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Flawed design fingered in Mt Polley tailings dam failure

31st January 2015

By: Henry Lazenby

Creamer Media Deputy Editor: North America

  

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TORONTO (miningweekly.com) – British Columbia will immediately act on seven critical recommendations of an independent panel that investigated the tailings storage facility (TSF) failure at Canadian base metals miner Imperial Metals’ Mt Polley copper/gold mine in August last year and launch a code review to consider the panel’s other recommendations.

The tailings pond breach took place in the early hours of August 4 at the mine, in south-central British Columbia, near the town of Likely, releasing millions of litres of mine waste into the pristine natural environment.

The Mount Polley Independent Expert Engineering Investigation and Review Panel on Friday delivered a final report on its investigation into the cause of the incident, saying it had found that the breach was the result of a failure in the foundation of the embankment.

The strength and location of a layer of clay underneath the Mt Polley dam was not taken into account in its original design. As a result, this layer became overstressed, causing the portion of the dam on top of that layer to collapse.

According to the report: “The panel concluded that the dominant contribution to the failure resides in the design. The design did not take into account the complexity of the sub-glacial and pre-glacial geological environment associated with the perimeter embankment foundation."

The report was delivered to the Ministry of Energy and Mines, the T’exelc First Nation (Williams Lake Indian Band) and the Xat’sull First Nation (Soda Creek Indian Band).

“I am relieved to know the cause of the failure at Mt Polley and want to thank the panel for their important work. Now we can devote all our energy to taking a leadership role in Canada and internationally to learn from this and ensure it never happens again,” British Columbia’s Minister of Energy and Mines and Responsible for Core Review Bill Bennett noted.

The panel also concluded that there was no evidence that the failure was owing to human intervention or overtopping of the perimeter embankments and that piping and cracking, which was often the cause of the failure of earth dams, was not the cause of the breach.

In regard to regulatory oversight, the panel found that inspections of the tailings storage facility would not have prevented its failure.

The panel made seven recommendations to improve practises and reduce the potential for future failures.

EMBANKMENT REVIEWS

In the wake of Imperial Metals’ Mt Polley mining disaster, the chief inspector of mines for British Columbia Al Hoffman ordered independent third-party reviews of 98 permitted tailings impoundments at 60 operating and closed metal and coal mines in the province.

Under the Health, Safety and Reclamation Code for Mines in British Columbia, the deadline for a normal yearly dam safety inspection would have been March 31, 2015, and would not have required an independent third-party review.

However, the chief inspector accelerated the date for inspections to December 1 and added the requirement for an independent review by a qualified, third-party professional engineer from a firm not associated with the tailings facility.

The inspections and reviews had not identified any immediate safety concerns.

Hoffman had issued a new order requiring all operating mines with TSF dams to provide a letter by June 30, confirming whether similar foundation materials to those used at Mt Polley existed below any of their dams.

If those materials were found to be present, the letters would also be required to confirm whether adequate investigations and testing had been completed to properly understand the strength and location of those materials and that the dams were designed to account for those conditions.

In reply to the panel’s report, British Columbia would move to implement a new requirement that all operating mines with TSFs in the province establish independent tailings dam review boards. These boards would support improved engineering practices by providing third-party advice on the design, construction, operation and closure of TSFs.

Some mines in the province already had similar boards in place.

The province also stated that it would immediately start a code review to determine how to best implement the panel’s remaining recommendations, including adopting best available practices and technologies.

The regional government made public the inspections and third-party reviews of TSFs at other permitted mines in British Columbia, ordered by the chief inspector last year following the TSF's failure.

Government explained that it was investing in improved information systems so that this information could be routinely made public in future.

The inspections included site investigations, interviews with TSF operators and reviews of instrumentation and monitoring data. Further to submitting inspection reports and third-party reviews, companies also submitted a letter of commitment to the chief inspector outlining how they planned to implement any recommended work.

According to the province, there were currently 98 permitted TSFs with 123 dams located across 60 metal and coal mines in British Columbia – 31 of these TSFs were located at operating mines or mines under construction. The remaining 67 were located at mines that were either permanently closed or under care and maintenance.

Edited by Tracy Hancock
Creamer Media Contributing Editor

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