Canadian Manufacturing

Poor design caused Mount Polley tailings dam breach: report

A government-ordered report says the dam design didn't account for the area's complex geological make-up

VICTORIA—The construction of a mine tailings pond on top of a sloped glacial lake weakened the foundation and was akin to loading a gun and then pulling the trigger, a report on the disaster says.

A government-ordered report released January 30 said the spill that gushed 24 million cubic metres of mine silt and water into nearby lakes and rivers in B.C.’s Interior was caused by an inadequately designed dam that triggered the disaster.

The tailings dam at the Mount Polley mine burst without warning at 1 a.m. last August 4, hours after workers had been at the foot of the structure.

Norbert Morgenstern, chairman of the panel that investigated the spill, said evidence indicates there was a glacial lake deposit under the foundation of the dam.

“The design did not take into account the complexity of the sub-glacial and pre-glacial geological environment associated with the perimeter embankment foundation,” he told reporters gathered for the release of the report.

Morgenstern, a geotechnical engineer who has worked on 140 dam projects worldwide, said not taking that glacial lake into account and building on a weak layer was like loading a gun.

“But if constructing unknowingly on this upper (glacial) deposit constituted loading the gun, building with a steep slope … pulled the trigger.”

Morgenstern said the two factors together constituted the root cause of the failure.

He is among a panel of three geotechnical experts appointed by the province a few weeks after the disaster to investigate the cause of the collapse and the role of government regulation and oversight.

B.C. Mines Minister Bill Bennett said his government would act on the report’s recommendations.

He told reporters that some changes would be made immediately, including that mines with the same dams have the foundations of their tailings ponds checked and that those mines establish independent tailings dam review boards.

“This obviously was an unprecedented, disastrous event that we must do everything we can to make sure it never happens again.” Bennett said.

The report also indicated that the failure was set off by construction of the downstream rockfill zone at the steep slope.

There was no indication that the dam was about to give way and no evidence that the failure was due to human intervention or so-called “overtopping” of the embankments, the report said.

Last month, B.C.’s chief inspector of mines allowed the owner of the mine, Imperial Metal Corp., to start repairs on the tailings pond.

The main concern for reconstruction was to ensure that increased water flow from melting snow this spring won’t cause further environmental or human-health impacts, the chief inspector said.

A report released in November said the cleanup from the dam’s breach will take many years to complete.

The gold and copper mine near Williams Lake, B.C., about 600 kilometres northeast of Vancouver, remains closed.

Bennett couldn’t say if or when the mine would reopen, but added that he realized many people in the Cariboo region want the mine to begin operating again.

“We are going to take the time that is necessary to review the application we have from the company and certainly the use of the existing tailings-storage facility is not something that is planned for any time soon.”

The minister said the government will wait for the results of two more reports before it will even consider allowing use of the existing tailings pond.

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