On August 4, 2014, the Mount Polley copper and gold mine tailings pond, in the Cariboo region of British Columbia, breached. This tailings dam is owned by Imperial Metals. The Mount Polley tailings storage facility released 10 million cubic metres of water and 4.5 million cubic metres of mining waste into the Polley Lake. This tailings slurry eroded river banks, tore down trees and flowed out of the Polley Lake into the nearby Quesnel Lake. This spill greatly affected the natural landscape of the surrounding area by causing the Polley Lake to rise by 1.5 metres and the nearby Hazeltine Creek to transform from a 2 metre wide stream into a 50 metre across “wasteland”. By the end of the fourth day, the tailings pond was virtually empty and the slurry of tailings had poured into the once pristine Quesnel Lake. Immediately after the tailings pond was breached, the Cariboo Regional District declared a local state of emergency over concerns of the quality of the drinking water. The tourism businesses in the surrounding areas remained open inspite of the tailings spill. On August 5 and 6, it was found that even with the tailings slurry in the Quesnel Lake, the water was not toxic to the rainbow trout. After the tailings dam failure, the water, sediment and fish were sampled and tested by the BC government. The government officials found that the fish contained elevated levels of selenium that exceeded levels safe for human consumption. The sampling also revealed elevated levels of arsenic and cooper in the fish, but not enough to be considered a threat to human health. The testing of the sediment revealed elevated concentrations of copper, iron, manganese, arsenic, silver, selenium, and vanadium, but these levels were already elevated before the dam breach.
On January 30, 2015, the Mount Polley Independent Expert Engineering Investigation and Review Panel delivered a final report detailing the findings of this independent investigation. This report was delivered to the Ministry of Energy and Mines, the T’exelc First Nation (Williams Lake Indian Band) and the Xat’sull First Nation (Soda Creek Indian Band).
The report states that there are three contiguous embankments that confine the Mount Polley tailings storage facility and that the perimeter embankment, where the breach occurred, was on the northern flank of the tailings pond. This report breaks down possible failure modes into four categories: Human intervention, overtopping, piping and cracking, and foundation failure. The investigation concluded that the Mount Polley TSF breach was due to the foundation failure of the embankment which showed visual evidence of bodily outward displacement and rotation of the embankment remnants that was consistent with foundation failure. One of the layers found in this investigation is especially weak and was not accounted for in the design of the TSF. Another is the presence of a brittle stratum that loses strength as it comes under load and becomes too weak to support the embankment and dam. Another possibillity is the presence of a layer that is compressible under the load and develops a high pore pressure, weakening an otherwise much stronger material.
Through surface investigations, subsurface investigations, and laboratory testing, the Panel concluded that the main reason for this dam failure is due to the design. The design did not take into account the complexity of the sub-glacial and pre-glacial geological environment. The foudnation investigations and associated site characterizations failed to identify the continuous GLU (glaciolacustrine layer) in the area surrounding the breach. This GLU layer is recognized to be susceptible to undrained failure when subjected to the stresses associated with the embankment. The Panel also reviewed the roles and responsibilities of all involved and found no regulatory oversight and that all the regulatory staff were well qualified. The panel found that TSF inspections would not have prevented the failure.
The conclusion of the report is that the safety of any dam, water or tailings, relies on multiple levels of defense. At the time of design of the Mount Polley TSF, the engineers were not able to design around this weakness in foundation which could have failed earlier due to overtopping of tailings in the pond or erosion. The Panel recommends that rather than following the prescribed Canadian Dam Association’s factors of safety, that there should be more individual requirements for site investigation, failure mode recognition, selection of design properties, and specification of factors of safety. Many dams in British Columbia have been functioning well, and similar failures have no occurred at other mine sites. Even though the probability of a tailings dam failing is small, the implications are not.
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