Mines are as unique as the places in which they’re found, yet we’re always looking to learn from the operations of others.
When a tailings basin dam at the Mount Polley mine in British Columbia, Canada failed last year, releasing millions of tons of watery tailings into nearby forest and waterways, it gave us an opportunity to make sure we’re doing all we can so nothing like that ever happens here. (Tailings are leftover, sand-like particles of rock that have been stripped of metals during processing and are stored permanently in large, engineered basins.)
To store our tailings from copper-nickel mining, we’ll use an existing taconite tailings basin that operated for 40 years. In fact, we’ll continue to use taconite tailings to build the walls of the basin, so in many respects it will continue to be similar to other taconite tailings basin in Minnesota, many of which have been around for decades and have shown to be safe.
The Final Environmental Impact Statement for the PolyMet project, that took 10 years to complete, acknowledges that the design of our tailings basin is “sufficiently different” than the Mount Polley tailings basin so “a direct comparison cannot be made.” It also says that modeling used to design PolyMet’s basin used “extreme scenarios such as maximum precipitation events and earthquakes” to ensure the basin would be stable even under the most unusual circumstances.
Based on a government investigation of the Mount Polley failure, critical modeling and design analyses that were required for the PolyMet project were unfortunately neglected in Mount Polley’s case. That investigation linked the BC failure to:
- A design that underestimated the amount of “loading” the ground beneath the tailings could sustain
- Faulty geotechnical modeling
- Unusually steep slope of the dam
The weight of the accumulating tailings eventually caused the dam to fail. The unusually steep slope of the Mt. Polley dam also contributed to the failure, according to the experts.
With that in mind, here are two key differences between PolyMet’s planned mine and the Mount Polley design:
- Necessary rock and soil strength analysis and geotechnical modeling has been completed, which is a step that was neglected in the Mount Polley design. The ground beneath the tailings can sustain the load of PolyMet’s tailings basin design.
- Geotechnical experts have performed a fundamental slope stability analysis as part of the PolyMet tailings basin design. PolyMet tailings embankments are significantly flatter than Mount Polley’s embankments to greatly spread the load over a broader area.
To learn more about the PolyMet tailings basin, read about the seven features that will help ensure it remains stable and safe now and over the long term.