The Integration of Borehole Geophysical logs for Geotechnical Risk Assessment at the Paardekraal 13-Level Ventilation Shaft Project

Downhole wireline logging and borehole radar surveys were undertaken at Paardekraal in two shaft geotechnical boreholes for the planning and design of ventilation shafts for underground platinum mining. Paardekraal is one of production areas of Anglo Platinum’s Rustenburg Section, Rustenburg Platinum Mines Limited, on the Western lobe of the Bushveld Complex. The horizons mined for platinum group metals are the Merensky Reef (MR) and the UG2. Old MR support pillars are targeted to provide stable ground conditions for a vertical shaft to access to the UG2 horizon, which lies stratigraphically below the mined out MR. Studies show that most structures (jointing) in the Bushveld Complex are steep-dipping and could pose a threat to mining operations even over short distances. Mapping of structures intersecting the borehole, their location and their orientation is used to assist in highlighting zones of potential hazard and to characterize rock formations. Key geophysical tools were deployed in the Paardekraal geotechnical boreholes to complement standard drill core geotechnical analyses. The acoustic televiewer records high resolution (2 mm pixel), oriented images of the full circumference of the borehole sidewall and maps in detail the location and orientation of structural features intersecting the borehole. Density and sonic velocity logs provide information on rock strength and competence. Flowmeter, differential temperature, neutron and fluid conductivity logs identify zones of possible ground water ingress (fluid pathways). A method for integrating the structural and geotechnical information, interpreted from the geophysical logs, into a single, visual log that highlights hazardous geotechnical zones was applied. This so-called Hazard Index (HI) is a weighted combination of the following parameters, namely: intact rock strength (IRS), shear wave slowness (SWAV), fracture frequency (FRAC), joint intersection rating (INTS) (based on fracture tilt combination), water ingress (FLOW) and fracture density projected within the shaft barrel (FDEN). Borehole radar in reflection mode penetrates the formations around the borehole to distances of up to 50 m (and sometimes further). This gives information about the condition of the rock mass surrounding the borehole and the lateral extension and continuity of major structural features and dykes. Critically, the radar data was analysed for information on the location of old MR workings with respect to the proposed position of the shaft excavations and the integrity of the MR pillar surrounding the shafts.