|Tailings dam design at Mount Polley didn't recognize the risk of an underlying, unstable layer of material.|
The massive tailings dam failure at Imperial Metals’ Mount Polley copper-gold mine in south-central British Columbia, Canada, on August 4, 2014, was the result of failure of a glaciolacustrine layer (clays and silts left behind by a retreating glacier) about 8 m below the base of the dam in the area of the breach.
The Mount Polley Independent Expert Engineering Investigation and Review Panel, appointed following the tailings dam failure, delivered its final report on January 30. The report stated, “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 subglacial and preglacial geological environment associated with the perimeter embankment foundation. As a result, foundation investigations and associated site characterization failed to identify a continuous glaciolacustrine layer in the vicinity of the breach and to recognize that it was susceptible to failure when subject to the stresses associated with the embankment.”
The report also indicated that the steep slope of the tailings dam’s downstream rockfill zone contributed to the failure of the dam. 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 dam failure.
The panel also concluded that there was no evidence that the failure was due to human intervention or overtopping of the perimeter embankments and that piping and cracking, which are often the cause of failure of earth dams, were 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 the failure.
The panel drew its conclusions and recommendations based on an extensive investigation undertaken between August 2014 and January 2015. The work included independent engineering field investigations, data compilation, laboratory testing, and analyses. It also involved inspection of related documents in the files of the Mount Polley mine, its consultants and the Ministry of Energy and Mines.
The panel made seven specific recommendations to improve industry practices and reduce the potential for future failures. The full report can be accessed at www.mountpolleyreviewpanel.ca/final-report.
Following release of the panel’s report, British Columbia Minister of Energy and Mines Bill Bennett announced that the government would act immediately on key recommendations of the panel and launch a code review to consider the other recommendations. The ministry’s chief inspector of mines is requiring all operating mines with tailings storage facility dams to provide a letter by June 30 to confirm whether or not 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.
The government also will move to implement a new requirement that all operating mines with tailings storage facilities in British Columbia 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 storage facilities. Some mines in British Columbia already have similar boards in place.
The government also will initiate a code review to determine how best to implement the panel’s remaining recommendations, including the adoption of best available practices and technologies.
Imperial Metals acknowledged the panel’s report and said the company continues to make good progress in repairing the effects of the tailings dam breach, working closely with British Columbia’s Ministry of Environment and in cooperation with First Nations and local communities.