An independent investigation into the tailings pond spill at Imperial Metals’ (TSX: III; US-OTC: IPMLF) Mount Polley gold-copper mine near Likely, B.C., found that a flaw in the dam’s design caused the breach last summer.
The expert panel concluded the dam, which unexpectedly broke on Aug. 4, discharging 25 million cubic metres of mine waste and water into Polley Lake and Quesnel Lake, sat on unstable ground.
“The design did not take into account the complexity of the sub-glacial and pre-glacial geological environment associated with the perimeter embankment foundation,” the panel said in its report released Jan. 30.
The original design failed to identify and assess the strength of a glaciolacustrine (GLU) layer called the Upper GLU, sitting 8 metres below the base of the dam. As the tailings pond grew, the Upper GLU, made mostly of clay, weakened.
“That layer of material has characteristics that make it unstable and prone to shearing under pressure. When it sheared, the portion of the dam on top of that layer collapsed,” Bill Bennett, B.C.’s minister of energy and mines, said at a press conference discussing the panel’s findings.
“If constructing unknowingly on this Upper GLU deposit constituted loading a gun, then building with a steep slope . . . pulled the trigger,” said Norbert Morgenstern, the panel chair.
Morgenstern was one of the three geotechnical experts on the panel formed by the provincial government weeks after the spill to investigate the breach.
There were no warning signs, Bennett said. “No one could have known this could happen unless they knew about the strength and location of those subsurface materials in that layer and apparently nobody did.”
It took the panel four months to fully understand the cause. “And no surface inspections could have detected this,” Bennett said.
The panel also investigated three other possible causes for the failure. It found there was no evidence that the dam broke due to human intervention, overtopping because of inadequate water control, or piping or cracking resulting in uncontrolled internal erosion.
But the panel did acknowledge that water accumulation contributed to the release of tailings.
In a statement, Imperial Metals said it had “recognized that water levels would increase in the tailings storage facility, and had taken steps commencing in 2006 to address this issue.”
It notes construction over the tailings storage facility’s 18-year life met the design criteria provided by the engineers of record and approved by B.C.’s Ministry of Energy and Mines.
Knight Piésold was the engineering firm of record of the tailings storage facility until February 2011, after which AMEC Earth and Environmental took over the role.
The panel published seven recommendations to reduce the potential of future failures. In response, the provincial government has taken three actions.
First, the chief inspector of mines will require all operating mines with tailings storage facilities in the province to confirm if similar subsurface layers exist below any of their dams in a letter to the ministry by June 30, 2015. If they do, the letter must indicate what work was done to understand and accommodate those materials in the design.
Second, all operating mines with tailings storage facilities must create independent tailings dam review boards to provide third-party engineering advice on the facility’s design, construction and operation.
(While the province contains 98 permitted tailings storage facilities with a total of 123 dams, only 31 of the facilities are at mines that are operating or under construction. The rest are at mines that are closed or under care and maintenance.)
Third, the province is initiating a code review on how to implement the other recommendations.
The panel did not make any recommendations on which individuals or parties should be held responsible for the failure. But a report by B.C.’s chief inspector of mines, which will be out in June, will pinpoint any individual or organization that may have violated the Mines Act or the health, safety and reclamation code for mines in the province.
Another probe by the ministry’s Conservation Officer Service will look at what, if any, laws were broken and will forward any recommendations for charges to the provincial Crown Counsel.
It is so sad that such an event occured. In a long term view the responsible are the current Greenies legislations of Environment which does not allow use of natural confinement facility like a Natural lake. Why there is a Lake? it is because water can not escape since it is surrounded by impermeable hard rock walls, most of the time.
Environmental regulators obliged industrial to put containment facilities up hill away from creeks and lakes and material is generally made of soil.
I think it is time the Environment people start thinking long term as within the next 50 to 75 years or more a bunch of other dams will spill in lakes and rivers as it is a natural process of erosion over time. Just think of our famous beaver and how they can affect the course a water body. Maybe Safety Factors should be significantly increase transforming all projects into uneconomical projects and Create a Huge National Parc all accross Canada…
In my opinion it could make more sense to move living species from the lake to another location and isolate the submerged talings from the rest to control its way of contamination and suspended matters to at the end of the day have it stabilized. The worst that can happen is that lake over time becomes a wetland peat bogs filtering and capturing CO2…capping the tailings. No No No you have to put these tailings storage up hill so if they fail they end-up in the lakes and rivers without control, such a better way to preserve the environment.
May Be I should start a Non Profit Organization where people could make donations to save the dams, I could make a business out of it giving presentation at 100K each time…
My 2 cents