Extraction of isolated blocks of ground with elevated levels of stress and/or seismicity is a common occurrence in South African gold mines. Isolated blocks of ground that are divorced from the main orebody and are bound by complex, dip and strike parallel, non-persistent geological structures dominate the Klerksdorp gold fields. No quantifiable methods exist to assess the risk associated with mining these and other classes of remnants. A case history, from a gold mine that planned to extract three remnant blocks hosted in a large complex fault system, is used to illustrate the decision making process based on a combination of underground observations, numerical and seismic techniques. Simple risk estimation matrices were developed and used to estimate remnant-mining risk and finally to quantify the decision on which of the three remnant blocks could be extracted at the lowest possible geo-mechanical risk.

Il est courant dans les mines aurifères sud-africaines d'extraire des blocs de terrain isoles qui possèdent des niveaux eleves de contraintes et/ou sismicite. Les regions aurifères de Klerksdorp sont dominees par des blocs de terrain isoles qui sont separes du gisement principal et qui sont retenus par des structures geologiques non continues, à parallèle de pendage et direction, et complexes. Il n'existe pas de methodes quantifiables pour evaluer le risque associe à l'extraction de ce genre et autres genres de residus. Une observation de cas, celle d'une mine d'or qui avait l'intention d'extraire trois blocs de residus qui se trouvaient dans un grand système complexe de faille, est utilisee pour illustrer le processus decisionnel base sur une combinaison d'observations souterraines, et de techniques numeriques et sismiques. Des matrices simples d'estimation de risque furent developpees et utilisees pour estimer le risque lie à l'extraction des residus et finalement pour quantifier la decision sur lequel des trois blocs de residus pouvait être exploite au moindre risque geo-mecanique.

Die Extraktion isolierter Gesteinsblöcke mit erhöhtem Spannugsniveau oder Seismizitat ist in suedafrikanischen Goldminen ueblich. Isolierte Gesteinsblöcke, die von der Haupterzader getrennt und durch komplexe, parallele abfallende und horizontale nicht persistent geologische Strukturen gebunden sind, beherrschen das Klerksdorp Goldfeld. Es gibt keine quantifizierbare Methoden die das Risiko welches mit dem Abbau dieser und anderer Klassen verbunden ist beschreiben. Eine Fallstudie einer Goldmine, welche die Extraktion von drei in einem großem komplexen Verwerfungssystem eingelagerten Einschlußblöcken plante, wird verwendet um den Entscheidungsprozeß zu illustrieren, der auf einer Kombination von Untertag Beobachtungen und numerischen und seismischen Techniken basiert. Einfache Gefahreinschatzungsmatrizen wurden entwickelt und angewendet um das Abbaurestrisiko abzuschatzen und letztendlich die Entscheidung zu quantifizieren, welcher der drei Einschlußblöcke unter dem niedrigst möglichen geo-mechanischen Risiko extrahiert werden konnte

Introduction

Remnants are essentially blocks of ground that pose elevated mining risk due to a combination of their shape, size, frequency and severity of rockbursts, anomalous stress and/or geotechnical conditions. The extraction of these blocks of ground is often risky and measures in addition to those used in normal mining practice must be instituted. A gold mine in the Klerksdorp goldfields, with workings at approximately 2500 m below surface, planned to extract three remnant blocks that were hosted in a large complex fault system. Due to the risk associated with the potential for large seismic events emanating from step faults that bound the remnant blocks, a decision had to be taken to determine which of the three remnants could be extracted at the lowest possible geo-mechanical risk. It is normal practice to use one or a combination of underground, numerical and seismic techniques to aid decision making in high-risk mining situations. This paper, describes by way of a case history the decision making process adopted using all of the aforementioned techniques. Simple risk estimation matrices were developed for the three remnant blocks to highlight areas of potential concern during extraction.

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