Misfire in a Development Heading
You are a shotfirer in an underground metalliferous mine. A development heading is being blasted with a burn cut. The round consists of 45 holes, including 4 empty parallel holes for the burn cut. The explosive is a bulk emulsion, initiated with electronic detonators. The blast is fired, and after the re-entry period and ventilation, you inspect the face. You observe that the burn cut has not pulled properly; there is a large bootleg (remaining explosive) in one of the cut holes, and several other holes have not detonated. There is also some smoke and fumes lingering. The site's misfire procedure requires that you wait 30 minutes before re-entry, which has passed. The heading is 4 m wide by 4 m high. You have a crew of two miners waiting outside. The shift supervisor is asking for an update.
1. What are the immediate actions you must take upon discovering the misfire?
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The shotfirer must immediately treat the situation as a misfire and follow the site's misfire procedure. First, ensure that no one enters the area except authorized personnel. The shotfirer should barricade the entrance and post warning signs. The shotfirer must not attempt to remove the bootleg or unexploded charges without a proper plan. The area should be made safe by ensuring that no ignition sources are present. The shotfirer should inform the shift supervisor and the blasting engineer. The misfire should be reported to the appropriate authorities as required by legislation. The shotfirer should also check for any injuries and ensure the crew is safe. The next step is to assess the situation: determine which holes misfired, the condition of the bootleg, and whether there is any immediate danger of accidental detonation. The shotfirer should not re-enter the face until it is safe to do so, and should consider using remote methods to inspect if possible. The shotfirer should also check that the ventilation is adequate to clear fumes. The misfire procedure may require waiting a further period before attempting to deal with the misfire. The shotfirer should document everything.
2. How would you safely deal with the misfire, and what are the key risks and controls?
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The safe handling of a misfire requires a systematic approach. The preferred method is to re-blast the misfired holes if possible, but this must be done with extreme caution. The shotfirer should first try to remove the stemming from the misfired holes using non-sparking tools, such as wooden or copper rods, and then place a new primer and explosive to re-initiate. However, if the bootleg contains explosive, it may be dangerous to drill or remove it. In that case, the shotfirer may need to use a controlled approach: drill a parallel hole at a safe distance (at least 300 mm) and charge it to dislodge the bootleg. Alternatively, use a high-pressure water jet to wash out the explosive, but this is only allowed if the explosive is water-resistant and the procedure is approved. The key risks include accidental detonation during handling, exposure to toxic fumes, and rockfall. Controls include: using non-sparking tools, wearing appropriate PPE, ensuring good ventilation, having a safety officer present, and following the site's misfire procedure exactly. The shotfirer must also consider the possibility of delayed detonation and ensure that no one is exposed. After dealing with the misfire, the shotfirer should inspect the area for any remaining explosive and clear it. All actions should be recorded. The shotfirer should also investigate the cause of the misfire (e.g., detonator failure, cut failure) to prevent recurrence.