The Mount Polley Independent Expert Engineering Investigation and Review Panel released its report on the cause for the breach of the tailings storage facility (TSF) at the Mount Polley mine on August 4, 2014. The panel found no evidence of failure due to human intervention, overtopping, or piping and/or cracking resulting in internal erosion. It indicated that the water accumulation within the TSF was not a cause of failure, but contributed to the release of tailings. The mine had earlier recognized that water levels would increase in the TSF, and had taken steps commencing in 2006 to address this issue.
The panel concluded that the perimeter embankment of the TSF failed because a glacio-lacustrine layer (GLU) lying approximately 8 m below the base of the dam in the area of the breach was not as strong as had been assumed in the design of the TSF. Construction over the 18-year life of the TSF was at all times carried out in accordance with design criteria provided by the engineers of record and approved by the Ministry of Energy and Mines for British Columbia. The panel noted that the omissions associated with site characterizations remained undetected notwithstanding the large number of experienced geotechnical engineers associated with the TSF over the years.
Design criteria accepted for the TSF required a factor of safety of 1.3 during the operating life of the mine, increasing to 1.5 at closure. Based on the calculations presented in the design reports, the factor of safety of the perimeter embankment where the failure occurred exceeded 1.5 at the time of failure. Had the GLU beneath the TSF been as strong as assumed by design criteria, this sudden and unanticipated failure would not have occurred.