The Mount Polley crisis was a long time coming. A report on a breach last year of the tailings dam at the Mount Polley mine said the dam had raised incrementally to meet the requirements for upcoming years. A lack of long-term planning eventually contributed to a catastrophic dam failure, the report said.
Here is a timeline from the report of events that led to the disaster:
Glaciolacustrine (GLU) fine sands, silts, and clays were observed in the main embankment foundation and established to be steady under pressure during the construction. (The embankment was one of three that made up the dam.)
Construction on the main embankment differed from the original design, and configurations were not in accordance with the stability analysis of the original design.
Each of the three dam embankments were to be constructed with a different material, but the Mount Polley Mining Corporation requested approval to use rockfill in all of them.
2001 to 2005
Mine operations were suspended for economic reasons
Renewed construction prompted questions from the Ministry of Energy and Mines about the effects of softer GLU foundation materials downstream on dam stability. Engineering consultant Knight Piésold confirmed the GLU deposits were isolated and would not affect dam stability.
A slope was placed at a steeper inclination than planned because of a delay in the delivery of construction material. A plan to rectify the steep slopes was never carried out.
Complications slowed the raising process. The main embankment buttress was not constructed as designed, turning out to be five metres below its design height and short of its design extent. A strength analysis was performed, and Knight Piésold concluded the embankment would remain stable. A tension crack appeared in 2010.
The 2011 geotechnical site investigation concluded GLU was not of concern.
Reducing the steep slope was deferred until the completion of the entire dam. The 2011 investigation showed the GLU went deeper than previously thought, but stability tests in 2012 did not take the material into account in all three embankments.
During the next months, water continued to rise in the dam. For years, the company managed to stay one step ahead of it by raising the dam, but on May 24, 2014, a small amount overflowed. The breach happened before a plan to add a new buttress to the perimeter embankment could be completed.