9.1. EXAMINATION OF SHOT
The shot firer is responsible for examining the site to ensure that no unfired explosives or additional hazards have been created and that it is suitable for work to commence again. Before examining the shot, consider the potential for hazards such as noxious fumes to be present and the ground integrity of the immediate and surrounding area. In surface blasting these hazards can be effectively controlled by using sight and smell for determination of risk acceptability. If no misfires are evident, the ‘all clear’ signal can be given and the blast guards dismissed.
9.2. MISFIRES
A written procedure or standard work instruction is required that provides a safe system of entry and inspection for misfires and their treatment. It is to include the method used for the detection of a misfire. The precautionary interval allowed before the shotfirer can conduct an examination of the site and the recording and treatment of misfires. A written record of the location and details of the misfired shot is to be kept. By definition in the Explosives Act 1999 an “explosive incident” is an event, including a misfire, with the potential to cause death or injury to persons or unexpected damage to property and as such is required to be reported to the Chief Inspector of Explosives (s55 Explosives Act 1999).
9.2.1. Determination of Misfires
Every hole that has been charged with explosives is considered a misfire until proven otherwise. Methods used to determine if a misfire has occurred are based on many factors, including appropriate training, standard operating procedures and guidance from AS 2187.2. There are certain events that indicate a misfire has occurred, these include:
(a) If using safety fuse, the number of shots counted is less than the number of holes fired or a disagreement on the count of shots fired.
(b) If damaged safety fuse, detonating cord, lead wires or unfired signal tube is exposed in a hole that has been fired.
(c) Evidence of cutoffs, butts or remaining portions of holes (e.g. boulders with drill holes) that are suspected of containing explosives has been shown to be free of explosives.
(d) Holes that have slumped between charging and firing due to dispersion of the explosive from water ingress or through joints and fissures.
(e) If during the normal excavation of the blasted ground, uninitiated or residual explosives are found or the load out machine encounters poor ‘diggability’ of the blasted ground.
The shotfirer should conduct a careful examination amongst the debris for explosives, which if present shall be removed to a safe place and disposed of in accordance with Section 10.1 of this Guidance Note.
9.2.2. Treatment of Misfires
The method used to treat a misfire should be based on a risk assessment and a combination of other factors, including appropriate training, standard operating procedures and information in AS 2187.2. The following methods can be utilised for the treatment of potential and determined misfires:
(a) To remove the hazard of residual explosives and blasting gases trapped within the blast it is best practice to use water sprinklers on the shot area immediately after firing.
(b) Stemming may be removed by applying water under pressure, compressed air, or a mixture of water and compressed air through a non-ferrous blowpipe. After removal a fresh primer can be inserted and the blasthole stemmed and fired. Note: The use of compressed air alone is not encouraged. Where it is used, special precautions should be taken to minimise the dangers from static electricity and impact (AS 2187.2)
(c) If the down lines are considered to be in good condition, an attempt may be made to refire.
(d) Drilling a relieving hole parallel to the original blasthole and charging and firing.
(e) If a misfire is suspected at any time during mining operations, the operations have to cease and a detailed inspection conducted by a shotfirer, or competent person.