| Summary: | A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the Degree of Master of Science in Engineering.
Johannesburg, 2018 === Face perpendicular preconditioning practice is one of the mechanisms used in deep level gold mines to transfer stress far ahead from the face on the development ends called de-stress cuts. Although face perpendicular preconditioning practice has been implemented, rockbursts are still considered as the most problematic issue faced by deep level gold mines. This has led to minor injuries, serious injuries, machinery damage and delays in the daily production of the mine.
This research attempts to highlight the effectiveness of face-perpendicular preconditioning practices that were developed for development ends in mechanized gold mines. The investigation involves the comparison between four face-perpendicular and five face-perpendicular preconditioning holes practice at a deep level gold mine. The research also attempts to provide a clear understanding of the orientation of long and short radial fractures at the vicinity of preconditioned holes in different geological conditions. Furthermore, studies on the occurrence and orientation of fractures ahead of the mining faces were investigated. To achieve the objectives of the study, an extensive literature review was conducted on the related studies. Underground observations (visual examination), recordings and data collection were also conducted within the sections. The position of preconditioning holes, depth of the preconditioning holes, fracturing on the stope face, stope sidewalls and hanging wall and general ground conditions were observed and measured, and photographs were taken after the observations. The advance per blast, drilling rate, tamping consumption, explosives used, preconditioning holes diameter and fragmentation were measured daily and a monthly report was generated.
Microseismic monitoring, Ground Penetrating Radar (GPR) and Borehole cameras were used for monitoring the effectiveness of face-perpendicular preconditioning practice. Vantage software was used for numerical modelling, and sigma 1 and the Safety Factor were analysed from 2014 to 2015. Abaqus Explicit numerical modelling software was also used to simulate sigma 1, Rate of Energy Release (RER) and ground closure. Lastly, Examine 2D was also used to simulate stress at the vicinity of the preconditioning holes, as well as to estimate the orientation of long blasting fractures.
The results of the study indicated that the five face-perpendicular preconditioning practice is much more effective than the four face-perpendicular preconditioning practice. The results were certified by adequate fracturing on the mining face, improvement of fracturing on the vicinity of the sockets (preconditioning sockets) and a rapid decrease on the hanging wall fracturing. GPR confirmed the improvement of face fracturing and sockets fracturing, and microseismicity showed less seismic events occurring during the implementation of the five face-perpendicular preconditioning practice than the four face-perpendicular preconditioning practice. Numerical modelling results have shown a gradual decrease in sigma 1, RER and the Safety Factor ahead of the mining faces from the four face-perpendicular preconditioning practice to the five face-perpendicular preconditioning practice.
The orientation of radial fractures at the vicinity of the preconditioning holes was discussed based on two case studies. The first case study was based on the mining face which consisted of a homogenous rock type. In this case, it was noted that long radial fractures were developed along the major principal stress, while short radial fractures were developed along the intermediate principal stress. The second case study was based on the mining face which consisted of bedding planes dipping at different angles. In this case, it was noted that long radial fractures were developed along or parallel to the bedding planes and short radial fractures were developed along the major principal stress.
Lastly, the borehole periscope observation of fractures ahead of the mining faces have shown that there were zones of fractured and unfractured rock ahead of preconditioned mining faces, which had a width of approximately 1m to 1.5m and continue as discrete entities for at least 14m parallel to the face. Furthermore, analysis has shown that the zones of fracturing do not form continuously as mining progresses, but tend to be developed from time to time, leaving a solid zone which has a width of approximately 1m or more between the fractured zones. It was recommended that future research should focus on the following studies; the development and optimisation of face-perpendicular preconditioning when mining towards major geological structures such as seismic active faults, the use of long face-perpendicular preconditioning holes on de-stress cuts near the dykes and develop appropriate preconditioning practice that can be implemented when mining remnants. === MT 2018
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