In order to make good decisions, as to whether large diameter raise bored
shafts can be constructed in blocky rock at a particular location, mine
planners, management, geotechnical engineers and contractors must understand and
consider a range of geotechnical issues.
Firstly, mine management must ensure that adequate time and resources are
available to find appropriate shaft sites. Deep weathering, adverse structures,
groundwater, etc might mean that the most desirable location, with respect to
other mine development, might not be suitable. Special site investigation
drilling, etc may be necessary. Even then, alternative construction options
might be necessary, other than just reaming through to the surface.
A site investigation
borehole should then be drilled to investigate specific and general ground
conditions along the raise alignment. Often, only small sections of weak blocky
rock dictate raise stability. While the empirical McCracken and Stacey method is
typically used in Australia to assess raise stability and assess geotechnical
risk, until recently it had no published performance data.
An Australian database of raise
performance now exists and is presented below, courtesy of an industry survey by
the Eastern Australian Ground Control Group. Raise diameter and ground
conditions (in terms of a lower bound Qr value) is presented for 50 cases versus