7
development with or without ground support update or
to stop development given triggered condition has been
achieved.
Drifting through the fractured zone in Extraction and
undercut level is generally take place in the Diorite domain.
Fractured observed in this zone is likely due to stress frac-
turing previously during cave propagation. As indicated
in Figure 10, the rock burst hazard in the fractured zone
is relatively low given the low stress magnitude. Risk of
subsidence and tensile failure into the cave direction is
anticipated. Therefore, heavy ground support, involving
surface support (FRS and heavy mesh) and high displace-
ment capacity bolt, has been recommended.
Controlling of Draw Rates to Preserve LWV
Pillar Integrity
During the writing of this paper, the extraction drift devel-
opments are about to enter caution zone (blue line bound-
ary in Figure 9). A continues drawing from PB1E increased
risk of personnel being exposed to potential instability
Figure 10. Beck model showing low stress magnitude near the cave and calibration result with
actual open hole survey (Beck-Engineering, 2024). The orange line representing the fractured
zone boundary in Figure 2 is indicated by the yellow cylinder
Table 2. Description of rock mass condition for cave zones
Zone Description Risk Action Plan
1 Elastic/ safe working zone: Very view open
cracks with no continuation or less than
two cracks over 8 m of borehole camera
survey length.
Low: Less likelihood that
extreme change in rock mass
condition. Low potency of
subsidence.
Continue blasting with normal round
and no change in the ground support
recommendation.
2 Fracture/ yielded zone: More than 25%
dislocation between fractured plane inside
the hole after the drilling rod pulled out. In
general, decrease in the rock mass strength.
Blocky ground may be expected during
drift excavation.
Moderate to high: High
possibility that the rock mass
strength degradation.
Continue blasting with short round.
Prepare ATE revision for update ground
support. Heading inspection is mandatory
after mucking out.
3 Cave back/ mobilized zone boundary:
Observed cave back or mobilized zone.
A clear indication that fragmented rocks
with gap that snatched from the drill
hole. Observed fragmentation may varies
depend on lithology and HOD (primary or
secondary fragmentation)
Very High: Subsidence is
certain, and support may not
be adequate to support drift in
the mobilized zone.
Stop blasting 5 m from the cave back
boundary. Notify the PB4 stakeholders to
halt development.
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