|Theme||Visible||Selectable||Appearance||Zoom Range (now: 0)|
South African gold mines are forcing the boundaries of ultra-depth ( 3500 m) for the mining of available gold reserves. Platinum mines are also following in the footsteps of gold mines. Most intermediate-to deep-level (1000 m to 3500 m depth) gold mines are reaching their final stages of life. The availability of locked-in reserves in pillars or remnants provides an opportunity for supplementing the declining production profiles and extending the life of these mines. In this paper, we provide an overview of current remnant mining practices in South African gold mines. The factors that contribute towards the formation of remnants are discussed, and the reasons for the extraction of remnants provided. The details of current support and mining (stoping) practices are discussed for the difficult and challenging case of remnant mining. INTRODUCTION The evaluation of remnant mining practices in the gold mining industry is necessary due to the high production tonnages currently sourced from remnant blocks, coupled with the high mining risks associated with remnants. As one progresses from mines in the Far West Rand region to the Klerksdorp and Free State regions, the average production from remnant blocks increases from approximately 5% to a high of 60% for some mines in the Free State (Rangasamy, 2004). Although the work upon which this paper is based was conducted in 2004, the findings are still relevant since the technical aspects related to remnant definition, classification, and mining have remained largely the same for the last 50 years.
ABSTRACT: The experience of seismic activity associated with stabilizing pillars at intermediate depth, approximately 2000 metres below surface on the Ventersdorp Contact Reef horizon, is summarized. One particular case study is used to describe certain aspects in detail. The evaluation is based On seismic data, closure-ride measurements and the extent and intensity of underground damage. Experience from pillar associated seismicity allows a review of stability criteria of stabilizing pillars at intermediate depth which seem to be dominated by the composition of the footwall strata.
Stabilizing pillars have proved to be the focus of large seismic events which resulted in considerable damage at Western Deep Levels South (WDLS). The gold mine is situated in the Western Transvaal, about 70 km west of Johannesburg. The mine forms part of the West Wits Line of mines which work two major conglomerate formations, the Ventersdorp Contact Reef (VCR) and the Carbon Leader Reef (CLR).
At present, at WDLS, only the VCR is mined between 2000 and 2400 m below surface. The reef dips approximately 21 degrees towards South with a varying channel width of 0,05 to 4,50 m. It is intersected by dykes and some faults of minor throw « 50 m). Both the VCR up dip to the North and the CLR some 900 m stratigraphically below are mined by the adjacent WDL-West and WDL-East mines. WDL has adopted longwall-mining as a general concept which allows a face-advance between 10 and 12 m per month.
The mine experiences a relative high incidence of seismicity. Stabilizing pillars became part of Western Deep Levels general mining layout to reduce stress concentrations at the stope faces and hence the incidence of seismic activity. Some major seismic events have occurred with catastrophic damage along these pillars but the circumstances which lead to them have until recently been poorly understood.
To shed light on this phenomenon, a study was carried out on pillars and their seismic activity, involving mainly the Lower Carbon Leader Reef at WDL-East and West mines (Lenhardt and Hagan, 1990). General results of this study and from Jantzon et al. (1990) can be summarized as follows:
ABSTRACT: A seismic event of magnitude 4.0 occurred in Western Deep Levels East Mine on May 5, 1996.It caused extensive, damage to the mining area. The event plotted below reef, at the intersection of the complex geology, ahead of actual mining. The mining area of interest is of great importance since the entire longwall is basically mining a large pillar caused by the extended mining of the mini-longwalls above and below. It will be shown that this seismic event changed the perception of the mining and rock engineering personnel concerning the negotiation of geological features and emphasized the importance of strict adherence to the recommended face configuration. All of this has resulted in a better understanding of the rock mass response to mining, support systems and an improved management of seismicity strategy.
RÉSUMÉ: Un incident seismique de magnitude 4.0 a eu lieu a Western Deep Levels le 5 Mai, 1996. Lesdegats etaient etendus dans la region miniere. Le trace de l'evenement a ete faite sous la veine, a l'interseeton de la “fault” zone, au dela de l'exploitation. Cette region d'exploitation est de grande importance du fait que l'entierete du “longwall” est en fait exploiter la plupart du restant cause par l'exploitation intensive des “mini-longwalls” dessus et dessous. Il sera demontre quecet incident sismique a change la perception du personnel minier at des mechaniciens des roches concernant l'approche des caracteristiques, L'accent a aussi ete mis sur l'importance d'adhere strictement a la configuration de front recommandee. Tout ceci a abouti a une meilleure comprehension de la reaction des masses rocheuses a l'exploitation, les systemes de support et une meilleure gestion de la strategie sismique.
