Assessment of Slope Stability for Deep Pits in Sedimentary Strata

Abstract

With advances in technology increasing the economic depth limit of open-cut mines, including those in sedimentary strata, the design of stable slopes has become more critical. Increased use of geophysical logging to compliment more traditional geological and geotechnical logging provides an unprecedented detailed description of the rock mass from surface to sub-seam. Making full and best use of this data in a quantifiable, repeatable and transparent manner for the stability assessment of slopes requires a step beyond simplifying assumptions during limiting equilibrium and numerical stress analyses. By describing a repeatable deterministic model with parameterised input data we provide the basis for detailed probabilistic studies for the quantification of uncertainty. This paper considers the implication on slope stability, as measured by a Factor of Safety, of describing the strata at near geophysical log resolution. A comparison is made when representing the rock mass at increasing levels of detail with justifiable property estimates. We find that for deep pits in sedimentary rock masses a detailed description of the strata at near log resolution together with explicit representation of the near surface blast damaged zone becomes more critical as pit slope angle is optimised. This approach is demonstrated to capture the composite failure mode in sedimentary strata so that the predicted failure mode becomes a combination of low angle slip on weak units with high angle shear failure through stronger rock units as observed in the field. Compared to models with a simpler description of the rock mass, our model predicts that the mass of rock at failure becomes increasingly shallow and localised and is displaced near horizontally out of the pit slope, a failure mode observed in the field.