By The Australian- news
October 07, 2006
October 07, 2006
BEACONSFIELD goldmine managers were warned of the potential for disaster three months before the Anzac Day rock fall that killed miner Larry Knight and trapped two others.
A confidential safety report by leading mining experts AMC Consultants, obtained by The Weekend Australian, warned in January that seismic damage to sections of the mine "extends beyond the support capacity".
"Based on AMC's understanding of seismicity at Beaconsfield, the potential exists for further large and damaging seismic events," wrote senior geotechnical engineer Glenn Sharrock.
The report was commissioned by mine management, which is controlled by Allstate Explorations, in the wake of a rock collapse on October 26 last year, only metres from the subsequent deadly rock fall on April 25.
AMC puts the cause of the October rock fall, which was a significant seismic event, down to mining activities and "unfavourable mine geometry".
AMC delivered its findings on January 30, urging improvements to ground support at the 1km-deep underground mine in Tasmania's north.
The October rock fall occurred in sections at 915 and 925 metres below ground, the same areas hit by the Anzac Day rock fall that killed Knight and entombed Todd Russell and Brant Webb for two weeks.
Miners had long complained about working in these production tunnels, or stopes, because of their instability.
However, miners yesterday told The Weekend Australian that some key improvements to mining methods made in the wake of the October rock fall were not applied in level 925.
Miners say work in 915 and 925 was suspended for four months after the rock fall.
Afterwards, they say ground support was improved by greater overlapping of mesh, which was bolted with razor straps, increased cable bolting (6m cables bolted to the rock face) and cone bolting (2.4m rock bolts installed with chemical agents) in some areas.
However, miners given the task of installing ground support said no cone bolting was carried out in 925, while some said cable bolting was not done in the critical area of the tunnel where Knight died.
Increased cable bolting was suggested by Dr Sharrock, while cone bolts were recommended by another consultant called in after the October rock fall.
Last night, the mine's chief geologist, Peter Hills, denied that some improvements to mining methods after the October rock fall were not made in level 925.
"That is not correct - mine management followed the recommendations of the report and discussed the implementation with the consultants regarding rock support at level 925," Mr Hills said.
He confirmed cone bolts were recommended, but said they were not "specifically recommended" for level 925.
Miners also said a new, safer mining method, known as checkerboarding, had only just begun to be implemented and was not used in 925.
They also claimed that a suggestion by Dr Sharrock to improve mine stability by increasing the size of unmined intervals between layered mine tunnels had not been applied in the area of 925.
Dr Sharrock found that shifting to checkerboarding - involving the extraction of parts of each level, rather than mining entire levels in sequence - could limit the prospects of further major rock falls.
"If the fault slip model is correct, then it should be a relatively simple task to adapt the checkerboard sequence to reduce the chances of a similar situation developing in the future," he wrote in January.
Mr Hills said checkerboard was in the process of being implemented in 915 and 925 after their reopening in March. "Safety has always been the first priority of management," he said.
However, miners said this far safer method of mining was only beginning in new, deeper reaches of the mine yet to begin full production.
There was about 10m between the floor of 925 and the floor of 915, leaving about five or six metres of unmined rock between the roof of 925 and the floor of915.
Miners said a far more cautious sub-level interval between production stopes had applied at higher levels of the mine, with the floor of some levels 25m below the floor of the level above.
However, miners claimed management had halved the size of these intervals as the gold seam became richer, to reduce the amount of gold left unmined.
This policy had seen the floor-to-floor distance reduce from 25m at levels above 680m to only 10m at 925.
Dr Sharrock's report found that: "Based on the modelling, there is scope to increase the
sub-level interval to decrease damage in pillars and around development."
Mick Borrill, a jumbo drill operator who worked in 925, claimed management ignored pleas by miners to leave support pillars in place.
"The miners told management time and time again 'you've got to leave pillars there' and they said 'there's too much gold there'," Mr Borrill said.
Allstate chairman Rodney Elvish said he could not comment as the mine accident was still the subject of a Tasmanian Government inquiry, headed by barrister Greg Mellick, and a coroner's inquest.
Mine manager Matthew Gill was in Canada examining new mining methods and unavailable for comment.
The accountant who controls the mine, Allstate administrator Michael Ryan, through a spokesman, said it would be inappropriate to comment while the matter was still before an inquiry and the coroner.
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