Late last week the independent review panel looking into the Mount Polley tailings dam breach released its report. Chief among the finding was that the dam was sited on top of a glacial lake. The weakness of the underlying clay eventually gave way.
The Mount Polley gold-copper mine 100 km northeast of Williams Lake belongs to Imperial Metals Corp. of Vancouver. The tailings dam burst at 1 a.m. last Aug. 4, sending 24 million m3 of material – tails, earth, trees – racing down Hazeltine Creek and into Quesnel Lake. The company has been cleaning up and remediating the area ever since.
The review panel consisting of three geotechnical engineers pointed squarely at the weak underlying glacial layer and the steep slope of the dam. In the words of the panel chairman, "The design did not take into account the complexity of the subglacial and pre-glacial geologic environment associated with the perimeter embankment foundation."
Imperial Metals issued a news release immediately after the conclusions of the panel were made public. It made its own investigation into the breach, and the company says its findings point toward the same cause: the dam failed because a glaco-lacustrine layer lying approximately 8 metres below the base of the dam in the area of the breach was not as strong as had been assumed in the design of the tailings storage facility.
Imperial also pointed out that the failure was not due to human causes, overtopping, cracking of materials or internal erosion. The water accumulating behind the dam was not the reason it failed.
That pesky layer of clay was assumed to be stronger than it turned out to be.
No one is laying blame that the original dam designers did less than their best work. No one is hinting that the enforcement of provincial regulations was in any way lax.
The Mining Association of Canada issued its own news release promising to review the information and recommendations of the panel with a view to enhancing tailings dam safety. MAC has already reviewed its own three tailings management guides and its Towards Sustainable Mining initiative.
MAC president and CEO Pierre Gratton said, "We believe that every effort must be taken to prevent failures and we support continual advancement in their design, operation and management."
So that's it in a nutshell. An unfortunate occurrence? Yes. An opportunity to sharpen everyone's knowledge about and design of tailings management facilities? Yes and with renewed determination.
In this writer's experience, Canadian miners are terrific problem solvers. This problem will be solved, too, and the chance of another similar one will be less for their efforts.
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