Leaving shaft pillars, in the deep-level gold mines of South Africa, is becoming prohibitive from both economic and mining points of view. The alternative option is to extract the orebody early in the life of the shaft and limit the ensuing damage by controlling the strata displacement. On the basis of some simple models, the paper describes the mechanisms of strata displacement along a shaft and the damage that may result from the early extraction of an orebody. The same method of modelling is utilised in generating numerical data to enable a comparison of measured and computed strata displacements brought about by an actual extraction.


Laissant les pilliers des puits, dans les mines d'or à grande profondeur en Afrique du Sud, devient inabordable au point de vue des aspects financiers et des operations minieres. L'option alternative est d'extraire la masse minerale tôt dans la vie d'un puits et de limiter les degats resultants en reglant le deplacement des couches. Sur le fondement de quelques modèles simples, la communication decrit les mecanismes de deplacement des couches le long d'un puits et les degats qui peuvent être provoques par l'extraction hative d'une masse minerale. La même methode de modelage est utilisee pour produire les donnees numeriques pour faciliter une comparaison des deplacements mesures et computes des couches provoques par une veritable extraction.


In den tiefen Goldminen in Suedafrika wird es sowohl vom wirtschaftlichen, als auch vom bergbaulichen Standpunkt aus zu aufwendig, die Schachtpfeiler stehenzulassen. Die Alternative ware, das Erz schon fruh in der Lebensdauer des Schachtes abzubauen, und Folgeschaden durch Kontrollierung der Gesteinsschichtverschiebungen zu begrenzen. Dieser Bericht beschreibt anhand einfacher Modelle die Mechanismen von Verschiebungen in Gesteinsschichten langs des Schachtes und den Schaden, den frueher Abbau des Erzes zur Folge haben kann. Die gleiche Modellmethode wird benutzt, um numerische Daten zu gewinnen fuer einen Vergleich zwischen gemessenen und errechneten Schichtverschiebungen, die in der Praxis bei einem Abbau entstanden.


The most common method of protecting shaft Systems from the damaging effects of mining is to leave the ore unmined in an area Surrounding the shaft; that is to leave a shaft pillar. The design method currently preferred for the design of shaft pillars in the deep-level mines of South Africa, which exploit narrow, flat-dipping gold-bearing reefs, was suggested by Salamon (1974). According to this method, at depths greater than 2,5 to 3,0 km, protection of vertical shaft systems by shaft pillars becomes prohibitive because of the large sizes of pillars required. An alternative option to leaving shaft Pillars is the removal of the mineral deposit early in the life of the shaft from an area that would normally constitute the shaft pillar area. However, the induced Vertical displacements in the hanging- and footwalls are substantial and the associated strata disturbance can cause considerable damage to the shaft. The scarcity of field observations and the lack of retrospective analysis of these data are the main reasons for the uncertainty in Planning the early extraction of an orebody around the shaft. In order to contribute to the solution of this problem, the present study is concerned with the mechanisms of damage involved and the comparison of observed and calculated effects of such an extraction on the hanging wall strata. It seeks information on the applicability of the elastic model in providing quantitative data for design purposes.


Stopping of a horizontal, tabular deposit close to any shaft will have a number of effects on the rock mass containing the shaft. The most obvious consequence is that the immediate hanging wall in the stope moves downwards and the footwall moves upwards. These movements result in relaxation of the rock mass for a distance along the shaft. As the extent of extraction increases, the movements of the foot- and hanging walls continue and the affected distances up and down the shaft increase until closure occurs. The consequences of these rock mass deformations are that, in some regions of both the foot- and hanging walls, components of the resultant stresses and induced strains become tensile in nature. Once closure takes place, virgin stress conditions tend to be re-established over the area of contact between foot- and hanging wall. The distribution of resultant stresses and induced vertical strain along the centre line of two parallel, diverging longwall stopes can be studied by accepting linearly isotropic elastic rock mass behaviour and using any appropriate numerical method. Such a distribution, in a two-dimensional section across the stopes just before closure, is depicted in Fig. 1. In this diagram the × and z axes are orientated horizontally and vertically respectively. Some additional and relevant information are presented in the diagram. Assuming the shaft to be located along this centre line, the effects of extraction on the vertical shaft can be indicated.

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