Mine layout, geological features and seismic hazard.

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 engi- neering personnel concerning the negotiation of geological features and emphasized the importance of strict adherence to the recom- mended 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. RESUME: 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’intersecton de la “fault” zone, au dela de l’exploitation. Cette re- gion 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 que cet 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 Ge- birgschlag verursachte erhebliche Schaeden im Abbaugebiet. Das Zentrum des Bebens lag vor der Abbaufront, am Schnittpunkt der zweir Verschiebungszonen. Die betroffene Abbauzone ist von grosser Bedeutung, da die angrenzenden Vortriebe unterund oberhalb in eine inselfoermige Lagerstaette vorstossen, die durch frueheren intensiven Abbau verursacht wurde. Es wird gezeigt, wie das oben- genannte Beben die Einschaetzung von Produktionsund Felsmechnikpersonal bezueglich des Vortriebes in geologisch gestoerte Ge- biete beinflusste, und dass 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. Appendix 1.4 Mining history The entire longwall C (area of interest – Figure 1) is basically mining a large pillar caused by the extended mining of the mini- longwalls above (A) and below (B). After a fire in 1987, the lower portion of the longwall C stood for approximately five years, while the lost production was made up by pushing the ad- jacent longwall, resulting in the creation of a highly stressed pil- lar – the area of interest here. 1.5 Seismic system The introduction of seismic monitoring networks in the early sixties yielded valuable insight into the origin and nature of seismic events and enabled more accurate estimates to be made of the energy content of these events. Seismic monitoring of the rock mass response to mining contributes to verifying mining layout design, to detecting potential instabilities and delineating new geology ahead of mining activity. To improve the quality of the seismic database, a digital seis- mic network was installed in the early 90’s. At present, twenty- three triaxial geophones are used to record seismic activity on the East Mine that covers an area of 16km. A geophone records the velocity of ground motion due to transient seismic waves as an electrical signal. An analog to digital conversion (A/D) is per- formed on this output voltage and transmitted as time series data (seismograms) to the central site on surface. The processing software, which is built into the system, provides all the impor- tant physical parameters describing the seismic source: size, seismic moment, emitted energy, static and dynamic stress drops, as well as the parameters describing ground motions, maximum amplitude, energy, power and duration of strong ground motion. Currently, the East Mine seismic system is recording in excess of 1000 seismic events per day, in the local magnitude range of –2 to 4.