Photo Courtesy of Imperial Metals

Work is ongoing to build up the height of the failed dam at Mount Polley Mine before the melt.

The Mount Polley Mine earthen dam in western Canada failed because of a faulty design that didn't consider the geology of the perimeter foundation, an independent review panel says. The Aug. 4 incident dumped 4.5 billion gallons of water and 10.3 million cu yd of mine tailings and construction waste into nearby lakes and rivers.

In its Jan. 30 report, the panel, established to evaluate the cause of the incident, ruled out as causes human error, inadequate inspections, overtopping, or piping and cracking in the embankment. The panel prepared the report for the British Columbia Ministry of Energy and Mines, the Williams Lake Indian Band and the Soda Creek Indian Band.

The breach at the mine occurred near Lively, British Columbia. Investigations into the spill continue by the Ministry of Environment Conservation Officer Service, among others, which could lead to charges or fines for the owner, Imperial Metals Corp., Vancouver, B.C.

Ongoing cleanup at the mine will cost an estimated $67.4 million, says Steve Robertson, an Imperial spokesman. Work is underway to bring the dam to its former height before this year's spring melt.

The Mount Polley Mine's dam "design did not take into account the complexity of the sub-glacial and pre-glacial geological environment associated with the Perimeter Embankment foundation," the panel concluded.

"The omissions associated with site characterization may be likened to creating a loaded gun. … If constructing unknowingly on the upper [glacio- lacustrine] constituted loading the gun," building the slope over the strata at too sharp an angle "pulled the trigger," the panel concluded.

"Had the downstream slope in recent years been flattened" to a more gradual slope "as proposed in the original design, failure would have been avoided," the report says. The slope was "on the way to being flattened" when the breach occurred, it adds.

In its report, the panel also made several general recommendations to the government to help ensure mine-dam safety: improve safety guidelines, expand design requirement, establish and enforce a stronger regulatory framework, and use more independent tailings review boards.

"There's a lot of work to be done on [tailings review]," Bill Bennett, minister of energy and mines, said at a Jan. 30 news conference. "It's complicated, but it's something we need to do and will do."

As a result of the Mount Polley failure, Bennet has asked all currently operating mines with similar tailings ponds to ensure that unsafe conditions do not exist. Mines will be required to provide information on foundation materials and testing at tailings ponds by June 30. Additionally, Bennett says he will make independent tailings review boards mandatory for operating mines.