Many tools exist to evaluate the stability of planar or wedge type bench-scale failure mechanisms, formed by the intersection of two or more geological discontinuities and these tools often include probabilistic features to evaluate uncertainty in discontinuity strength and orientation. Failure mechanisms at the inter-ramp scale are dependent on both rock mass and discontinuity properties as blocks can span several benches in size and are, geometrically, more complex. Stability assessment of these complex blocks/mechanisms is typically conducted using more advanced numerical methods (finite element, discrete element) with specific approaches to account for the multiple components (anisotropic strengths, ubiquitous joint constitutive models, discrete features). These advanced techniques are too computationally prohibitive to investigate the mechanisms at a probabilistic level and do not permit a full sensitivity analysis of the distribution of structural and rock mass parameters. A computationally more efficient approach that couples discrete fracture network modeling, a stress-based factor of safety assessment, and a novel slip surface identification algorithm is outlined in this paper.
Open pit slope design involves stability analysis at the bench, inter-ramp, and overall slope scales. Stability at the bench scale is generally structurally controlled and tools such as SWedge (RocScience, 2019) or RocPlane (RocScience, 2019) are applied to conduct a kinematic analysis of single bench-scale blocks. At the inter-ramp (and overall slope) scale, composite mechanisms involving failure through rock mass and along discontinuities are more likely. These complex mechanisms may involve shear along multiple discontinuities, faults, and portions of varying rock mass which all contribute to the instability of the slope.
Analyses involving these more complex failure mechanisms are typically accomplished through advanced numerical techniques including continuum- and discontinuum-based approaches. In addition to the computational challenges of applying these tools in a scalable way for probabilistic evaluations, the ability to develop physically accurate representations of discontinuities such as faults, bedding, and joints in these tools is limited. Continuum approaches to modeling discontinuities include developing anisotropic strength estimates or incorporating a select number of features into the tools through the use of interfaces, isotropically weak elements, or discrete joints. Discontinuum-based techniques allow for more detailed inclusion of discontinuities in models, at the cost of computational performance, but a true physical representation of a fractured rock mass in numerical modeling software remains a challenge. The impact of uncertainty in persistence, distribution, and orientation of discontinuities on the formation of potential failure mechanisms is therefore difficult to quantify when using more advanced techniques to assess a single wedge or planar mechanism.