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Rock Mechanics.pdf - Mining and Blasting

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Figure 17.16 (a) Ground motion<br />

recorded during sympathetic detonation<br />

of charges; (b) reduction of vibration<br />

amplitude as a result of overbreak<br />

(after McKenzie, 1988).<br />

Figure 17.17 Malfunctions in an<br />

underground blast (after McKenzie,<br />

1987).<br />

EVALUATION OF EXPLOSIVE AND BLAST PERFORMANCE<br />

is necessary to consider transducer polarity <strong>and</strong> the records of both the upper <strong>and</strong> the<br />

lower transducer arrays. It transpires that packet 3 is associated with delay 10 in hole<br />

A <strong>and</strong> packet 4 with that in hole B.<br />

One of the most significant outcomes of near-field vibration monitoring <strong>and</strong> analysis<br />

of the type outlined has been the identification of a high frequency of occurrence<br />

of charge malfunctions in production blasts (McKenzie, 1987, 1988). Traditionally,<br />

a misfire is defined as an explosive charge which does not detonate, typically due to<br />

an initiation system fault. Recent experience has shown that such misfires are relatively<br />

uncommon. More common causes of charge malfunction are delay inaccuracy,<br />

instantaneous detonation, sympathetic detonation (Figure 17.16a), explosive desensitisation<br />

or overbreak (Figure 17.16b). In the underground production blast recorded<br />

541

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