Letšeng Diamond Mine (LDM) is the highest open-pit diamond mine in the world located at 3 100 masl. Two kimberlite pipes, the Main Pipe and the Satellite Pipe, are currently being excavated and are called the Main Pit and Satellite Pit respectively. Two failures of approximately 3 000 and 2 000 t of material occurred in August and September 2016 respectively in the southern portion of the Main Pit at Cut 3 West (MC3W) along a major shear zone that intersects the southern edge of both pits. An investigation was undertaken to determine the causes of failure in order to mitigate and prevent future failure occurrence. The investigation involved a detailed area inspection, geological mapping, rock mass characterisation, discontinuity survey, kinematic analysis and a back-analysis. The field inspection revealed that the shear zone is composed of sheared and friable kimberlite, calcite, pyrite and altered basalt. Rock mass classification confirmed that the shear zone is made up of a weak rock mass with low rock mass rating. It is evident from geological mapping and discontinuity survey analysis that the shear zone consists of a combination of faulting, kimberlite dykes and closely spaced sympathetic joints which are liable to toppling failure. Stereonet based kinematic analysis using Rocscience Inc. DIPS shows only one cluster, which has an average dip and dip direction of 85°/175°. The mode of failure was a combination of toppling failure on the major planes coupled with unravelling of friable material. Back-analysis using RocTopple suggests that failure may have been triggered by groundwater conditions as a result of infiltration of rain water into the relative weak rock mass.
The stability of a rock slope is governed by the local geological and geotechnical characteristics of the rock mass, the hydrogeological, geomorphological conditions, and other environmental factors such as freeze and thaw cycles (Wyllie and Mah, 2004; Verma et al., 2011). Structural geology plays a major role in the design and maintenance of an open pit mine as the presence of structural defects affects the mechanical and hydrogeological properties of the rock mass (Bye and Bell, 2001). Therefore, the assessment of geological structures forms an essential part of rock slope stability analysis and slope management.