Page 381 - Rock Mechanics For Underground Mining

UNDERGROUND MINING METHODS

Mount Isa Mine, Queensland, Australia, to replace the cut-and-fill method that was
formerly used in the steeply dipping silver-lead-zinc orebodies (Villaescusa, 1996).
In this case, initial drilling and extraction drives are mined along the length and width
of the orebody. A bench slot is created between these two horizons at the end of
the orebody by enlarging a cut-off raise or longhole winze located near the footwall.
The slot so created is used as an expansion void into which the remainder of the
bench stope is formed by the sequential blasting of down-holes. In most cases, the
production holes are drilled in rings parallel to the dip of the orebody between the two
drives. Mining proceeds by the sequential firing of production rings into the advanc-
ing void and the ore is mucked remotely from the extraction horizon (Villaescusa,
1996).
In the case of the less regular and flatter dipping orebodies mined by benching
methods at the Neves Corvo Mine, Portugal, vertical transverse primary bench stopes
are developed and extracted from the footwall to the hangingwall, mucked from the
extraction drift and then filled mainly with paste fill. Secondary bench stopes are then
mined between the primary stopes on the current mining horizon. Depending on the
orebody geometry, further bench-and-fill lifts may be developed and mined above the
initial benching horizon.
Following mucking, bench stopes are backfilled with rock fill, hydraulic fill, paste
fill or a combination of fill types (e.g. Been et al., 2002, Villaescusa and Kuganathan,
1998). Clearly, the permissible stope sizes, the support and reinforcement required
for the accesses and the drilling and extraction drives, the lengths of hangingwall
that can be left unsupported, and the mining sequence including the filling sequence,
will all depend on the geotechnical conditions, their understanding and management.
Because of these factors, the bench-and-fill method may be used at a number of scales
and with a number of variants. In some parts of the world, it has become the preferred
method of narrow vein mining, for example.

12.4.7 Longwall mining
Longwall mining is the preferred method of mining a flat-lying stratiform orebody
when a high area extraction ratio is required and a pillar mining method is precluded.
The method is applicable to both metalliferous mining in a hard-rock environment and
coal mining in soft rock. In both mining situations, the method preserves continuous
behaviour of the far-field rock. Different modes of response are induced in the stope
near-field rock. For both cases, longwall mining requires an orebody dip of less than
20 , with a reasonably uniform distribution of grade over the plane of the orebody. A
◦
highdegreeofcontinuityoftheorebodyisnecessary.Aparticularstructuralgeological
requirement is that the throw of any faults transgressing a mining block must be less
than the thickness of the orebody.
In hard-rock mining, the stoping method seeks to maintain pseudo-continuous
behaviour of the near-field rock mass, although significant fracturing may be induced
in the stope peripheral rock. Thus a basic requirement is that the orebody footwall and
hangingwall rocks be strong and structurally competent. The schematic representation
of the method shown in Figure 12.11 indicates the main elements of the operation.
Mining advances along strike by blasting rock from the face of a stope panel. Ore is
drawn by scraper down dip into a transport gully, excavated in the foot wall, through
which it may be scraped to an ore pass. Temporary support, perhaps provided by
yielding props, is emplaced near the mining face, while resilient supports, such as
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