Abstract:
Rock mechanics and rock engineering are disciplines that deal with the study of rock behavior and the design of stable excavations. The design should evaluate the risks associated with excavations developed in the rock. The aim of this study was to ensure that harmful effects of rockfalls and rockbursts are minimized through designing of stable excavations. The research was conducted at a greenfield platinum project. The mining method used for this prefeasibility study is longhole stoping. The depth of operations ranges from 600m to 1100m. Geological structures present include faults, joints and shear zones. The host rock is dominantly pyroxenite which is a competent rock. Excavations that need to be supported include stopes, development tunnels, drifts and shafts. Rock mass classification methods were used in a bid to quantify the quality of the rock mass. Geotechnical data was used to propose a preliminary support design at prefeasibility stage of the project. Both empirical and numerical methods were used for stability evaluation to inform reliable support design. Prefeasibility studies of this nature are crucial as they guide financial decisions on whether or not to commit more resources for further detailed investigation.
Introduction
Prefeasibility studies normally utilize low cost technology to assess the viability of a project and hence determine if further detailed work on a project is economically justified. Amongst crucial aspects of a prefeasibility study are the investigations into stability of excavations and the accompanying support design, the anchors of safety in underground mines. This paper seeks to describe the geotechnical environment together with rock engineering requirements and design for a South African platinum mine greenfield project. The paper will give a brief geology, Rock Mass Classification methods, analysis together with support design and risk assessment of the project.
Geology
The project area is located in the northern limb of the Bushveld complex in South Africa. The orebody varies in thickness and lithology. The rocktypes present include pyroxenites, norites, chromitites, gabbronorites and harzburgites. The mining depth varies from 600m to 1100m. In the project area, low amplitude folds exist. The presence of upthrow fault and rolls weakens the ground conditions. There are also sympathetic joints present with altered orientations and dips. Two main aquifers exist in the research area, alluvial and the Rustenburg layered. Hydrological survey shows that there is less groundwater hence less effect in cohesion. The joint water reduction factor was therefore considered to be one. Various measures will be put into place in problematic areas such as regions truncated by faults.
