A dissertation submitted to the Faculty of Engineering and Built Environment, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science in Engineering. 2017 / Even after years of mining, a vast gold resource remains in the Carletonville area of
the Witwatersrand basin. Mining in this area nevertheless predates 1934 and the
accessible ore-reserves on the primary reefs are being depleted. Mining of secondary
reefs and remnants has therefore become important in recent years. Of particular
interest is multi-reef mining as it will become much more prevalent in this part of the
gold mining industry. Very little rock engineering research on multi-reef mining
scenarios at great depth is available in the literature. Owing to the lack of proper
understanding and guidelines for remnant selection and extraction in a multi-reef
environment, a study was conducted by the author of this dissertation. The key
references of earlier work are discussed in the literature survey.
This study compared two displacement discontinuity boundary element codes,
MINSIM and TEXAN, to simulate multi-reef environments. The preliminary analysis
proved to be valuable and indicated that a small middling with a large overlap of
mining is required to achieve a significant effect on ERR and average pillar stress on
the remnant pillar. Care should be exercised with the so-called “45° rule” when
destressing pillars and modelling is required in all cases. The study found that there is
no significant difference between the results obtained from the two numerical
modelling codes.
To better understand the process of understoping, a simplified geometry of a pillar
being understoped was investigated. An analysis of the incremental stress evolution
in the middling between two reef horizons, as the pillar was being incrementally
understoped, revealed valuable information. A zone of very high major principal
stress and low minor principal stress develops between the two reefs. This indicates a
high risk of violent shear failure between the two reefs. The reason for successful
historic extractions of actual multi-reef remnants was therefore explored in more
detail.
As part of this analysis, an “extended” ERR concept introduced by Napier in 1991,
was investigated. It was found that bedding planes and lithology played a crucial part
in the stable dissipation of energy in these multi-reef remnant geometries. The study
indicated that the stope convergence and the various energy components are affected
by the presence, position and properties of a bedding plane. The energy solutions are
very complex and sometimes counterintuitive. Care should be exercised when
modelling specific cases. The modelling was nevertheless valuable to indicate that
energy dissipated on weak layers, such as bedding planes, may reduce the risk of
violent failure in a multi-reef mining scenario. This is an important novel contribution
of this study.
As a further step, actual multi-reef remnants were selected and back analysed to
verify the hypothesis of weak layers being useful to make multi-reef mining less
hazardous. Aspects such as seismic history, numerical modelling and photographic
records were studied. From the analysis it was found that multi-reef extraction was
successful in cases where a weak rock type was present in the immediate stratigraphy
or where pertinent bedding planes were found. This supported the hypothesis as
presented above.
An important practical outcome of this study is that much greater emphasis needs to
be placed on studying the rock mass surrounding multi-reef environments at depth to
determine the risk of extraction in these conditions. This should be added to the list of
remnant selection criteria. / MT 2018
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/25735 |
| Date | January 2017 |
| Creators | Esterhuyse, Johann Christoff |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf |
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