Early shaft reef extraction — The Western Deep Levels South approach to shaft protection

Early shaft reef extraction would appear to provide a key to successful protection of vertical shafts and major service excavations in mines at great depth. This concept has been adopted at Western Deep Levels South Mine in preference to a more traditional shaft pillar approach. While high compressive stresses and strains would cause problems in a shaft pillar, total extraction of the near shaft reef will result in a dislocation to the line of the shaft at the reef intersection and the shaft axis is placed in a state of induced tension. Three dimensional boundary element modelling has been completed to evaluate elastic rock movements, which can be restricted by backfill placement in stopes. Near shaft seismic activity is of major concern and an estimate of likely occurrances has been made through the computation of Energy Release Rates and comparison with present VCR mining sections. Attention has been paid to the design of the support system for securing the rock mass around the shafts at the reef intersection. Some non-elastic rock movements are expected and during sinking a slot has been cut around each shaft and filled with an engineered material in an attempt to restrict such movements.