Its design failed to take into account the complexity of the unstable underlying glacial and pre-glacial layers under the retaining wall.
A second report from B.C.'s chief inspector of mines will look at who should take the blame for the massive breach, but Jack Caldwell, a leading North American expert in the field of tailings dams, says it's clear that key questions remain unanswered.
Caldwell, who has more than 40 years' experience, questions why the tailings pond was built atop unstable ground in the first place — and indicated a perfect storm of little problems led to the massive breach at Mount Polley.
"It raises some very interesting questions, whether a reasonable engineer could have, should have, understood those properties and taken them into account in their design," Caldwell told CBC News.
Tailings facility built atop unstable glacial till
It appears the engineers and designers overlooked or dismissed that the ground a mere eight to 10 metres below the tailings pond and dam wasn't bedrock.
The tailings facility was built atop glacial till — a combination of clay and silt sediment left behind by an ancient glacier.
The fact the mine itself was built upon glacial till wasn't all that important, but it was crucial information needed for the construction of the tailings pond and its dam.
As the pond grew larger, and the load increased on the dam, the foundation suddenly gave way and spilled contaminated water into Polley Lake, Hazeltine Creek and Quesnel Lake.
Friday's report reveals Knight Piesold, the engineering and consulting firm that designed the tailings pond, knew about the glacial till after it did boring tests, but decided the deposit was spotty and would "not adversely affect the dam stability".
"They misinterpreted the test results and didn't really understand the nature of the strength, and the nature of how it could get weaker, and the nature how in fact, it did, fail," Caldwell said.
'Spectacularly horrible failure'
Bill Bennett, the province's mines minister, agreed.
"Those tests were done and they were missed, they weren't done properly in this case back in the mid 1990s," he said.
"This was a spectacularly horrible failure and it is one of a kind."
But the report released on Friday warns there could be other tailings ponds in B.C. built on glacial deposits and unless they're fixed, the province could expect to see two similar failures every decade.
In an interview with CBC Vancouver host Andrew Chang, Bennett said his government has requested all operating and inactive mines in the province file an annual dam safety report and have independent engineering consultants access and review those reports.
Bennett said the ministry will also reach out to all mining operators in the province to see if their mines might have been built on glacial till and if so, whether they were built with that instability in mind.
"If there is any question at all of similar circumstances to what existed at Mount Polley on your mine site, you need to tell us so that we can do further inspections," Bennett said.Suggest a correction