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PRODUCTION BLASTING Diagnostics of Production Blasts in a Deep Underground Mine Study reveals significant discrepancy between the vibration energy released and what was measured in highly stressed stopes By B. Mohanty, D. Zwaan and F. Malek An extensive blast monitoring program has been carried out to study explosive performance and the effectiveness of blast designs in production blasts in a deep underground mine. Comprehensive monitoring of blasting vibrations through multi-station high-capacity, high-frequency accelerometers, and borehole deformation history have been the main tools of such analysis. The mining activity involves relatively unstressed stopes as well as highly stressed stopes. More than 30 production blasts have been monitored, and the respective vibrations from close-range as well as distant seismic stations have been analyzed, and the vibration parameters (amplitude and frequency) have been correlated with explosive charge weight in the holes. The correlation between blasting rounds and the corresponding vibration amplitudes often have been poor for a very significant portion of the blasts. These point to either explosive charge malfunction in those specific blast holes, or inadequate blast design, or the effect of high insitu stresses, or any combination thereof. Introduction Misfires or any other type of malfunction in a surface blast, be it construction, quarrying, or large open-pit operation, is easily detected. Once the source has been identified, remedial action can be taken to ensure productivity and safety (Farnfield, 1966; Segarra et al, 2009). However, the situation in an underground operation is very different, especially in stope blasting or any other mass mining operations. In such cases, fan drilling and vertical retreat mining (VRM) are the usual methods employed. The boreholes in this case could have a single column of explosives or incorporate several explosive decks with various delays. Any missing or misfired holes in this case are extremely difficult to detect after the blast because it is not possible to uniquely identify the source of a poor blast. In such cases, the presence of over52 E&MJ; • AUGUST 2013 Figure 1—Typical ring blast layout. size fragments cannot be ascribed to the blast in question. Standard blast monitoring through commercially available vibration monitoring is sometimes employed to monitor some of these blasts, but for reasons explained in this article, it is not possible to carry out a truly diagnostic analysis of such blasts and identify any particular malfunctioning blast hole. Experimental Program The study was carried out at a deep underground mine. A typical cross section of a stope blast design at the mine is shown in Figure 1. The borehole diameter was 162 mm, with varying depths as shown. A booster-sensitive packaged water-gel slurry explosive with a density of 1.15 g/cc was used throughout. The quoted effective RWS (i.e. gas expansion up to 100 MPa) by the manwww.e-mj.com