Page 567 - Rock Mechanics For Underground Mining

MONITORING SYSTEMS

Figure 18.1 (a) Single point conver- convergence values. An example of the use of convergence rates is given in the
gence measurement with a tape exten- case history discussed in section 18.3.2 below. Mechanical convergence monitoring
someter, Telfer Gold Mine, Australia
stations are robust, simple and inexpensive to install, and the results obtained are
(after Thompson et al., 1993); (b)
usually reliable. However, because of potential obstruction to accesses and working
three-point convergence measurement
with a tape extensometer, PT Indone- places, they are generally not suitable for remote or continuous reading. Regular
sia’s Deep Ore Zone Mine, Indonesia manual reading of large numbers of stations can be time-consuming.
(after Rachmad and Widijanto, 2002).
18.2.4 Multiple-point borehole extensometers
Among the most useful measurements of rock mass performance are those made
using multiple-point borehole extensometers (MPBXs) (Figure 18.3). A single-point
borehole extensometer gives the relative displacement between an anchor point inside
the rock mass and a measuring point, generally located at the excavation boundary. A
multiple-point extensometer can also give the relative displacements between several
points at different depths in the borehole. In this way, the distribution of displacements
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