Fundamental research into the stress and energy changes associated with the mining of deep tabular deposits shows that the most effective means of keeping these changes small is to employ a system of strategically-spaced barrier pillars. Analysis of seismic observations in a deep level gold mine confirms the theoretical concept. It was found that the level of seismicity can be reduced by a factor of two to four depending on the size and spacing of these pillars. The work described forms the basis of new design concepts for deep mines.
L'etude fondamentale des variations de contraintes et d'energie associees à l'exploitation minière des depots profonds indique que la meilleure manière d'eviter les dangers de coups de terrain est de reduire au minimum la perte de volume des zones exploitees. Cet article decrit l'effet des piliers de stabilisation sur la frequence et l'amplitude des evènements sismiques dans une mine d'or à grande profondeur. L'analyse d'informations sismiques montre que les piliers de stabilisation peuvent reduire l'activite sismique de moite ou des deux tiers. Les resultats de cette etude constituent une base pour la planification des nouvelles mines.
Grundlegende Untersuchungen ueber den Einfluβ des Abbaus tiefer flözartiger Lagerstatten auf die Spannungsveranderungen im Gebinge haven gezeigt, daβ die Gebirgsschlaggefahr durch regelmassig angeordnete Bergfesten maβgebend beeinfluβt werden kann. Die Arbeit beschreibt den Einfluβ von Bergfesten auf die Frequenz und Gröβe von Gebirgsschlagen in einem tiefen Goldbergwerk. Die Analyse von Gebirgsschlagdaten welche ueber einen Zeitraum von 4 Jahren gesammelt wurden zeigt, daβ die Gebirgsschlagtatigkeit um einen Faktor von zwei bis vier verringent werden kann. Die Arbeit stellt die experimentelle Grundlage fuer die Planung des Zuschnittes tiefer Gruben dar.
The frequent occurrence of violent rock failures, commonly known as rockbursts, is one of the most serious problems which faces deep level hard rock mining. During the past fifteen years considerable effort has been devoted in South Africa towards studying this burning problem. Basic research into the stress and energy changes resulting from the mining of deep tabular deposits have shown that these changes are directly related to the volumetric closure in the mined out area, Salamon (l963). Based on this finding Cook et al (l966) proposed that mining systems which minimised volumetric closure would be best suited to alleviate rockburst hazard in deep gold mines. Of the various possible methods of minimising stope closure in the mined out area a system of regularly spaced stabilising pillars offered the greatest advantages. However, in view of the inherent dangers that are generally associated with mining systems employing a number of highly stressed pillars to support the weight of the superincumbent rock strata a cautious approach was needed to minimise the possibility of pillar failure and the development of an unstable situation which could result in the release of large amounts of potential energy. In close collaboration with the management of one of the deepest gold mines in South Africa a system of 70 m wide stabilising pillars was designed for the lower sections of the mine. An underground seismic network was installed to monitor seismicity in the area protected by the stabilising pillars and to evaluate the long term behaviour of these pillars. Apart from these aspects the seismic network was used also for fundamental studies of seismic source mechanism by the Bernard Price Institute for Geophysical Research of the University of the Witwatersrand.
To appreciate fully the role of stabilising pillars as far as the alleviation of the rockburst hazard is concerned some basic results concerning the energy changes induced by mining are discussed briefly, Salamon (l974). To deform mine support, to create new rock fractures, or to generate seismic waves energy is required. Thus, a fundamental approach to the control of these phenomena is to study the energy balance of mining. Consider the situation where a mining excavation is enlarged. In the process of enlarging the excavation some energy becomes available to do some work. The sources of this energy are the potential energy change, W" and the strain energy content of the rock which was removed in the course of enlarging the excavations, Um. Thus the total energy available is Wt + Um. In the process of enlarging the excavations the rock mass surrounding the excavation is strained further and a portion, U., of the total energy available is stored as strain energy. The remaining part of Wt + Um is lost to the system in one form or another. From the above follows that the change in the potential energy is fully expended in the straining of the rockmass and the deforming of the supports. Hence, if no rock failure takes place, the enlargement of underground excavations in small steps is a quasi-static, stable process which cannot result in the release of seismic energy.
