In a Jan. 30 media release, an independent review panel delivered the final report on its investigation into the tailings storage failure at Mount Polley mine in British Columbia last summer.
The Mount Polley Independent Expert Engineering Investigation and Review Panel – made up of three geotechnical experts – concluded the Aug. 4, 2014 breach was caused by a failure in the foundation of the embankment.
The breach released about 31 million cubic yards of metals-contaminated matter into surrounding water systems, threatening valuable salmon fisheries in the process.
The report states “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 GLU layer in the vicinity of the breach and to recognize that it was susceptible to undrained failure when subject to the stresses associated with the embankment.”
The report also concludes the failure was triggered by construction of the downstream rock fill zone at a steep slope. Had the slope been flattened, the Panel reported a failure would have been avoided. The slope was in the process of being flattened when the breach occurred.
However, the report finds no evidence that the failure was due to human intervention or overtopping of the perimeter embankments, and that there were no signs of piping and cracking. Inspections of the tailings storage facility would also not have prevented failure, the Panel concluded.
Examining the historical risk profile of other tailings dams in British Columbia, the Panel concluded the adoption of Best Applicable Practices and Best Available Technology would avoid future risk. This would include improving corporate design responsibilities, the adoption of independent review boards, using more compacted, unsaturated tailings, and reducing the use of water covers in a closure setting.
Following the report’s release, the B.C. Ministry of Energy and Mines issued orders for all mines in the province utilizing tailings storage facilities and dams to confirm whether foundational materials similar to those used at Mount Polley are being used. There are currently 98 such facilities in the province, and 123 dams at around 60 metal and coal mines. The Ministry further announced it would act immediately on key recommendations outlined in the report.
The full report, appendices and supporting information can be found at http://www.mountpolleyreviewpanel.ca.
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