The Treatment of Misfires in an Underground blasting operation - Introduction

A misfire is an unplanned and avoidable event.

Obviously, it is safer to prevent misfires than it is to treat and handle them, hence every means to prevent them must be exercised. Most misfires, although not all, occur as a result of some form of initiation system failure.

Modes of Misfires/Cut Offs

Common Causes of Failure

• Poor Design
• Poor Application
• Incorrect lace up technique
• Earth Leakage when using electric detonators
• Detonators not connected
• Tube or wire damaged during stemming
• Tube or wire damage to the lead-in firing system
• The whole unit is mishandled (e.g. Trodden on or thrown to the ground)
• Cut-off by cratering before the initiating process is complete
• Cut-off by rock movement (Inadequate timing design) - similar to the above
• Cut-off by ground shift within the blasthole
• Misuse of detonating cord
• Product failure, both explosives and initiation products.
  • This includes dynamic desensitisation of explosives and dynamic pressure damage to detonators

Almost all misfires that occur in mines relate to human error and incorrect product application. All explosive companies provide details of the best practices to be used for their products, and these instructions should be strictly adhered to.

SUSPECTED MISFIRE refers to:

1. Any remaining blasthole or portion of a blasthole which contains damaged shock tube, electric/electronic detonator wires, detonating cord or safety fuse.
2. Any remaining portions of a charged blasthole which is suspected to contain explosives, i.e. "Cutoff" or "Bootleg/Socket".
3. Any suspected unexploded charge.
4. A situation where safety fuse has been used and the number of shots counted is less than the number of blastholes fired.
5. Any situation with any reason to suspect that explosives charges have failed to explode, i.e. poor advance, uneven face shape, unusually large fragmentation and uneven floor conditions.

Misfires in Long Holes

Should a misfire or partial misfire be identified in a long-hole blasting operation or suspected to be a misfire, handling procedures differ from those associated with more conventional methods of blasting underground.

In the case of long hole raises that are not mucked out immediately after firing:

1. it is not possible to make a site examination close to the location of the misfired explosives,
2. the misfired explosives may be a considerable distance down the blasthole making it difficult to establish their condition, and
3. unfired explosives in the muckpile may well be broken up, dispersed or water desensitised in the normal process of mining.

It is not uncommon in long hole blasting operations for there to be more than one detonator in the blasthole, similarly, the blasthole may be "lined" with a length of detonating cord. It is particularly important to know this prior to treating any misfire of this nature.

In most cases where a misfire occurs in an uphole, unless you can access the area safely (which would be unlikely), the most efficient means to treat this would be to drill a relief hole, subject to the procedures stipulated at your operation, or in local legislation.

Misfires in Shaft Sinking

Identifying and finding misfires in shaft sinking operations are obviously more difficult once the blast has been taken. Generally, the misfire will only be found once the excavation process has begun. In most cases when the misfire has only been detected after the mucking-out process has begun, the most efficient and safe manner to treat these would be:

1. wash them out with water and compressed air and remove the old explosives, taking care with the primer removal, and
2. retrieve the old product and remove these from the shaft bottom and
3. handle the old explosives in the manner specified in your local procedures, or as per the local legislation directs.

Mucking out can then continue as normal and the resultant "bump" in the shaft bottom can be handled in a practical manner, either by the removal of the higher rock profile by drilling and blasting as a separate event, or in conjunction with the next blast. It is most likely that the latter will be the preferred method from a practical point of view.

1. Where it is unsafe to remove the primer cartridge, then the drilling of a relieving hole, subject to your local rules with be the safest way to treat these.

Identifying Misfires in the Workings

Successfully identifying misfires in a blast is a critical part of the post-blast inspection process. Apart from the blast completely failing, the following indications should be looked for:

1. Any visible evidence of undetonated explosives within, or on the post-blast muckpile,
2. Any visible part of the face indicating unbroken ground,
3. Course material in the muckpile,
4. Any overhangs in the side and back of a tunnel or stope.

When misfires occur that were undetected in the standard post-blast inspection, these will become evident in the loading out/mucking of the broken rock. The following would be indications of a misfire during this process:

1. Any visible evidence of undetonated explosives within the muckpile,
2. Visual evidence of tubes or wires hanging from the face or stope,
3. Coarse material in parts of the muckpile, generally inconsistent with the rest of the rock.