A roadheader was developing new entries at the mine when it came across a dyke (fault), and a decision was made to use full-face shotfiring to overcome the hard rock obstacle.
According to the safety alert, the initial shotfiring proceeded without incident but as the job was near completion there were three misfires.
While the detonators were correctly initiated, unexploded cartridges and parts of cartridges were found during mucking out and inspection.
From an investigation, assisted by the explosives supplier, it was found that the most likely cause of the misfires was desensitisation of the explosives.
NSW Industry and Investment’s Mine Safety Operations Branch said desensitisation could occur through transmission of the shockwave from an earlier firing hole, as well as physical deformation of the shothole from the earlier firing hole.
The safety alert outlined the following cases where there is a greater risk of desensitisation occurring:
- When full-face firing – the energy is not dissipated to a free face
- On softer ground – where plastic deformation can occur
- With overcharged faces – where hole spacing is too close
- With a poorly designed burn cut that does not provide adequate relief.
In this case, the mine safety branch said the most likely cause was a tight shotfiring pattern with high energy levels.
“This would also have been impacted by the changing ground conditions which resulted in softer ground at the time of the misfires,” the branch said.
As part of recommendations from the injury-free incident, the branch said shotfiring patterns should be designed or reviewed by personnel with the relevant technical expertise and knowledge of blasting in underground coal mines.
“This is particularly important with full-face firing.”
The second recommendation was that mines implement an audit system to ensure the drilled face matches the design of the shot.
