ABSTRACT

Pillars are invariably used in the shallow hard rock tabular mines in South Africa. Crush and yield pillars, which are designed to exceed their elastic limit, as well as conventional pillars are most commonly employed. Insitu investigations have shown that crush and yield pillars in their post-peak state can provide more than adequate support resistance in shallow mining conditions.

RÉSUMÉ:

Les piliers sont utilises invariablement dans les mines peu profondes à roche tabulaire en Afrique du Sud. Les piliers conventionnels ainsi que les piliers à expansion conçus de manière à exceder leur limite elastique sont les plus employes. Les investigations in-situ ont montre que les piliers à expansion peuvent procurer un soutenement de resistance beaucoup plus adequoit dans les conditions des mines peu profondes.

ZUSAMMENFASSUNG:

Pfeilerausbau wird immer angewandt in den relativ seichtliegenden, plattenförmigen Hartgesteinsgruben in Suedafrika. Meistens werden konventionelle Pfeiler sowic Brunch- und Verformpfeiler benutzt. Brunch- und Verformpfeiler sind so konstruiert daβ ihre elastische Kapazitat ueberschritten wiid. Insitu Untersuchungen haben gezeigt, daβ Bruch- und Verformpfeiler in weniger tiefen Bergwerken auch nach Überschreiten ihrer elastischen Kapazitat noch mehr als ausreichende Stuetze bieten.

1
INTRODUCTION

In South africa, all the mines operating at depths less than 1000 m use pillars for regional and local support purposes. The compelling need for using pillars is dictated by the hanging wall conditions, which can be characterized as a rock mass containing well defined discontinuities subjected to deadweight tension. When unsupported, the hanging wall becomes susceptible to "backbreaks" if critical spans are exceeded. Only pillars of unmined ore can provide the required robust and stiff support resistance. The current design of pillar systems at shallow depths in South Africa is mostly empirical, each mine developing its pillar design methods based mainly on practical experience. There are four main types of pillars, namely non-yield, crush, yield, and barrier pillars, which can be identified as being used in relatively shallow mines (Figure 1) and it is the subject of this paper to discuss the design and application of these pillars.

2
NON- YIELD PILLARS

At very shallow depths, the tensile zone in the hanging wall of a tabular stope can extend up to surface and the design of pillars under these conditions is similar to the design of bord and pillar workings in coal mining. That is the main consideration is to ensure that the strength of the pillars at all times exceeds the maximum average pillar stress (APS), which is imposed by the cover load of superincumbent strata, by a suitable factor of safety.

3
CRUSH PILLARS

As depth increases, extraction ratios associated with coal type bord and pillar layouts become increasingly unfavourable. The increasing mining depth, however,, results in a relatively smaller portion of the hanging wall to be supported, and this in turn permits the safe use of reduced levels of support resistance: the support no longer needs to carry the full weight of overburden, but rather only the weight of superincumbent strata reaching up to the furthest active weak parting in the hanging wall. The design of crush pillar layouts is usually based on experience and, depending on the performance of the new layouts, the pillar dimensions' and spacing are adjusted until the pillars provide the required behaviour. Alternatively, the initial design w/h ratio is specified to be 2 and if pillars do not crush initially, the w/h ratio is decreased until stable crushing is achieved. The w/h ratio normally does not exceed 2,5 for crush pillars and should not be less than about 1,5 in narrow stope widths.

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