ZUSAMMENFASSUNG: Ein seismisches Beben der Staerke 4.0, ereignete sich in der WDL East Mine am 5. May 1996. Der Gebirgschlag verursachte erhebliche Schaeden im Abbaugebiet Das Zentrum des Bebens lag vor der Abbaufront, am Schnittpunkt der zweir Verschiebungszonen. Die betrotTeneAbbauzone ist von grosser Bedeutung, da die angrenzenden Vortriebe unter- und oberhalb in eine inselfoennige Lagerstaette vorstossen, die durch frueheren intensiven Abbau verursacht wurde. Es wird gezeigt, wie das obengenannte Beben die Einschaetzung von Produktions- und Felsmechnikpersonal bezueglich des Vortriebes in geologisch gestoerte Gebiete beinf1usste, und class die strikte Einhaltung von Empfehlungen bezueglich Layout unbedingt notwendig ist. All dies fuehrte zu einem besseren Verstaendnis der Reaktion des Felsmasse zum Abbau, zu Stuetzmassnahmen und zu einer verbesserten Strategie zur Verhinderung von Gebirgschlaegen.
Western Deep Levels, as one of the deepest gold mines in the World, is faced with a relatively high level of seismicity. The amelioration of the rockburst hazard is the single most important rock engineering task. In May 1996 a local magnitude 4 tremor occurred. There was no accident associated with the event, but it served as a catalyst for the development of new methods for the analysis of seismicity in mines and for a new stope support strategy.
Concepts and methods for the analysis of mining induced seismicity have developed rapidly over the past two decades since Mc Garr (1976) related seismic moment to the volume of elastic closure and Mendecki (1993,1997) brought mine seismology into the realms of rock mechanics and rheology. Some practical applications implemented in day to day operations in mines Were described by Amidzic and Glazer (1995). Specific procedures for the analysis and interpretation of seismicity in South African gold mines are, however, by no means standardised. Methods described here may contribute in this regard.
In this paper we give an overview of the circumstances around - and nature of the large event.
Although the application of destress blasting as an active rockburst mitigation measure is not yet commonplace in hard coal longwall mining, the method is assuming heightened importance due to increases in mining depth and high horizontal stresses in the rock mass. The main goals of destress blasting are the softening of competent rock layers, the reduction of strain energy storage and rock mass stress release, which together contribute to minimizing rockburst occurrence and risk. One such region in which destress blasting in competent rock is applied is the Upper Silesian Coal Basin, mainly in its Czech part.
Here a case study of destress blasting is presented and evaluated in terms of rockburst prevention, focusing on a thick coal seam (about 5 m) subject to longwall mining under very unfavourable geomechanical conditions (great depth – 1200 m, competent rigid rocks between coal seams, unfavourable stress field due to long-term mining). Destress blasting stages were carried out in a group of boreholes (4–8; diameter 95 mm) with the total explosive charge ranging from 2100 kg to 3750 kg, detonated regularly at a distance of 43 m to 114 m ahead of the advancing longwall face. Evaluation of destress blasting based on seismic monitoring revealed that the longwall blastings were very successful in terms of rock mass stress release and decreasing the rockburst risk, with only one rockburst occurring on a roadway 210 m from the longwall face. The following longwall section was mined safely without rockburst occurrence.
In the hard coal reserve of the Upper Silesian Coal Basin (USCB), which is shared by the Czech Republic and Poland, longwall mining is the dominant underground mining method. The Czech part of the USCB, known as the Ostrava–Karvina Coalfield (OKR), lies in the northeastern part of the country (Figure 1). The exhaustion of the upper seams due to the continuation of coal mining activities for around 200 years has shifted activity to a greater depth (> 800 m). Under existing mining and geological conditions in the Karvina sub-basin of the USCB, underground extraction of coal is typically accompanied by rockbursts, the first of which occurred in 1912 (Pelnar 1938). A number of attempts have been made to address rockburst activity in both the Czech (e.g. Straube et al., 1972; Holecko et al., 1999; Takla et al., 2005; Holub, Rušajová, and Holecko, 2011) and Polish parts of the USCB (e.g. Dubinski and Konopko, 2000; Drzewiecki and Kabiesz, 2008).