DEVELOPING VIABLE TECHNOLOGY FOR ULTRA-DEEP LEVEL MINING IN SOUTH AFRICA

Text of article published in Mining Magazine, May 1999 issue.
Southern Africa Focus, pages RSA4-RSA11

The DEEPMINE Programme was launched in July 1998, with total funding over a four year period of R70 million. It aims to create a technological and human resources platform which will make it possible to mine gold safely and profitably at depths of 3000 to 5000 m. It has been estimated that the gold resource which resides at these depths (perhaps as much as 40 000 tonnes) is similar to the amount recovered date from the reefs of the Witwatersrand basin during the last century. The Programme has four key objectives:

	To acquire knowledge of and develop appropriate technology for new ultra-deep level mines
	To stimulate education and training
	To establish a culture of innovation
	To encourage rapid technology transfer and implementation

This unique research programme was first conceived by CSIR Miningtek and the University of Witwatersrand. The great technical and human challenges of mining at ultra-depth has led to unprecedented co-operation between mining companies, research institutions, government, labour and universities. While DEEPMINE's focus is on mining at ultra-depth, it is anticipated that there will be many spin-offs which will benefit mining at current depths.

One of the most important advantages of the way this project is managed, according to Dave Diering, chairman of the Technical Management Committee and head of mining engineering at AATS, is that its direction is being set by the needs of the industry, and not dictated by the researchers. The mining companies involved are Anglogold, Durban Roodepoort Deep Limited and Gold Fields Limited.

The scope of the programme is summarized by the DEEPMINE Technology 'Wheel', shown here. This divides deep mining technology into 15 separate elements, arranged in three tiers - those which are critical to ultra-deep mining, those required to access ultra-deep resources, and those that are not specific to mining or access, but are nonetheless critical to efficient ultra-deep mines. Within each element, the current situation will be one of three. In the best case, technology exists and requires ongoing monitoring and refinement. Otherwise, applied research or development is required. Finally, in the third situation, fundamental research is needed. Using the technology wheel as a framework, 145 research tasks have been defined.

The main objective of the first year of the Programme is the rigorous assessment of the capability of the industry to mine at ultra-depth. Exhaustive reviews have been carried out to identify the "state of art" with regard to technology and best practice, both in the SA industry and world-wide. A great deal of effort has been devoted to the extrapolation knowledge of rock mass behaviour from depths where mining is currently practised to ultra-depth. Once critical knowledge and technology gaps are identified, work will start to fill the gaps. DEEPMINE places great emphasis on an integrated understanding the entire mining system. For example, it is no good having a layout which is wonderful from a rock engineering point of view, if it cannot be adequately ventilated or cleaned.

Forty-five tasks involving about 200 researchers were tackled during the first year of the programme. The tasks mostly focused on the first two tiers of the technology wheel (i.e. issues critical to mining and gaining access to the orebody at ultra-depth). The major obstacles to mining at ultra-depth are believed to be heat and seismicity, and many tasks addressed these issues. The research tasks conducted during the first year are briefly reviewed below.

Technology Element 1: Occupational Health & Safety

It is recognised that internationally accepted standards of safety and health are essential for mining at ultra-depth. The objective of this project is to anticipate, recognise and evaluate those hazards which could compromise the health, well-being and safety of employees and to provide solutions to these hazards.

The first task investigates the physiological effects of increased barometric pressure on exposure to airborne pollutants. Appropriate threshold limit values have been determined for gaseous pollutants at 5000 m, as well as the permissible safe limit for methane. The second task investigates the influence of heat stress on human performance and cognitive ability, particularly the effect of heat stress on the ability to be alert to hazards. The design of the protocol for controlled experimentation is in progress. The last task investigates the impact of ultra-deep mining on the occupational culture of workers, employing both original ethnographic research to provide a detailed description and analysis of daily working life of miners, and focus group research to identify coping mechanisms of miners. As an orientation exercise, four researchers spent two weeks living in hostels at Elandsrand Gold mine, and going underground daily with the stope teams to the working places. The next phase will involve participant-observer research in the Upper and Lower Carbon Leader stopes at Western Deep Levels gold mine (i.e. less than 3000 m and greater than 3000 m below surface, respectively).

Technology Element 2: Delineation and characterization of geological structures ahead of mining

A foreknowledge of geological structures is considered to be crucial for safe and productive mining at ultra-depth. The objective of this element is to develop routine tools and techniques to prevent a situation arising where geological structures are encountered unexpectedly or have an unexpected effect on mining.

It is envisaged that the reef will be imaged by geophysical tools placed in boreholes. The first task investigates the drilling and behaviour of boreholes. The experience of contractors drilling holes in areas that are highly stressed as a result of mining (equivalent to depths of 3 to 5 km) has been captured, and the deformation of these boreholes monitored. The second and third tasks seek to integrate and optimise seismic and electromagnetic techniques. Information on the physical properties of the reef horizon and surrounding strata have been collected. Initial field experiments have been conducted, with promising results obtained using borehole radar to image the Vaal and Basal Reefs. The last task involves the development of an integrated geological and geotechnical database for deep mining areas. A needs analysis has been conducted in order to identify the kind and format of data and the appropriate hard/software, and a combined geophysical, geological and rock mechanical working group for the exchange of existing and newly generated data has been initiated.

