'Failure resides in the design': Mount Polley Review Panel
Mount Polley's tailing dam failed because of design failures, wrote the government appointed panel that released its findings today.
The panel said that ". . . there was no evidence that the failure was due to human intervention or overtopping of the perimeter embankments and that piping and cracking, which is often the cause of the failure of earth dams, was not the cause of the breach."
Rather it was design failures that were the main cause:
The breach of the Perimeter Embankment on August 4, 2014 was caused by shear failure of dam foundation materials when the loading imposed by the dam exceeded the capacity of these materials to sustain it. The failure occurred rapidly and without precursors.
Deposited in a complex geologic environment, the weaker glaciolacustrine layer was localized to the breach area. It went undetected, in part because the subsurface investigations were not tailored to the degree of this complexity. But neither was it ever targeted for investigation because the nature of its strength behaviour was not appreciated.
Another conbributor was the construction of the downstream rockfill zone at a steep slope. The reports authors said if the downstream slope had been flattened, failure would have been avoided.
"The slope was in the process of being flattened to meet its ultimate design criteria at the time of the accident."
The panel did not place any blame with inspections or current regulations stating that the "failure was without precusors."
"Despite having a strong regulatory process and personnel, the Perimeter Embankment of the Mount Polley TSF still failed. It was a sudden failure without precursors. Additional inspections of the TSF would not have prevented the failure.
The media briefing is embedded below.