Page 374 - Rock Mechanics For Underground Mining

MINING METHODS AND METHOD SELECTION

in a regular grid array, to simplify planning, design and operation. Since personnel
operate continuously under exposed roof spans, close observation of the performance
of roof spans and pillars is required. Immediate roof rock may be unsupported, or
supported or reinforced artificially, using methods described elsewhere. The pillars
may be permanently unmined. Alternatively, pillar ore may be recovered in the orderly
retreat from a mine panel or district, inducing collapse of the immediate roof of the
mined void and caving of the superincumbent strata.
Room-and-pillar mining is applied in flat-lying stratiform or lenticular orebodies,
although variations of the method can accommodate an orebody dip up to about 30 .
◦
The orebody must be relatively shallow, to prevent commitment of excessive ore in
pillars. Mechanised mining operations require a fairly uniform orebody thickness, but
the method is sufficiently flexible to accommodate some local variations in thickness
of the mineralised zone. It is one of the two methods suitable for recovery of thin,
flat-lying deposits. Orebody heights greater than about 6 m are generally worked by
multiple passes. A top slice is mined conventionally, and the underlying ore is then
mined by an underhand method, such as downhole benching.
The geomechanical setting suitable for implementation of room-and-pillar mining
consists of a strong, competent orebody and near-field rock medium, with a low
frequency of cross jointing in the immediate roof rock.
Close control of product ore grade is possible in room-and-pillar mining, since
the method admits highly selective extraction of pockets of ore. Variability of
grade distribution can be accepted, with low-grade ore being left as irregularly dis-
tributed pillars. Barren rock produced during mining can be readily stowed in mined
voids.

12.4.2 Sublevel open stoping (Figure 12.5)
Ore is produced from a stope block in which extensive development has been un-
dertaken prior to stoping activity. Stope pre-production development consists of an
extraction level, access raises and drifts, drill drifts, slot raise and stope return airway.
An expansion slot is developed by enlarging the slot raise, using parallel hole blasting,
to the width of the stope. Ore is fragmented in the stope using ring-drilled or long
parallel blast holes, exploiting the expansion provided by the stope slot. Broken ore
reports to the drawpoints for extraction. Stope faces and side walls remain unsup-
ported during ore extraction, while local and near-field support for the country rock
is developed as pillars are generated by stoping.
Bighole open stoping is a scaled-up variant of sublevel open stoping which uses
longer blast holes with larger diameters of 140–165 mm (Figure 12.6). Holes to depths
of 100 m may be drilled using the in-the-hole (ITH) technique. The large diameter
ITH holes may be drilled relatively accurately so that the vertical spacings between
sublevels can be increased from typically 40 m for sublevel open stoping to 60 m for
bighole stoping (Hamrin, 2001).
Open stoping is applied in massive or steeply dipping stratiform orebodies. For an
inclined orebody, resulting in inclined stope walls, the inclination of the stope foot
wall must exceed the angle of repose of the broken rock by some suitable margin.
This is required to promote free flow of fragmented rock to the extraction horizon.
Since open stoping requires unsupported, free-standing stope boundary surfaces, the
strength of orebody and country rock must be sufficient to provide stable walls, faces
and crown for the excavation. The orebody boundaries must be fairly regular, since
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