Technology Element 3: Mining layouts & methods

This project seeks to establish criteria for optimal mining layouts and to develop appropriate in-stope processes for each geotechnical area expected at ultra-depths, taking into consideration rock engineering, resource productivity and environmental criteria.

The first task involves the establishment and quantification of the critical rock engineering criteria for stoping. Four categories of critical parameters have bee identified and quantified: exploration (rock mass information), mining (extraction ratio, average pillar stress, angle between face and discontinuity, fault negotiation), support (energy release rate, backfill, peak particle velocity), and monitoring (seismicity). It was noted that some criteria, such as ERR and peak particle velocity, are ill-defined. In the second task, the critical non-rock engineering systems criteria are established. Three major systems have been identified: environmental, mining/rock breaking, and transport of men, materials and rock. The outputs of the first and second tasks are integrated in the third task which evaluates specific mine design alternatives. Decision support software to evaluate a complex multi-variable system has been used to create a prototype package for evaluating different layouts using the criteria. Work has also commenced on simulations of three mine-layouts (longwall mining with strike stabilising pillars, sequential grid mining, and down-dip mining with closely-spaced narrow dip pillars) to derive realistic cost-sensitivity analyses. The final task investigates whether or not in-stope process technologies can meet the specified systems criteria within the appropriate mine design.

Technology Element 4: Stope Support For Ultra-Deep Level Mining

The objective of this project is to design and develop cost-effective and user-friendly stope support systems which will enable safe and economic mining at ultra-depth depth under static and dynamic loading conditions, whilst minimizing material transportation requirements. The stope support system must integrate into the appropriate mining method and regional support system within each geotechnical area.

The first task attempts to predict the static and dynamic rock mass behaviour at ultra-depth, and its impact on the behaviour of stope support. No new rock types are expected at ultra-depth, though the thicknesses of the various strata will change, and the orebody becomes increasingly quartzitic. The joint and fault pattern will be similar at ultra-depth to areas of current mining. Although the intensity of fracturing will increase significantly with depth, it is expected that the fracture envelop will remain much the same. Convergence is expected to be greater, although the rate of closure will probably be mush the same as at present. The second task then establishes quantitative rock engineering criteria for effective stope support at ultra-depths. It is predicted that areal coverage will become increasingly important as depth increases. The third task then assesses the ability of existing deep-level rockburst resistant support to meet these criteria. Gullies are recognised as particularly vulnerable areas, hence a task is devoted to the evaluation of alternatives to conventional gully pack support for an ultra-deep mining environment. It has been found that there is an abundance of suitable supports for gully edges. Present problems with gullies are due to the absence of lines, poor blasting, unstable foundations, incorrect rigging of scrapers, blasting damage, and excessive stiffness of some packs. It has been concluded that solutions are presently available for gully support at ultra-depth.

Backfill is a currently widely used to improve stability, and there are four tasks looking at its implementation at ultra-depth. One task seeks to establish the inter-relationship between backfill and face area support units in terms of backfill to face distances, a second task involves a technical and economic evaluation of backfill in current deep-level mining, while a third task evaluates the current backfill compositions and systems in terms of their ability to meet the defined rock engineering criteria.
It has been found that aggregates/tails offer high potential for good performance at ultra-depth in terms of particle-size distribution and porosity. Comminuted waste is only marginally less effective. Classified tailings need only be used in specific areas based on rock engineering criteria. Indications are that comminuted waste provides the most attractive option for ultra-depth. Lastly, there is a task looking at the utilization of run-of-mine waste for backfill. It has been found that the break-even cost for underground utilisation (rather than waste hoisting) is favourable at ultra-depth mining, but varies according to the underground application.

Technology Element 5: Seismic Management

Rockbursts are one of the most serious obstacles to mining at ultra-depth. The objective of this project is to acquire the necessary understanding of seismicity, and develop techniques to manage seismicity, so that rockbursts do not prevent productivity and safety objectives from being met.

Research is being carried out to investigate the relationship between seismicity and depth, mining rate, and mining method (conventional drill and blast, and non-explosive methods such as the impact ripper, diamond saw). Preliminary analysis has been carried out on data from several mines. A fourth task seeks to integrate of seismic monitoring and numerical modelling. Three sites are being used for detailed back analysis. Finally, the "state of the art" with respect to seismic prediciton and hazard assessment is being reviewed. It has been concluded that seismic events cannot, with current technology, be accurately and reliably predicted, although predictions are 2-3 times better than random. It was found that hazard assessment is applied on times scales of hours to months, and do make deep-level mines safer. It was not possible to determine whether or not certain mining methods or layouts are more amenable to prediction than others.

