Geotechnical characterisation and design processes for anisotropic rock masses in open pits : a case study of an iron ore deposit in orientation biased anisotropic host rocks

Abstract

Slope design and mining in foliated in-situ rock masses can be a major logistical and geotechnical hindrance. The Rock Mass Classification parameters need to be considered in relation to the anisotropic rock masses. What data acquisition methods should be considered, how much geotechnical data is needed and how should the data be represented so have to produce an optimum design which considers all the classification parameters? Data collected in foliated rock masses needs to be characterized along with specific laboratory rock testing to give an accurate indication of the extent and effect the anisotropy will exhibit on the rock mass. Geotechnical logging parameters gathered on site are dependent on the classification system to be applied for the geotechnical design requirements. Rock Mass Rating system (RMR), Mining Rock Mass Rating system (MRMR) as well as Geological Strength Index (GSI) are used, however the addition of the In-situ Rock Mass Rating system (IRMR) and the Japanese Geological Society Engineering Classification System (JGS) can allow for a downgrading of the rating value to account for the presence of foliation or other anisotropy in the rock masses. The geotechnical investigation done on this project area in Mozambique undertakes defining these additional data parameters required for adjusting rating systems for anisotropy as well as a holistic pit slope design consisting of geotechnical models such as geological model, structural model, rock mass model (material properties and classifications), kinematic, Swedge and limit equilibrium analyses. Limit equilibrium modeling of isotropic models inherently shows overall slopes being more stable than those of anisotropic models where the failure of the slope occurs due to the orientation (dip and drip direction) of the exposed rock material fabric on the slope in relation to the direction and angle of the overall slope. Application of anisotropic slope model designs would produce worst-case scenario outcomes, which if used to generate final slope geometries would result in more conservative slope parameters being applied in the slope design.