Mount Polley Mine report calls for changes in the industry
An expert engineering panel determined the breach at the Mount Polley Mine tailings pond was the result of a failure of the foundation of the embankment and recommended some major changes for the mining industry in British Columbia.
The report by Dr. Norbert Morgenstern, Steve Vick, a Colorado geotechnical engineer and Dr. Dirk van Zyl, a University of British Columbia mining engineering professor, determined “the dominant contribution to the failure resides in the design,” which failed to take into account “the complexity of the sub-glacial and pre-glacial geological environment associated with the perimeter embankment foundation.”
The panel indicated that the failure was triggered by construction of the downstream rockfill zone at a steep slope, which if flattened, would have averted the catastrophe. “The slope was in the process of being flattened to meet its ultimate design criteria at the time of the accident,” according to the report.
The experts concluded there was no evidence that the failure “was due to human intervention or overtopping of the perimeter embankments and that tipping and cracking, which is often the cause of the failure of earth dams, was not the cause of the breach.”
The panel concluded “business as usual” cannot continue and the industry must move away from storing mine waste under water behind earthen dams, The Vancouver Sun reported.
The B.C. government-appointed panel pointed to the Greens Creek, Alaska, mine that uses filtered tailings, sometimes called dry stacking, saying it is the “prime candidate” to eliminate the risk of tailings dam failures. There is no dam and no water stored on the tailings. That mine also uses a synthetic liner to prevent leakage, a practice not common in B.C.
The panel noted that dry stacking is more expensive than water storage.
While B.C. Energy and Mines Minister Bill Bennett endorsed some of the panel’s recommendations, he had a muted response to the call to move away from storing finely-ground rock that contains potentially toxic metals under the conventional method used in the province. There are no examples of dry stacking in B.C.’s 60 operating and closed mines.
In its 156-page report, the panel said that simply put, dam failures are reduced by decreasing the number of dams that can fail.
The report said the number of tailings dams could be reduced by storing waste below ground in mined-out pits or as backfill for underground mines. But surface storage using filtered tailings is a prime candidate for best available technology, added the report.
Bennett said independent dam review boards (used in the Alberta oilsands, which add another level of scrutiny) would be made mandatory and regulations will be brought in that likely make dam design criteria more prescriptive.