A roof fall that occurred in an underground coal mine in New South Wales has highlighted the need for quick response when a tell-tale detects roof convergence.
The incident occurred over a 24-hour period and was first noticed when a tell-tale detected movement in the roof of a mining place.
This movement was continually monitored by the panel deputy and undermanager on shift. Mining proceeded until monitoring indicated an increase in displacement to a level which started a trigger action response plan (TARP).
9am
The undermanager, in accordance with the TARP, withdrew the continuous miner back to the 10m chainage mark and commenced upgrading the roof support to code red.
At this point, senior management was notified and confirmed the actions being taken.
12.45pm
The undermanager notified the geotechnical engineer that the tell-tale had moved to 40mm and continued installing additional support with the concurrence of the geotechnical engineer.
5pm
The undermanager notified the geotechnical engineer that the tell-tale had continued to move to 60mm and that roof deformation was noticeable.
As a precaution, the continuous miner was pulled back to the intersection and the erection of additional support was stopped.
9pm
The geotechnical engineer inspected workings and saw visible signs of active roof deformation, roof support loading and audible strata noise. The tell-tale at this point read 110mm.
All workers and machinery were withdrawn and a breaker line chock timber was installed on the inbye side of the intersection and the place was barricaded.
9am the next day
A fall occurred inbye of the tell-tale at 28m chainage, about 12 hours after all workers were withdrawn, and extended towards the face.
Regulatory response
The NSW Resources Regulator investigated and said the incident highlights the need for prompt action when strata monitoring equipment indicates that roof convergence is occurring.
“Site specific ground support TARPs should incorporate the timely installation of appropriate standing support where rates of convergence indicate accelerating deterioration in the stability of the monitored area,” the Regulator said.
“The ongoing improvement of geotechnical inputs into ground support design has significantly reduced the incidence of such failures over the last decade, however strata failure remains a fundamental risk with localised geological anomalies potentially affecting the adequacy of the ground support as it was designed and installed.
“Ground support designs will ideally be able to tolerate localised changes that demand more load-carrying ability than the installed ground support.”
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