Page 580 - Rock Mechanics For Underground Mining

MONITORING ROCK MASS PERFORMANCE

Figure 18.13 Examples of con- Unanticipated or anomalous stresses elsewhere. Point 3 in Figures 18.13 a and b
vergence rate contours, production shows a high displacement rate away from the undercut boundary in an area in which
level, PT Freeport Indonesia’s DOZ
high stresses would not be expected to occur. It was found that this area was in the
Mine, Indonesia (after Rachmad and
location of an old exploration drift which had been backfilled. The monitoring data
Widijanto, 2002).
showed that the backfilled drift did not have the same load carrying capacity as the rock
left in situ around the production level excavations. Point 4 in Figure 18.13b shows an
atypical convergence rate contour for a point well inside the undercut area. Such an
occurrence provides warning of a potential problem that must be investigated. If it is
not associated with a weak rock mass, as in the case of the backfilled drift, something
else must be causing an anomalous stress to be transmitted to the production level in
what should be a caved area. The main possibilities are packing or compaction of ore
that has not been drawn continuously, and a pillar or stump of incompletely blasted
ore being left in the undercut. In the case indentified by point 4 in Figure 18.13b,
it was found from visual inspection of the undercut area and probe drilling that the
problem was caused by a stump of unbroken ore. The stump was then drilled and
blasted from the extraction level (Rachmad and Widijanto, 2002).

Abutment stress width. Knowledge of the width of the stress abutment associated
with the advancing undercut front is important for planning and scheduling purposes.
In planning the DOZ mine, it was assumed on the basis of experience in the earlier
block cave and at other mines, that the width of the abutment zone would be 20 to 30 m.
Data such as that shown for one convergence monitoring station in Figure 18.14 may
be used to validate and refine this assumption. The figure shows that no convergence
was recorded at the station for some time before the undercut approached the station
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