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Mount Polley mining pond disaster investigation

News | February 12, 2015 | By:

Poor design ’loaded the gun,‘ steep slope ’pulled the trigger’

Building an unlined tailings pond over a weak foundation was step one leading to a disastrous dam breach that unleashed millions of cubic meters of sludge into adjacent lakes and rivers in south-central British Columbia, Canada.

The widespread destruction occurred in the early morning hours last August 4 at the Mount Polley copper and gold mine.

Investigators termed the step-one pond embankment construction as “loading the gun.” Step two-“pulling the trigger”-was building a downstream rockfill zone at a steep slope of 1.3 horizontal/1.0 vertical.

The official 156-page “Report on Mount Polley Tailings Storage Facility Breach” was commissioned by the British Columbia Energy and Mines minister. The independent investigation panel issued its report Jan. 31, 2015. Its conclusions stated:

…the dominant contribution to the failure resides in the design. The design did not take into account the complexity of the sub-glacial and pre-glacial geological environment associated with the perimeter embankment foundation. As a result, foundation investigations and associated site characterization failed to identify a continuous GLU [upper glaciolacustrine] 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 specifics of the failure were triggered by the construction of the downstream rockfill zone at a steep slope of 1.3 horizontal to 1.0 vertical. Had the downstream slope in recent years been flattened to 2.0 horizontal to 1.0 vertical, as proposed in the original design, failure would have been avoided. The slope was on the way to being flattened to meet its ultimate design criteria at the time of the incident.

In its concluding recommendations, the report urged a migration to “best available technology” (BAT). One panel member specifically cited a multi-metal mining operation in Alaska-the Greens Creek mine on Admiralty Island in the far southeastern part of the state. He noted that this mining site included a “dry stack tailings facility” constructed with “a geosynthetic liner” that prevented seepage, protecting groundwater.

Ron Bygness is the senior editor for Geosynthetics magazine.

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