Crown pillar failure mechanism – case study based on seismic data from Palabora Mine

This detailed back analysis of crown pillar failure at Palabora is based primarily on seismic data. Other cave monitoring devices installed in boreholes that could provide data had been lost as a consequence of the response of the rockmass to mining. Analysis of the seismicity emanating from the crown pillar between the caving rockmass and the base of the open pit revealed the existence of a destressed zone of fractured rockmass below the base of the open pit when the crown pillar reduced to 100 m thickness. At this stage the crown pillar was considered to have failed. The intensity and orientation of fracturing and jointing in this failed rockmass volume directly below the open pit did not allow the creation of a stable arch and hence an air gap over the caved rockmass. The cave finally broke through into the open pit in April 2004 with no evidence that there was any significant air gap above the caved rockmass. The existence of the failed zone below the open pit became apparent during reconstruction of the mainly a-seismic crown pillar failure mechanism. At the time this zone was created there was only a very limited amount of seismic data and only the analysis of the later seismicity provided sufficient conclusive evidence of its existence and extent. At present, a similar process to that which occurred in the crown pillar is taking place below the underground production level at the base of the block cave. The extension of the failed zone below the underground production level, however, was monitored by the seismic network right from its initiation and data referring to this process was collected on a daily basis. It therefore provides additional data that can be used to analyse the failure mechanisms of crown pillars that may be applicable to all present and future cave mines, regardless of the lift number.