A large hard rock rib pillar was recently preconditioned at Mt Charlotte
mine using a novel blasting technique. The motivation for trialling this
procedure was a history of past pillar failures, with associated seismicity.
Design studies had shown that a new proposed pillar would be at risk of
violent failure, and preconditioning was used in an attempt to reduce this
risk. Preconditioning by blasting involves physically modifying a
selected zone of ground in advance of mining, then forming the pillar so
that it incorporates that zone of ground. The modified rock in the zone is
intended to allow extra closure (yield) of the pillar in response to
mining-induced stress changes. The ground modification was done by tight firing a blasthole array to
fracture the rock. The blast design comprised pairs of pre-split blastholes
to create a series of short cracks across the target zone, followed by main
blastholes to fracture the rockmass between each pair of cracks. A
monitoring program was used to establish whether or not the
preconditioning was effective. It included conventional extensometers,
stress change cells and closure stations, plus a seismic monitoring system.
To-date, stoping is complete on one side of the pillar and has just
commenced on the other. Results show significant movements within the
preconditioned zone and on geological structures in the pillar, as well as
lower stress in the pillar compared to an adjacent non-preconditioned site. The preconditioning technique can be applied elsewhere, provided its
purpose, mechanism and side effects are understood. Benefits are likely
to include more assured production, and successful pillar recovery even in
high stress situations.