Borehole periscope observations of rock fracturing ahead of the preconditioned mining faces in a deep level gold mine

摘要

South Africa is one of the biggest gold producers in the world, with most of the gold mines found within the geological setting of the Witwatersrand basin. This basin hosts narrow and tabular reef deposits, which have been extensively mined for the past 100 years at shallow depths, using conventional mining techniques. Cur-rently, mining has progressed towards ultra-deep levels with the use of mechanized and conventional mining techniques. As mining depth increases, the stress levels ahead of the mining face increase rapidly, compromis-ing the stability of the tunnels and intersections of the excavations. These extreme high stresses increase the probability of face bursting. Owing to that, the face-perpendicular preconditioning blasting technique has been applied in order to redistribute the stress peak further into the solid region ahead of the mining face by elimi-nating the strain energy 'lock-ups' in the asperities of the pre-existing or mininginduced fractured. The objec-tives of the study were to identify the pattern of mining-induced fractures ahead of the preconditioned min-ing faces. To achieve the objectives of the study, borehole periscope observations and Ground Penetrating Radar were used to identify the fracture frequency ahead of the preconditioned mining faces. The results of the study have indicated that there were zones of fractured and un-fractured rock ahead of preconditioned mining faces, which were approximately 1m to 1.5m wide and continue as discrete entities for at least 14m parallel to the face and at least 7m to 8m vertically. Furthermore, analysis has shown that the zones of fracture do not form continuously as mining progresses. There is usually a solid zone of approximately 1m or more be-tween the fractured zones. The work presented in this paper is part of a Master of Science in Mining Engineer-ing by dissertation study at the University of the Witwatersrand, Johannesburg, and School of Mining Engi-neering, conducted by the first author.