In Part One of our blog about PolyMet tailings, we shared how PolyMet will use an existing tailings basin to permanently store our tailings – the silt and sand-like crushed and ground material that’s left over after copper, nickel and other economic metals have been removed from the ore. We also explained how the basin will be built and the nature of the water and tailings that will be stored there.
In Part Two, we look at seven features of the PolyMet tailings basin that help ensure the basin remains stable and safe over the long term.
- Tailings foundation conditions at the PolyMet site have been explored and analyzed repeatedly. This analyzation began more than 50 years ago and has more recently been done with sophisticated methods of exploration. This has given us a thorough understanding of the geotechnical conditions in the area and the basis for solid planning and design.
Last August, a tailings basin dam in British Columbia at the Mount Polley copper mine failed, releasing millions of tons of watery tailings into nearby forest and waterways. A subsequent government investigation linked the failure to faulty geotechnical modeling and a design that underestimated the amount of “loading” the ground beneath the tailings could sustain. The weight of the accumulating tailings eventually caused the dam to fail. The unusually steep slope of the dam also contributed to the failure, according to investigators.
We have performed all of the rock and soil strength analysis and geotechnical modeling that was neglected in the Mount Polley design. Our geotechnical experts performed a fundamental slope stability analysis as part of the PolyMet tailings basin design.
- The region around our tailings basin is void of seismic activity. Still, a series of internal concrete-like walls will be built into the existing tailings basin before mining operations begin to further increase the basin’s overall resistance to slope movement from a major earthquake, even though the probability of one occurring is extremely low.
- As we plan the project, we do our best to anticipate events that could cause a failure. Such events include earthquakes, major storm events, pipe breaks and so on. We design the structure to reduce the risk of those events, and we have contingency action plans in place to assist us with identifying early warning signs, potential consequences and required actions. A draft of this plan is included as an attachment to the April 2013 NorthMet Project Flotation Tailings Management Plan, which is a reference in the supplemental draft Environmental Impact Statement.
- The Minnesota Department of Natural Resources (DNR) will complete safety inspections on a regular basis. Inspections include those of the tailings dam under the provisions of our future permit and will continue throughout operations and even after closure. PolyMet staff and independent geotechnical experts will supplement DNR inspections with thorough and frequent assessments.
- PolyMet’s engineering firm has a proven past. Minnesota-based Barr Engineering, PolyMet’s engineering firm, has been designing tailings basins for mines in northern Minnesota since the 1960s and now works on tailings basin projects around the world. Independent geotechnical experts will continue to assist in the specification of future engineering requirements once production starts.
- There will be ongoing monitoring using top-of-the-line instruments. High-tech instrumentation will be installed in and around the basin to systematically and continually monitor water movement and slope stability.
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