ABSTRACT:

This paper describes a numerical modelling exercise carried out for a massive tabular orebody mine. Mining to date has taken place by means of room and pillar mining, with the recent introduction of backfill and pillar extraction. To access extensions to the orebody, mining has to progress to greater depth, rendering the current mining method uneconomic and unsafe. Numerical modelling techniques were used to evaluate existing and proposed mining methods. Back-analysis using observations of stope hanging- and sidewall failure zones, and vibrating wire stressmeter instrumentation data enabled calibration of the numerical models.

RESUME:

Cet expose decrit une modelisation numerique effectuee pour nne mine ayant un corps mineralise massif et tabulaire. La mine fut exploitee par chambre et piliers et I'on a recemment commence à exploiter Ie remblayage et les piliers. Pour atteindre les prolongements du corps mineralise, il faudrait I'exploiter à un niveau plus profond, ce qui ne sera it cependant plus rentable et suffisamment sûr avec la methode actuelle. Des techniques de modelisation numeriques furent utilisees pour evaluer la methode d'exploitation actuelle et les nouvelles methodes proposees. Les modèles numeriques furent calibres à I'aide d'une retroanalyse basee sur des observations de zones de rupture des roches superieures et laterales, ainsi qu'à l'aide des donnees obtenues de l'equipement de mesure à til d'acier vibrant.

ZUSAMMENFASSUNG:

Die vorliegende Untersuchung beschreibt ein numerisches Modellier-verfahren, das fuer ein Bergwerk mit einem massiven, tafelförmigen Erzstock verwendet wurde. Das Erz wurde bisher durch Strecken_ und pfeilerbau gewonnen und ein Hinterfuellungs- und Pfeilerabbauverfahren wurde vor kurzem eingefueht. Um zu den Verlangerungen des Erzstockes zu gelangen, muesste das Erz in einer grösseren Tiefe abgebaut werden, was jedoch mit der heutigen Abbaumethode unwirtschaftlich und nicht sicher genug ware. Numerische Modellierverfahren wurden verwendet, um die heutige ebenso wie neue Abbaumethoden zu bewerten. Die numerischen Modelle konnten mit Hilfe einer Rueckanalyse, bei der Beobachtungen von hangenden und seitlichen Bruchzonen verwendet wurden, sowie aufgrund der Mess-ergebnisse einer Schwingdraht-DehnungsmessanIage kalibriert werden.

1 INTRODUCTION

The Otjihase copper mine for which this study was carried out is situated ± 40 km north-east of Windhoek, in the Republic of Namibia (Figure 1). The primary commodity mined is copper, with a run of-mine tonnage of 55 000 tons per month. Some silver is also produced as a by-product. The orebody consists mainly of pyrite and Pyrrhotite, with the copper ore mineral being chalcopyrite. The geologic setting is in the Matchless Schist Belt, and the host rocks are quartz mica schists, which are fissile in nature. The orebody is lenticular and tabular, trending East-West, with a dip of±14° NW and a plunge, along the trend axis, of ±7°. The average stoping width is ±5,5 metres. Two payable shoots occur but for the purposes of this study, only the more extensive of the two was considered, as the mining of the secondary shoot does not appear to have any influence on mining the primary, down-dip shoot. The orebody outcrops at surface and is accessed by means of a surface adit, with are being crushed underground and brought to surface by conveyor belt. Due to the gentle plunge of the orebody, the depth of mining has been increasing progressively over the life of the mine, hence changes to the mining method have gradually been implemented to cope with the increasing overburden load. Apart from early attempts at Witwatersrand type tabular stoping, all mining has been by the room-and-pillar method, whereby 10m wide stopes are developed on strike, leaving 10m rib-pillars between them to support the hangingwall. This mining method is illustrated in Figure 2.

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