Rockburst source studies-small particle formation

The research project comprising GAP 524 involved detailed examination of a group of 3 rockburst ruptures or burst-fractures discovered in a VCR stope panel on a peninsular remnant on Mponeng Mine at a depth of 2550m below surface.
A careful visual study of the exposures was made, samples were taken and minerological and geochemical analyses conducted on the fractured rock and the finely comminuted fault gouge.
The most important conclusions were:
  • In every important detail the 87-50 6E ruptures were effectively identical to the type example studied in 1974 on ERPM Ltd.
  • This confirms that the shear rupture type of origin is an important sub-class of rockburst source mechanism.
  • This type of rockburst is probably more common than previously believed and does not require any unfavourable geological structure or ‘bad ground’ as a necessary pre-condition for its occurrence.
  • The relative direction of shear movement can easily be determined even without a stratigraphical marker and the sense of the movement is normal, dip-slip.
  • There are strong indications that the rupture has its origin at some considerable distance from the reef plane.  The fracture front is driven through pristine rock at high velocity, towards the stope abutment.
  • Although there are intense frictional effects and probably quite high temperatures on the sheared surface; no chemical or mineralogical changes occur.
  • In particular no coesite was found in the finely comminuted rock flour
  • Considerable similarity exists between the mechanics of the rupture process and that involved in crustal earthquakes.
  • As a result of this study a better foundation of understanding of rockburst source mechanism now exists on which to build future conjecture regarding important practical matters such as the magnitude of ground motion parameters like free- surface particle velocity and amplitude.
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