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Design flaw caused Mount Polley Dam Breach

JOC Digital Media
Design flaw caused Mount Polley Dam Breach

The Mount Polley tailings pond breach that released a slurry of mining waste into the Cariboo region of Central B.C. was caused by faulty design, an independent review panel concluded.

"The design did not take into account the complexity of the sub-glacial and pre-glacial geological environment associated with the perimeter embankment foundation," read a report released by the Mount Polley Independent Expert Engineering Investigation and Review Panel on Jan. 30.

The pond breached last August, releasing tens of millions of cubic metres of contaminated water and 4.5 million cubic metres of fine sand.

Designers failed to identify the deep layer of clay, which collapsed when stressed.

The report also indicated that the failure was triggered by construction of the downstream rockfill zone at a steep slope.

The panel concluded that had the downstream slope been flattened, failure would have been avoided.

The slope was in the process of being flattened to meet its ultimate design criteria at the time of the incident.

In its report, the panel also concludes that there was no evidence that the failure was due to human intervention or overtopping of the perimeter embankments and that piping and cracking. These factors are often the cause of the failure of earth dams, but 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 failure.  Regulatory staff are well qualified to perform their responsibilities and they performed as expected.

The panel examined the historical risk profile of other tailings dams in B.C. and concluded that to avoid risk in the future requires the adoption of the best applicable practices (BAP) and the migration to the best available technology (BAT).

The Ministry of Energy and Mines announced it would act on some of the key recommendations.

All operating mines with tailings dams will have to provide a letter by June 30 to confirm whether foundation materials similar to those at Mount Polley exist below any of their dams.

If those materials are present, the letters must also confirm whether sufficient investigations and testing were completed to properly understand the strength and location of those materials and that the dams were designed to account for those conditions.

"I am relieved that we have a cause," said Minister of Mines Bill Bennett. "Now that it has been identified, we can move forward."

The province will also move to implement a new requirement that all mines with tailings ponds establish Independent Tailings Dam Review Boards. These boards will support improved engineering practices by providing third-party advice on the design, construction, operation and closure of tailings ponds.

Lastly, the province will initiate a code review to determine how to best implement the panel’s remaining recommendations.

There are still two other reports due later this year that will examine the failure of the dam.

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