Design Failure Caused the Mount Polley Disaster

It’s been nearly six months since the tailings storage facility (TSF) at Imperial Metals’ (TSX:III) Mount Polley copper-gold mine was breached, releasing tailings into the surrounding area. However, the public is only now receiving answers about what went wrong

The Mount Polley Independent Expert Engineering and Review Panel, which is comprised of three geotechnical experts and was formed on the day of the breach, released its final report on the cause on Friday, noting that “the breach was the result of a failure in the foundation of the embankment, a failure that occurred in a glaciolacustrine (GLU) layer of the embankment’s foundation.”

That might sound confusing, but a recent CBC article provides a good breakdown of the report, explaining that, put simply, the disaster was caused mainly by design failure. Specifically, the design of the TSF “failed to take into account the complexity of the unstable underlying glacial and pre-glacial layers under the retaining wall.”

While not initially a problem, the instability of those layers became an issue as the TSF grew more and more full. Ultimately, the weight of the material in the TSF caused them to give way, with the breach being the result.

Elaborating to CBC, Jack Caldwell, an expert on tailings dams, said that Knight Piesold, the engineering and consulting firm that designed the TSF, knew what material the TSF was built on, but “misinterpreted … test results and didn’t really understand the nature of the strength, and the nature of how it could get weaker.”

According to the panel, that all means the breach was not caused by human intervention; regulatory oversight was also not the problem. Indeed, it emphasizes that inspections of the TSF would not have stopped it from failing. Imperial Metals is unsurprisingly pleased with the news, and said in a statement released on Friday that the panel’s determination of the cause of the breach matches the results of an investigation conducted by Mount Polley Mining, the mine’s operator.

Of course, key questions now are whether other TSFs may also be ticking time bombs, and what can be done to prevent future TSFs from not being built propertly. On that note, the panel includes in its report seven recommendations. They are as follows:

• Use a phased approach to implement the best available technology at new and existing TSFs. 
• Improve corporate governance at companies looking to operate TSFs.
• Expand the requirements for permits for TSFs.
• Use independent tailings review boards more frequently.
• Assess current TSFs in the province and determine whether they are at risk.
• Push the Association of Professional Engineers and Geoscientists of British Columbia to put together guidelines aimed at improving site characterization for tailings dams.
• Improve safety guidelines for TSFs.

At close of day Friday, Imperial Metals’ share price was up 6.04 percent, at $8.95. Prior to the breach, it was sitting at $16.80, and immediately after it was at $9.70. The company said Friday that it “continue to make good progress” in repairing the damage done by the breach.

Securities Disclosure: I, Charlotte McLeod, hold no direct investment interest in any company mentioned in this article. 

Related reading: 

Imperial Metals Down 42 Percent After Tailings Pond Breach