Page 379 - Rock Mechanics For Underground Mining

UNDERGROUND MINING METHODS

a working platform and temporary support for the stope walls. The method is directly
comparable with cut-and-fill stoping, with broken ore temporarily fulfilling some of
the functions of backfill.
Shrink stoping operations follow the cyclic pattern of drilling and blasting, ore
extraction, and scaling and supporting. Ore is broken in the stope by stripping the
crown of the excavation, with miners working directly under the stope crown. To
maintain adequate work space in the stope, broken ore must be drawn from the base
of the stope after each blast. The amount drawn is related to the swell, or increase
in void ratio, which occurs when rock is blasted. Since the void ratio of blasted,
displaced rock is about 50–55%, only 30–35% of the freshly broken ore can be drawn
after any stope blast This aspect of the method clearly has production scheduling
disadvantages. Once the stope has been mined to its full design height (which may
take many months or years), ore is drawn until either the stope is empty or until
dilution due to stope-wall collapse becomes excessive.
The orebody type, orientation, geomechanical properties and setting suitable for
shrink stoping are virtually the same as described for cut-and-fill stoping. However,
in the case of shrink stoping, there are additional restraints on the physicochemical
properties of the ore to be satisfied. The orebody rock must be completely inert, with
no tendency for oxidation, hydrolysis, dissolution or development of cementitious
materials. It must also be strong and resistant to crushing and degradation during
draw. All these properties are necessary to ensure that once the ore is mobilised by
blasting, it will remain mobile and amenable to free granular flow during its residence
time in the stope. Chemical and physical degradation of ore, recementing and binding
of ore are all promoted by mine casual water (introduced by drilling, for example),
which may percolate through the broken ore mass.
Pre-production development for shrink stoping resembles that for cut-and-fill stop-
ing, except that no ore-pass development is required in the orebody foot wall. Instead,
an extraction system must be developed at the base of the stope. This may consist of
a slusher drift, with associated finger raises, or drawpoints suitable for mucking with
mobile front-end loaders.
Shrink stoping is a labour-intensive mining method that was used more widely
before the advent of large-scale mechanised mining. Currently, more productive sub-
level stoping, vertical crater retreat and cut-and-fill methods can usually be used
under conditions in which shrink stoping may have been used previously. Neverthe-
less, shrink stoping remains one of the few methods that can be practiced effectively
with a minimum of investment in machinery but is still not entirely dependent on
manual capacity (Hamrin, 2001).

12.4.5 Vertical crater retreat (VCR) stoping (Figure 12.9)
VCR stoping is a variation on the shrink stoping procedure, made possible by develop-
ments in both large-diameter blasthole drilling technology and explosive formulation.
Details of the method are provided by Mitchell (1981) and Hamrin (2001). It is a non-
entry stoping method, the use of long, subvertical blast holes eliminating the need
for entry of operating personnel into the developing mine void. Upward advance of
the stope crown is effected by a series of cratering blasts using short, concentrated
charges in the large-diameter blast holes. After each episode of blasting, sufficient
ore is drawn from the stope to provide a suitable expansion void for the subsequent
blast.
361