Design flaw led to Mount Polley breach say investigators

An independent review panel has concluded that the Aug. 4, 2014 breach of the Mount Polley tailings storage facility (TSF) in British Columbia was the result of a design flaw, not human intervention, overtopping or cracking. The panel determined that designers took into account the strength of the existing foundation of unconsolidated glacial materials, but didn’t account for those changing as the dam grew higher and the load on the foundation increased. Further, they noted the “unprecedented steepness” of the perimeter embankment, which contributed to the conditions. The combination of the steep embankment and weakened foundation material created a situation the panel called “a loaded gun.”

Regular inspections would not have prevented the breach, concluded the panel, which also wrote that the failure happened despite a “strong regulatory process and personnel.”

The panel had several recommendations, including improving corporate governance, strengthening regulatory operations and to use the Best Available Technology (BAT) to move toward a zero failure rate for tailings dam facilities. “The Panel firmly rejects any notion that business as usual can continue,” the authors wrote.

The Canadian Minister of Energy and Mines, Bill Bennett, said that the panel’s recommendations would be put into effect right away, and has ordered working mines to determine if similar foundation material exists in their tailings dam facilities. Additionally, all mines operating with TSFs must establish Independent Tailings Dam Review Boards.

Members of the panel included three experts in tailing management facilities, Norbert Morgenstern, Steven G. Vick and Dirk Van Zyl.

After the breach, Knight Piésold Ltd, the designer of the Mount Polley TSF, stated that its design had undergone independent reviews during construction (1995-1997) and again in 2006. Knight Piésold handed over responsibility for the design, construction and monitoring of the dam to another company in 2011.

Knight Piésold Ltd is one of the consultants that has worked on the tailings dam design for the proposed Pebble mine.

More about tailings storage facilities (PBS.org, July 2012)