An explosive incident occurred recently where an unplanned detonation was initiated at a development face within an underground metalliferous mine. While the operator was lowering the jumbo drill rig's front jacks, they heard a loud 'bang'. The operator depowered the rig and inspected the area.
Immediately below the jack leg, there was a visible depression, as well as a short length of detonator signal tube. A small amount of smoke also surrounded the drill rig's undercarriage, but there were no signs of mechanical or structural damage to this machinery.
Fortunately, the operator was unharmed.
The exact amount of detonated explosive material was not able to be determined.
There were no signs of misfire or unusual ground conditions, and it was not conclusive whether the detonator in question had been intended for use as part of a previously fired blast.
Figure 1: Floor depression below Jumbo Jack Leg
Unplanned initiation of explosives can create significant risks to the health and safety of employees and independent contractors, through exposure to harmful energy, flying objects and contaminated atmospheres.
Recommended ways to control risks
To reduce the risk of unplanned initiation of explosives, mine operators must:
- perform a risk assessment to identify all potential hazards and assess the adequacy of all considered control measures, including the potential for unplanned initiation of explosives
- ensure processes and systems to identify uninitiated explosives (i.e. misfires) are implemented and effective
- ensure processes and systems to control the risk associated with uninitiated explosives are implemented and effective
- ensure processes and systems are in place for managing and accounting for explosive stock levels during each stage of the blasting activity, including storage, transport, pre and post blasting
- ensure pre-task inspections target uninitiated explosives post blasting activity
- ensure employees are provided with information, training and instruction about how to identify and manage uninitiated explosives in a mine
Mine operators should consult with employees and include a broad range of expertise and personnel when developing risk assessment teams to ensure all aspects of risk are considered.
Under the Occupational Health and Safety Act 2004, employers must, so far as is reasonably practicable:
- provide and maintain a working environment that is safe and without risks to the health of employees and independent contractors
- provide employees with the necessary information, instruction, training or supervision to enable them to do their work in a way that is safe and without risks to health
- ensure that persons other than employees of the employer are not exposed to risks to their health or safety from the conduct of the undertaking of the employer
This means employers must eliminate, so far as is reasonably practicable, the risks associated with explosives in the workplace. Where the risk cannot be eliminated, it must be reduced so far as is reasonably practicable.
Under the Occupational Health and Safety Regulations 2017 (OHS Regulations), mine operators must:
- identify all mining hazards at their mine and assess associated risks, so far as is reasonably practicable
- implement risk controls that eliminate or reduce risks, so far as is reasonably practicable
- establish and implement a safety management system for the mine, which provides a comprehensive and integrated management system for all identified risks
Under the Dangerous Goods (Explosives) Regulations 2011, shot firers must:
- not use explosives unless the shot firer has first prepared a blast management plan in accordance with the applicable requirements of AS 2187.2
- after blasting has been carried out, ensure the site is carefully inspected for the presence of unfired explosives or misfires, and the shotfirer, or the person authorised by the shotfirer, is satisfied that no misfires have occurred, before any further work is commenced at the site
- treat any misfire as required in the blast management plan