The Unexploded Cartridge
You are the shotfirer on a bench blast. The pattern is 60 holes, loaded with emulsion, electronic detonators, and stemmed. Twenty minutes before firing, the blast crew report that one hole, number 37, has been loaded but the detonator lead cannot be found. The hole is in the middle of the pattern. The exclusion zone is set at 200 m and the site is clear. A contractor's lorry has arrived at the weighbridge, 150 m from the bench, and the driver is walking towards the site office. The weather is clear and still; the blast is due at 14:00 to meet a customer deadline.
1. What is the immediate hazard and what must happen before the blast can proceed?
Read the worked answer
The immediate hazard is a misfire or unplanned initiation: hole 37 is loaded but its initiation lead is lost, so the firing circuit is incomplete and the hole may not fire as designed, or may fire unpredictably.
Before proceeding:
- Stop and secure the area. The exclusion zone must be enforced; the contractor's driver must be stopped and removed to a safe location beyond 200 m, or held at the weighbridge with the gate closed.
- Locate and trace the lead - carefully expose the detonator lead at the collar without disturbing the charge. If it cannot be found, the hole must be treated as a misfire.
- Do not fire the pattern until hole 37 is resolved. Firing a pattern with an unresolved hole risks a sympathetic initiation, flyrock, or a later unexploded charge in the muckpile.
- Record the situation and inform the quarry manager.
The customer deadline is irrelevant to the decision. The blast can be delayed.
2. If the lead cannot be recovered, outline the safe procedure for dealing with hole 37.
Read the worked answer
Treat as a misfire and follow the site's misfire procedure, which should include:
- Wait the specified time (typically 30 minutes) before approaching, in case of a slow or delayed initiation.
- Re-enter only with the shotfirer and a second competent person.
- Carefully remove the stemming and, if safe, the explosive, using non-sparking tools; or re-prime and fire the hole in a controlled way if the charge is intact and can be safely initiated.
- If the charge cannot be safely recovered, it must be flushed or neutralised under the supervision of a competent person, or the area around it clearly marked and excluded until it can be dealt with.
- Search the muckpile after firing for any unexploded explosive; any found must be made safe by the shotfirer.
- Record the misfire, the action taken and the outcome in the blast record.
3. How would you manage the contractor's lorry and the exclusion zone while the situation is resolved?
Read the worked answer
- Stop the lorry at the weighbridge and close the gate; the driver must not proceed towards the bench.
- Brief the driver on the exclusion and the reason, and keep them in a safe place with a radio or phone contact.
- Maintain the 200 m exclusion with sentries at all access points; the zone must not be relaxed because the blast is delayed.
- Communicate with the site office so no other traffic enters.
- Only release the lorry once the shotfirer confirms the blast has been fired and the area is safe, or the blast is cancelled and the area made safe.
A delayed blast is still a blast; the exclusion zone is not time-limited.