A 2010 dam safety inspection at the Mount Polley gold and copper mine identified several concerns, including the discovery of a 10 to 15-metre long “tension” crack in the earthen dam.

The crack was discovered in the perimeter wall, the same embankment where a section failed Aug. 4 of this year, releasing millions of cubic metres of water and tailings containing potentially toxic metals into Hazeltine Creek and Quesnel Lake.

“A tension crack does not necessarily indicate a plane of weakness in fill materials but it can’t be ignored either,” Knight Piesold, the company’s geotechnical engineering firm at the time, said in its report obtained by The Vancouver Sun.

Knight Piesold recommended a stability assessment be carried out, and told the company the crack should have been reported to them immediately. It was noticed by a grader operator two months before the engineer’s inspection on Oct. 7, 2010, and had been partly covered over with dirt.

No signs of distress were identified at the tailings embankments other than the tension crack, the report said.

Other concerns identified by Knight Piesold included that 40 per cent of 92 instruments that measure water pressure in the dam were broken and needed to be replaced before the height of the dam was raised. The height of the dam is raised on an almost continual basis to contain rising levels of water and tailings.

Replacing the instruments, called piezometers, had already been identified as an issue in 2006 during a more detailed dam safety review conducted by engineering firm AMEC.

Source: The Vancouver Sun - See full article