ABSTRACT:

A technique for assessing on a daily basis the level of seismic risk prevailing in a given area of a deep-level mine has been developed. The technique relies on having an adequate history of seismicity recorded from the area, to allow for the accurate determination of the behaviour of a number of seismic risk parameters in relation to the occurrence of potentially damaging seismic events. The usefulness of the technique has been investigated at two sites on deep-level South African gold mines, with positive results.

RESUMÉ:

Une technique pour valuer journellement Ie niveau dominant de risque sismique dans une region ou une mine ô niveau profond à ete developper. La technique depends essentiellement sur Ie fait qu'il y ai eu prealablement des rapports adequats d'enregistrement sismique dans la region pour permettre de determiner Ie comportement du nombre de risque sismique en relation à l'existence du potentiel qui pourrai endomager les evennements sismique. L'utilite de la technique à ete examiner dans deux chantiers de mines d'or à niveau profond en Afrique du Sud, avec des resultats positifs.

ZUSAMMENFASSUNG:

Ein Verfahren, um das seismische Risiko in einem gewissen Gebiet einer tiefen Mine zu bestimmen, ist entwickelt worden. Die Voraussetzung der Methode ist, daß man eine ausreichende Aufzeichnung der seismischen Vorkommnisse des Gebietes hat, um die genaue Bestimmung des Verhaltens einiger Parameter, die das seismische Risiko in Relation zu potentiell schadigenden seismischen vorkommnissen reflektieren, zu ermöglichen. Die Nuetzlichkeit dieses Verfahrens ist anhand von Daten aus zweier Gebiete in tiefen Goldminen in Suedafrika untersucht worden, mit positiven Resultaten.

1 INTRODUCTION

Most gold mining in the Witwatersrand, South Africa involves the extraction of thin, tabular, shallowly dipping ore bodies at considerable depth (in excess of 2000 m below surface). The rock mass response to the high stresses induced by mining under such conditions can take the form of rock bursts, which continue to be a serious problem on deep-level South African gold mines, causing death or injury to underground workers and damage to underground excavations. While well-designed mine layouts and suitable support measures can be effective in combating the rockburst problem, there are areas in which more direct measures are required. The CSIR Division of Mining Technology is conducting research towards the development of a proactive rockburst control technique called preconditioning, in which explosive charges are detonated ahead of an advancing stope face, so as to transfer the high stresses away from the working areas (Lightfoot et al, 1996). A seismic risk assessment method has been developed under the auspices of the preconditioning project, to investigate the feasibility of determining periods of increased seismic risk accurately, so that the practice of preconditioning can perhaps be tied in to such periods. Earlier stages of development of the method have been described by Stewart and Spottiswoode (1993) and by Stewart (1995).

2 DATA SETS

The seismic risk assessment method has been developed from and tested on the seismic data recorded from preconditioning sites on two deep level South African gold mines, which will be referred to hereafter as Mine A and Mine B. 2.1 Preconditioning sites The preconditioning experiment on Mine A is being conducted at a site where a strike-oriented pillar is being extracted (Figure 1). The seismicity from the site is being monitored via a Portable Seismic System (PSS), which has been described by Pattrick et al (1990). The deployment of the geophones around the site is shown in Figure 1. The preconditioning at this site is performed by means of long, large diameter holes (up to 20 m long and typically 89 mm in diameter) drilled parallel to and about five metres ahead of the advancing stope face. The explosive charge is detonated separately from the normal production blasts which are used to advance the face.

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