Geotechnical assessment of a kimberlite pipe in Greenstone belt granites

Abstract

The potentially hazardous nature of open pit mining requires the application of sound geotechnical engineering practice to mine design, for the purpose of permitting safe and economic mining of any commodity within any rock mass. The Lerala Diamond Project is situated in the south west of Botswana near the Martin’s Drift Border Post. A 2m-soil cover made surface mapping of geological features impossible, so a number of geotechnical holes were drilled to evaluate the characteristics of the kimberlite pipes and the Granite/gneiss host rock. The Lerala Diamond Project is a typical example of the geotechnical assessment of a kimberlite pipe in Greenstone belt granites. The explosive nature of the formation of these pipes was seen in the various types of joint and fracture pattern identified during this study that could have an influence on the stability of the open pit. Estimating the stability of rock slopes is required by the mining engineering industry for a wide variety of projects. Of importance in this regard is the preliminary evaluation of slope stability at the feasibility stage, excavation stage, and operating stage. The Lerala Diamond Project is currently undertaking a preliminary evaluation as part of a feasibility study. The aim of the geotechnical assessment was to divide the local rock into easily identifiable types that could be geotechnically evaluated. Two classification systems were used during the quantification of the rock mass types. These are the Rock Mass Rating (RMR) system of Bieniawski (1976) and the Mining Rock Mass Rating (MRMR) system of Laubscher (1990). Observations and recordings of the drill core were carried out and these, in conjunction with laboratory results, enabled the determining of the characteristics of the rock mass that will be exposed in the slopes. Computer modelling programmes such as ROCKPAK III were used to test the designs against potential failures. The various potential failures were identified for the different highwalls. Recommendations including the continuous logging of geotechnical features were proposed for the purpose of developing a sound geotechnical model for identifying potential unstable areas within the pit.