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Rock Mechanics.pdf - Mining and Blasting

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Figure 18.1 (a) Single point convergence<br />

measurement with a tape extensometer,<br />

Telfer Gold Mine, Australia<br />

(after Thompson et al., 1993); (b)<br />

three-point convergence measurement<br />

with a tape extensometer, PT Indonesia’s<br />

Deep Ore Zone Mine, Indonesia<br />

(after Rachmad <strong>and</strong> Widijanto, 2002).<br />

MONITORING SYSTEMS<br />

convergence values. An example of the use of convergence rates is given in the<br />

case history discussed in section 18.3.2 below. Mechanical convergence monitoring<br />

stations are robust, simple <strong>and</strong> inexpensive to install, <strong>and</strong> the results obtained are<br />

usually reliable. However, because of potential obstruction to accesses <strong>and</strong> working<br />

places, they are generally not suitable for remote or continuous reading. Regular<br />

manual reading of large numbers of stations can be time-consuming.<br />

18.2.4 Multiple-point borehole extensometers<br />

Among the most useful measurements of rock mass performance are those made<br />

using multiple-point borehole extensometers (MPBXs) (Figure 18.3). A single-point<br />

borehole extensometer gives the relative displacement between an anchor point inside<br />

the rock mass <strong>and</strong> a measuring point, generally located at the excavation boundary. A<br />

multiple-point extensometer can also give the relative displacements between several<br />

points at different depths in the borehole. In this way, the distribution of displacements<br />

549

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