Technology Element 6: Refrigeration and ventilation for ultra-deep level mining

High rock temperature is one of the most serious obstacles to mining at ultra-depth. The objective of this project is to identify, evaluate and develop those technologies and systems which will enable cost-effective ultra-deep mining to take place in acceptable environmental conditions. The emphasis of these technologies and systems should be on effective ventilation and cooling at worker locations.

The first task seeks to determine appropriate criteria for acceptable environmental conditions. It has been found that the design parameters pertaining to occupational health do not differ drastically from those used at current mining depths. The second task reviews the applicability and limitations of existing environmental design software. About 20 different packages have been identified, and evaluation is underway. It has been found that no single set of software utilities deals comprehensively with all the anticipated environmental control issues at great depth. It has already been recognised that there is a need to develop simulation software for the prediction of dynamic heat loads. Work has commenced on the derivation and validation of mathematical models. Empirical models may prove necessary if analytical models are found to be inadequate or too complex.

Losses of coolth has serious cost implications. It has been found that there is the potential for a 50% reduction in heat load if the airway is fully insulated, falling to 30% if partially insulated, to only 15% reduction with a wet or damp footwall. A smooth finish can reduce flow resistance significantly. One task seeks to develop viable airway insulation products Another key task involves the a performance and cost analysis of the cooling generation systems in all deep gold mines. Allied to this work is a task evaluating the efficiency of chilled water distribution systems and air coolers. The quantification of dynamic losses by dams, including the conduction of heat from permanently submerged rock, conduction from tidally submerged rock, heat and mass transfer from air, and thermal radiation from exposed rock surfaces, has been completed. It was found that the long term component is driven by the virgin rock temperature, while the short term transient component is due to varying water levels. A final task is evaluating appropriate in-stope air cooling appliances. Several prototypes have been tested.

Technology Element 7: Access development and support

The objective of this project is to evaluate and develop techniques for safe, cost-effective and rapid access to ore bodies, such that these access-ways are conducive to the rapid transport of men, materials and rock and remain functional for their required life.

The understanding of the fracturing and associated damage around circular openings in highly stressed rock at ultra-depths is fundamental. Underground observations have been made of excavations (tunnels, shafts and ore passes) in highly stressed areas and in squeezing rock conditions. Laboratory tests and numerical modeling of the relationship between the stress and fracture zone, and the stability of break-out zones, have also been carried out. A second task involves the cost sensitivity analysis of different methods and rates of access development. Coupled with this task is an evaluation of rapid access technologies, as well as technologies for the removal of broken rock from the face in both flat and inclined development ends.

The support of tunnels in highly stressed rock is an important issue. The stress history of tunnels in different mining layouts have been numerically evaluated. The depth of tunnel rock mass instability has been empirically determined, and projected to ultra-depth. Preliminary findings are that greater yieldability of support systems (rock bolts and fabric) will be required at ultra-depth. An increased shear capacity of reinforcement will be required in areas of large stress reduction. It is thought likely that shotcrete will form part of the support system, hence a task is investigating the supply and distribution of materials for full coverage shotcrete. Preliminary findings indicate that wetcrete offers more scope for optimization, both with regard to transportability and quality control. For deep mining, underground comminution, batching and distribution to multi-end developments appears to become more and more viable, and these aspects need more research.

Technology Element 12: Transport of men, material and rock

The objective of this project is to identify and develop those technologies which will ensure the rapid and safe transport of men and the efficient transport of materials and rock between surface and the working place.

The suitability of existing rock handling systems (rail, conveyors, mono-rails, self-steered wheeled vehicles, endless rope haulages, pumping systems etc) for ultra-deep mines is being reviewed. A second task is evaluating current practices in the design, support and systematic maintenance of shaft rock passes. It was concluded that the ideal rock pass at ultra depth should be in competent rock, have a suitable ratio of size rockpass:material, be raise bored, inclined between 60ÅŸ and 70ÅŸ, kept full, and regularly monitored and maintained. Finally, the development of a practical and cost-effective system for the lateral hydraulic transport of rock is being investigated.

Technology Element 13: Energy systems

The objective of this project is, through technological innovation, to avoid incremental power costs so that total power costs for ultra-deep mines are lower than current deep mine costs. Two tasks have recently commenced: the first seeks to establish the impact of hydro-power on ultra-deep mine ventilation and refrigeration requirements, and the second involves a technology audit of energy recovery devices to discharge to a back pressure.

The findings of the first year of research are now being rigorously evaluated by the Technical Management Committee and industry experts. The DEEPMINE sponsors have confirmed their commitment to the second year of the Programme, and additional sources of funding are being explored. Work will continue on many of the tasks started in 1998. About twelve new tasks are scheduled to commence in 1999, mainly dealing with the transport of men, materials and rock. Funds have been set aside to facilitate the involvement of universities and technikons in the Programme. The DEEPMINE Programme is now well set to provide the technology and expertise required to unlock the gold resources residing at ultra-depth within the Witwatersrand Basin.

http://deepmine.csir.co.za/Press%20Clips/clip01.htm

---