western Australia are hosted in highly anisotropic rock masses. For iron ore, these comprise strong banded iron formations discretely interbedded with very weak shales. Gold and other precious metals deposits in the same region are hosted by interbedded siltstones, sandstone and quartzites. Slope instability mechanisms generally involve sliding along bedding planes combined with joints or faults acting as release planes.

Slope stability modelling techniques applied to highly anisotropic rock masses have developed significantly over the years from basic kinematic analysis in the 1990’,s through to two-dimensional limit equilibrium analysis and numerical modelling in the 2000,s with the available software increasing in functionality and complexity over time. Limit equilibrium analysis software now offer a range of options to model the behaviour of anisotropic rock masses. The results obtained by these different models can vary significantly. It has been found that selecting either inappropriate anisotropic shear strength models for a given rock mass or using poorly calibrated models typically result in overly conservative slope designs. This paper presents case studies which illustrate the importance of geological interpretations, correct constitutive model selection, the use of non-linear shear strengths, and 2D and 3D modelling approaches.