Crews were working to raise the tailings dam at the Mount Polley mine by up to four metres before the structure failed and sent a torrent of waste and debris into surrounding waterways.
Imperial Metals Corp., the Vancouver-based company that operates the mine, had also asked the Ministry of Environment for a permit to release more treated effluent from its tailings pond. That permit was pending when the dam gave way on Aug. 4.
The capacity of the tailings pond, and measures the company was taking to increase it, are now part of an investigation of the breach, which resulted in an emergency water ban (since largely rescinded) and worries about how chemicals could affect water, fish and wildlife. Cleanup cost estimates have been as high as $200-million, and a class-action lawsuit has been filed in connection with the breach.
Now engineering firm AMEC, Imperial Metals and the B.C. government are in the spotlight over an incident that has given a black eye to the mining sector and spurred the province to review its policies on tailings dams.
AMEC, the London-based, multinational company that is the engineer of record for the tailings dam, was handling the recent construction.
AMEC spokeswoman Lauren Gallagher told The Globe and Mail in an e-mail on Tuesday the company is limited in what it can disclose because of the investigation.
"The performance and stability of a dam is dependent on many factors including design, construction, operation and maintenance as well as the potential for unforeseen conditions. Determining which of these factors contributed to the Mount Polley dam breach requires a thorough investigation," Ms. Gallagher said.
"While AMEC serves as the engineer of record on the most recent raising of the dam, implementation of the AMEC design has not been completed and some construction activity was still taking place to complete our design," she added.
In a telephone interview on Thursday, Imperial Metals vice-president Steve Robertson said the construction, begun in May, was part of an annual routine.
"We usually do a three- or four-metre raise on the impoundment just to increase the capacity for tailings," Mr. Robertson said.
He said the dam was "within its design parameters" when the breach occurred.
To get permits for tailings dams, operators typically must maintain a certain level of "freeboard" – a buffer zone above a maximum fill level – to prevent overflow. For the Mount Polley mine, that is about one metre. In May, the company exceeded height limits in its tailings pond, resulting in an advisory from the province. After the breach, the province released details of that advisory, and four others issued to the company since mid-2012.
Such construction work had been done nearly every year since the mine has been in production, Mr. Robertson said. About 100 kilometres northeast of Williams Lake, the Mount Polley copper-gold mine opened in 1997, closed in 2001 and re-opened in 2005.
Asked whether it is safe to keep raising the height of the dam, Mr. Robertson said Imperial relied on its engineers' expertise.
"We feel very confident the engineers would have felt this was the appropriate structure for the storage that was required," he said.
The tailings dam was about 35 metres high when it was breached.
Mr. Robertson would not speculate on the cause of the breach, saying multiple investigations are under way and it would be premature to draw conclusions. The tailings pond is about four kilometres long and four kilometres wide.
Environment Minister Mary Polak has said it is unlikely the spill would have been prevented if the province had authorized the company to release the treated water from the tailings pond.
B.C.'s Mines Minister Bill Bennett told The Globe in an interview the investigation will look at all players. "Most, if not all, mines have engineers of record who are independent, arms-length engineering companies who, when they put their stamp on something, they're on the hook in terms of being responsible and being liable," he said.
The province is expected to announce updated policies for tailings ponds as early as Friday.
With reports from Andrea Woo and Sunny Dhillon in Vancouver