Benefits and limitations of applying directional shear strengths in 2D and 3D limit equilibrium models to predict slope stability in highly anisotropic rock masses

The bedded iron ore deposits in Western Australia are hosted by highly anisotropic rock masses that typically comprise strong banded iron formation discretely interbedded with very weak shales. Slope instability mechanisms within these bedded units generally involve sliding along bedding planes combined with joints or faults acting as release surfaces. Slope stability modelling techniques have significantly developed over the years from basic kinematic analysis in the 1990s to complex two-dimensional limit equilibrium analysis and numerical modelling in the 2000s. Limit equilibrium analysis software now offers the possibility of modelling the behaviour of anisotropic rock masses in either 2D or 3D. The results obtained by these different modelling methods can vary significantly. Whilst the choice of either a 2D or 3D modelling code will generally be dictated by the geometry of the situation, it has been found that selecting either inappropriate anisotropic shear strength models for a given rock mass or using poorly calibrated models can result in overly conservative slope designs, regardless of the modelling code used. This paper presents case studies which illustrate the importance of geological interpretations and correct constitutive model selection.