Page 421 - Rock Mechanics For Underground Mining

OPEN STOPE-AND-PILLAR DESIGN AT MOUNT CHARLOTTE

results in a decrease in damaging seismicity and a substantial reduction in seismically
induced falls of ground. The point illustrated by the sequence is the importance of a
conceptual model of stress interaction with the mine structure and the way this can
be managed by a disciplined advance of the mine abutments.
Implicit in the development of an extraction sequence for an orebody is the pro-
tection of the major installations, external to the deposit, which sustain operations
within it. These include ventilation shafts and airways, workshops, haulages, and
drives and passes for placement of backfill. Although some of these installations are
inevitably damaged or destroyed as mining proceeds, the objective is to maintain their
integrity for as long as is demanded by extraction operations. This is accomplished,
in the geomechanical assessment of any proposed extraction scheme, by estimation
of potential rock mass response around these excavations. The analyses of stress
conducted in the sequencing studies may be used for this purpose. Other modes of
rock mass response, including rigid body instability of large wedges identified in the
zone of influence of mining, or slip and separation on penetrative planes of weak-
ness, must also be considered in a global analysis of the mine structure and its near
field.
A final objective in the development of an orebody extraction sequence is to limit
the amount of work, by mine personnel, in areas subject to high stress or potential
instability. The extraction scheme should seek to avoid the generation of narrow
remnants, by such activity as the gradual reduction in the dimensions of a pillar. As
an alternative, a large-scale blast involving the fragmentation of pillar ore in a single
episode is usually preferable.

13.8 Open stope-and-pillar design at Mount Charlotte

The Mount Charlotte Mine is located on the Golden Mile at Kalgoorlie, Western
Australia. Its value as a case study is that it illustrates some of the problems of
stope-and-pillar design in a relatively complicated geometric and structural geological
setting, and the need to consider appropriate modes of rock mass response in stope
and excavation design and extraction sequencing. The following account of rock mass
performance is based on that by Lee et al. (1990).
The orebody considered in the study by Lee et al. was the Charlotte orebody. The
long axis of the orebody strikes north. It is bounded on the north–east by the Flanagan
fault, and on the south–west by the Charlotte fault. A fracture system related to the
Beta fault trends north–south and dips west at about 45 . The rock materials are strong
◦
and stiff, and the rock mass is infrequently jointed. Estimated friction angles for the
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Charlotte and Flanagan faults were about 20 , and about 25 for the Beta fault.
◦
From beneath the original open-cut mine of the surface outcrop, the orebody had
been mined progressively downwards, by open stoping and subsequent pillar blasting.
After rib and crown pillars in a block were fragmented in single mass blasts, the broken
ore was drawn from beneath coarse granular fill which was introduced at the surface
and which rilled into the active extraction zone. The stoping block considered in
the study was the G block, located between the 19 and 24 mine levels, which were
about 630 m and 750 m below ground surface. The mining sequence for the G block
shown in Figure 13.31b indicates large stopes (G1, G2 etc.) and rib pillars beneath
a continuous crown pillar. The stopes were 40–80 m wide, 35 m long and up to
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