The anatomy of a tragedy
|Published
Churchs and community Join in prayers at the mine for the the three Lily Mine workers still trapped underground.285 Photo: Matthews Baloyi 21/02/2016 Churchs and community Join in prayers at the mine for the the three Lily Mine workers still trapped underground.285 Photo: Matthews Baloyi 21/02/2016
Barberton - The geology of Lily Mine is known, but the reason for the failure that left three miners trapped underground can only be understood after a full investigation.
This is according to Kim Ncube-Hein, a professor of mining geology at Wits.
Each mine has its own geological constraining problems and there are multiple reasons for failure for any given mine.
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Given that mines are inspected and that should have happened at Lily Mine, and given that the mine has engineers and geologists who understand rock stresses and potential for failure, the cause of disaster at Lily Mine is not immediately apparent, she said.
Ncube-Hein said it could, for example, “be as simple as the rock mass dehydrating because South Africa has had an unseasonably hot summer and the ground has dried”.
“Or the opposite: the rock has become water saturated in a major downpour.
“Ground shaking, the rumbling of trucks, inherent weaknesses in the ground can also be the cause. We will need a full assessment.”
Ncube-Hein said when a mine collapses, geologically, it’s because the rock strength, meaning the ability of rock to support weight, is overcome by the force of gravity and it collapses.
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In the case of Lily Mine, where a sinkhole formed, Ncube-Hein said this was technically not a mine failure but gravity.
A sink occurs because the underlying substrata are eaten away by ground water, such as carbonate leaving a hole, she said.
In the case of mining, the miners are eating away spaces, development drives and panels to recover the gold.
In Lily, the miners had eaten away the rocks at depths three levels below the failure level, which is the volume of rock between, much like a three-storey building, but its floor is quite thick, metres thick.
She said the rock mass left between the surface and the top level is called the crown pillar – it’s the crown that sits on top of the underground workings.
It’s left to preserve the stability of the underground mining, said Ncube-Hein.
At Lily Mine, it seems this fractured, failed and collapsed, closing all the levels below as it collapsed inward.
The container slid into the hole as it was collapsing, she added.
When asked about contingency plans, she said these varied.
“Mostly, the mine is evacuated completely, but it depends on the speed of collapse.
“Most collapses are monitored using laser to make sure no rock mass is moving and if it is, the region is designated a no-go zone.
“That region is just left to fail and no workers may enter,” Ncube-Hein said.
She said it was important to understand that all mines would eventually collapse.
But this is managed with the back filling of spaces with waste rocks and the like.
In the case of Lily, the failure was completely unexpected and rapid.
There was no warning or signs of potential failure as far as I know, she said.
Regarding whether mines understand that such collapses can take place, Ncube-Hein said failure is mitigated, but the type of failure experienced at Lily Mine was unexpected. Crown pillars should never fail.
Something went wrong at this mine that was completely unforeseen, but only an investigation could establish that conclusively, she said.
Ncube-Hein also emphasised that the state of a particular mine was discussed in an extensive meeting every morning.
“I stress every day, and every mine does this. It’s standard practice and compulsory. Management is fully aware of the risks.
“In my many years in mines, I have never known a business that spends more time worrying about stability and safety than the mining industry,” Ncube-Hein said.
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