10/04/2015
CHALLENGES OF HARD ROCK MINING
Mitigating the risk of rockbursts in the deep hard rock mines of South Africa:
South Africa hosts two of the world’s richest ore bodies, the gold-bearing conglomerates of the Witwatersrand Basin and the platinum-bearing pyroxenites of the Bushveld Complex. Both ore bodies extend to depths of several kilometers. Gold was discovered near present-day Johannesburg in 1886. Mining-induced seismicity and its hazardous manifestation, rockbursting, were first encountered in the early 1900s when extensive stopes reached depths of several hundred meters.
It has been observed that to mitigate the rockburst risk has focused on three main areas which are enumerated hereunder:
(i) development of macro-layouts (e.g. sequential grid) and regional support (e.g. backfilling) to control the release of seismic energy through the geometry and sequence of mining.
(ii) development of support units and systems (e.g. rapid-yielding hydraulic props, prestressed elongates) to limit rockburst damage and
(iii) mine seismology, which seeks to develop techniques to continually assess the seismic hazard and control seismic activity (e.g. through the rate of mining).
Implementation of these technologies, together with improvements in training, work organization and regulation, have reduced fatality rates and made it possible to mine successfully.
Risks posed are:
Mining-induced seismicity and its hazardous manifestation, rockbursts, were first encountered in the early 1900s when extensive stopes, supported solely by small reef pillars, reached depths of several hundred meters. The dipping conglomerate “reefs” were found to persist to depths of several kilometers. Rockbursts have remained one of the most serious and least understood problems facing deep mining operations, claiming the lives of thousands of mine workers. Despite many technical advances, rockbursts continue to pose a significant risk which includes:
• Tunnel in a deep South African gold mine damaged by an ML=3.6 tremor
• Apartment block in Welkom destroyed by an ML=5.2 tremor
• Buildings in Stilfontein damaged by a ML=5.3 tremor
• Anatomy of a typical deep Witwatersrand Basin gold mine
• Delay-time analogue computer used to find focus of seismic events
• Plan (1000 ft grid) of F longwall East and G longwall West, ERPM, and foci of 445 seismic events.
• Dip section of F longwall East, ERPM, and foci of located seismic events
Though the main responsibilities are focussing on three main disciplines which endeavour the following:
• Mine layout: aimed at minimizing the effect of rock pressure at the design stage.
• Support units and systems, aimed at reducing falls of ground and the extent of rockburst damage.
• Rockburst control, which was concerned with developing instruments to monitor seismicity and engineering techniques to control the rockburst risk.
For more comprehensive views, one can envisage as:
• Basic scientific research: This may be conducted to determine the properties and behavior of the rock mass and the engineering materials that are used to stabilize and support it.
• Rock properties and rock mass behavior: This involves field and laboratory investigations and numerical simulations. Advanced computer codes, such as WAVE (simulation of wave propagation and the elastodynamic interactions between faults and stopes) and DIGS (simulation of large-scale assemblies of interacting cracks) were developed and used to improve the fundamental understanding of rock failure mechanisms.
• Mine seismology: Techniques were developed to analyze seismograms and seismicity patterns.
• Engineering research: This are required to be conducted to produce practical methodologies and technologies to improve safety.
• Layouts and design criteria: These are a major change in layout philosophy in deep gold mines.
• Support components and systems: Many new components and systems are to be evaluated.
• Mining methods: Preconditioning was field-tested. It is required to be demonstrated to reduce the hazard of face-bursting and was widely implemented.
• Transfer of knowledge and technology.
• Semi-controlled earthquake-generation experiments.
• Knowledge dissemination.
• Short-term hazard assessments are also required to be issued daily.
• Human Capital Considerations: Perhaps the greatest concern now facing the gold industry is the degree and rapidity of shrinkage in personnel. The ongoing implications of this include increased expenditure on medical insurance and disability cover and higher indirect labour costs through reduced productivity, higher absenteeism, and the need to train and replace labour.
• Policy Considerations: In addition to financial, human and geological challenges, the South African gold mining industry has also been required to embrace various pieces of national legislation, designed to address the inequalities of South Africa’s apartheid past. Labour issues are also main attributes in mining industry.
• The Beneficiation Debate: In South Africa, the concept of beneficiation as technically defined by the Department of Minerals and Energy refers to the various processes that involve upgrading, improving, processing or treating a primary ore by the removal or separation of impurities from the economic minerals.
• In South Africa, there is a widely accepted need to redress the past destructions and distortions to individual career paths and education, and to widen access to opportunities both in terms of employment and the country’s inherent mineral wealth and entrepreneurial prospects.
• Another serious problem is that of heat. In the deeper mines refrigeration of the intake air is often necessary to keep conditions tolerable and this is now becoming necessary on some platinum mines which, although shallower, have a higher geothermal gradient. The South African mining industry is frequently criticized for its poor safety record and high number of fatalities. The reason for the difference is quite clear; the gold mines are much deeper and conditions are both more difficult and dangerous than on the shallower platinum mines.
• Falls of ground dominated the causes at machinery, transportation and mining accidents caused and the remainder were classed as general. Of the falls of ground, approximately two thirds were on the deep gold mines, a reflection of the extreme pressure at depth and continual movement to country rock. Amongst the machinery, mining and transportation fatalities were working on grizzlies without safety belts, working below loose rock in ore passes, getting crushed by that deadly combination of a loco and a ventilation door frame (the clearance between the two is only a few inches) and working on running conveyors, all direct contraventions of safety instructions. Drilling into misfires was also mentioned, a clear example of sloppy and unsafe mining.
• It is difficult to see how falls of ground can be eliminated given their frequent unpredictability, which is increasing with depth and the difficulties in providing continuous roof support as on longwall coal mines due to the violence of a face blast in the hard rock of the gold mines but clearly much can be done to improve training and to instill a sense of safe working practice in the miners, many of whom are relatively inexperienced.
