The Vancouver Sun reports as follows about the Mt Polley tailings failure:
Likely resident Gerald MacBurney worked at Mount Polley for seven years, the last two he says as a foreman directing work on the tailings dam.
He says AMEC instructed the company to bring in five million tonnes of rock to shore up the outside of the dam in order to handle the increased amount of water in the tailings pond.
He said the company never carried through, perhaps only bringing in one million tonnes of rock.
That’s because they didn’t want to take their large equipment — big haul trucks that can carry as much as 120 to 200 tonnes — away from delivering ore to the mill, according to MacBurney.
Global BC reports MacBurney as saying:
“They needed to put in five million tonnes [of rock] around the dam, because they added, once they went to a bigger mine life, they added five times the amount of water,” adds MacBurney. “That dam was never designed to hold five times that amount of water.”
“Five million tonnes, well we got maybe a couple hundred thousand. And that’s it, in two years. I’d had enough.”
“They carried on going up instead of wider and doing the proper way of doing it,” says MacBurney.
Constructing a buttress of rock around the perimeter of a tailings facility embankment is not new or for that matter uncommon. If the embankment slopes are steep and going higher than original planned, it is reasonable to construct a buttress around the toe of the dam. If water is seeping out of the impoundment and emerging as flow at the toe of the embankment, you first place a filter and then the mass of the buttress.
A decision to construct a filter and buttress at the toe of a tailings facility embankment is not taken lightly. It is inevitably expensive to build. But where there are valid concerns for the stability and integrity of the perimeter embankment, that is what is done.
If, as MacBurney says, a decision to stabilize the embankments was made, clearly somebody had concerns about the stability of the facility. There must somewhere be a letter, report, or drawing showing what had to be done and explaining why it had to be done. Such a document would surely have been provided to the regulators–for so significant a change from the original design would have required their concurrence.
And when timely construction of the buttress was not happening, surely somebody besides MacBurney, must have been concerned. One would have expected the regulators to at the least have written to the company urging completion of the stabilization works.
I am not convinced that a buttress would have prevented the failure. I suspect the upstream circular arc failure zone that is so apparent in the videos I have seen indicates high pore water pressures in the upstream part of the embankment–high enough pressures to have cause the failure. Such a failure would not have been precluded by a buttress.
But if the two embankment that came together at the corner where failure occurred were moving apart, then the buttress may have indeed limited the movement. Here is a picture of a recent failure in Hungary where the two perpendicular embankment both moved outwards and away from each other. Maybe this is part of the Mt Polley story.
This mystery of the missing buttress could easily be cleared up if we had ready access to the reports. Or if all such document were made public.