An independent investigation has determined the failure of the Mt Polley mine tailings dam was caused by a failure in the foundation of the embankment caused by its design.

The report, which was released on Friday morning in Victoria, said the design failed to take into account the complexity of the instability of underlying glacial and pre-glacial layers under the retaining wall.

But the authors, geotechnicians Norbert Morganstern and Steve Vick, did not blame provincial inspectors for not detecting the problem beforehand.

The investigators said the dam was built on a weak layer of glacial deposits that was undetected at the time, making the dam like a ‘loaded gun.”

When the slope on the embankment was steepened as the dam raised, that was “like pulling the trigger,” causing the failure last summer.

The volume of water in the tailings dam did not cause the dam to fail, but it did cause more tailings to be released once it did fail.

The investigators also made several recommendations to improve the safety of tailings dams, including updating the way they are designed in B.C.

B.C.’s Minister of Energy and Mines Bill said he was thankful the panel was able to narrow down the failure to a specific cause.

“I am relieved that we have a cause,” said Bennett. “Now that it has been identified, we can move forward.”

Bennett said the government would take action based on the report, including the inspection of existing tailings dams for similar risks, the establishment of review boards for existing dams, and a core review of existing regulations.

“There should be zero failures of this nature….Everything must be done to ensure this does happen again.”

Bennett said there are still two other reports due out later this year into the failure of the dam.

source: Government of BC