Shaft sinking risk analysis through the integration of borehole radar and acoustic televiewer data in deep geotechnical boreholes

Abstract

The 2.06 Ga Bushveld Igneous Complex of South Africa is the world?s largest layered igneous intrusion. Situated to the north of the Johannesburg, it extends ~450 km from east to west and ~300 km north to south (Simmat et al., 2006), and has a maximum thickness of ~8 km. It is estimated to contain over half the world?s Platinum Group Element (PGE) resources (Cawthorn, 1999). The two PGE-bearing chromitite layers in the Bushveld Complex, the Merensky Reef and the UG2, occur within pyroxenites of the Critical Zone, show high lateral continuity and commonly dip at relatively shallow angles (2°-20°) towards the centre of the complex. These are overlain by a succession of norite and gabbronorite formations of the Main Zone, representing a more mafic phase of the intrusion. The project area is the location of a planned new Anglo Platinum mine on the western limb of the Bushveld, south of the Pilanesberg Alkaline Complex, a later volcanic event. Major capital investment is required to sink the twin shafts, accessing the underground platinum reefs from ~700 m depth. Due to the proximity to the Pilanesberg, dykes and faults are pervasive throughout the deposit. Several dyke varieties occur and these differ in composition, texture and orientation, and consequently represent different geotechnical hazards. Dolerite dykes tend to be hard and competent with similar rock strength properties to that of the surrounding Bushveld rocks, and are not expected to represent significant geotechnical problems. Lamprophyre dykes, however, are often soft, weather easily when exposed and demonstrate significant contrast in rock strength properties to the Bushveld rocks. Shaft geotechnical boreholes were drilled along the two proposed shaft axes and were logged with a suite of geophysical tools to detect potentially hazardous structures that may influence shaft sinking operations. This paper examines the integration of borehole radar (BHR) and acoustic televiewer (ATV) data down the proposed main- and ventilation shaft positions.