Initial and east cave breakthrough events at Palabora Mining Company

摘要

Understanding cave induced seismicity is important to better understanding the processes of block cave mining. This seismicity is a natural process indicating that the cave is progressing and is connected with rock mass fracturing ahead of the undercut and propagating cave. The space and time distribution of seismicity, as well as changes in their source parameters over time, are directly associated with what is happening in the rock mass around the cave and underground mining infrastructure. The initial cave breakthrough into the overlying open pit took place in May 2004 while the east breakthrough took place during the months of May and June 2008. There is a great deal of similarity as regards stress changes (as-indicated by the energy index time histories), migration of seismicity, seismic energy release as well as seismic deformation patterns, with respect to the two breakthrough events. There are also some differences but they are easy to explain as the initial breakthrough took place 25 months after the stress caving process was initiated while in case of the east breakthrough, this time span had increased by a further four years. The continuous caving process during these four additional years had to significantly change the conditions of the rock mass around the cave that resulted in these differences. The energy index time histories for the two breakthrough events are very similar, indicating a stress increase prior to the breakthrough with a rapid stress release post-breakthrough. The main differences relate to the stress levels. Based on the energy index time histories the stress levels prior to the initial breakthrough were higher than those prior to the east breakthrough. The rapid stress release period in both cases was about six times shorter than the period during which the stresses were high. In both cases the pillar failure, and thereafter the breakthrough, resulted in major changes in the stress patterns around the mine. After the failure of the crown pillar larger size seismic energy releases started to appear below the extraction level. When the east pillar failed and broke through there were also some larger seismic energy releases taking place below the extraction level. In both cases the two breakthrough events resulted in an increase in the seismic hazard. The analysis presented shows that not only can seismic data analysis be successfully used to monitor the caving process, but also confirms the reliability of the process.