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Questions and answers RE: “Backfill depleted open-pit mines…”

February 1st, 2011 / By: / From Our Readers

Editor’s Note: A variety of comments and questions arrived following the April/May 2010 publication of Allan Breitenbach’s article, “Backfill depleted open-pit mines with lined landfills, tailings, and heap leach pads.”

Comments

From: Ana
Posted: May 17, 2010
Re: Excellent article

Very interesting and complete.
Thank you!

From: Herman
Posted: Oct. 5, 2010
Re: Pit floor?

Please can you help me on:

What factors cause failure to achieve floor level in most surface mines?

What are the effects of failure to achieve and maintain pit floor level?

What to be done to achieve and maintain level pit floor?

From: K. A. Pandian
Posted: Oct. 15, 2010
Re: Excellent articles

Dear sir,

You have provided excellent technical articles. Can you provide more technical details about geosynthetic application on mining open pit and overburden dumping areas?

Response

Author Breitenbach responds:

For lined tailings impoundments in the pit floor: The pit is dewatered during operations, so at the end of operations, dewatering continued until the tailings could be slurry deposited by pipeline and spigot disposal, with the groundwater level allowed to rise, but maintained below the level of the saturated tailings beach fill materials and surface water pool.

The water pool is recirculated back to the plant for reuse, but typically has an approximate 3m minimum depth to decant clear water to the plant. The controlled rise in the groundwater level to its natural level below the tailings beach and water pool level helps to hydraulically contain the tailings with reduced risk of leakage compared to land-based, above-ground containment with dams.

For solid waste landfill impoundments in the pit floor: Current regulations in the U.S. require dry conditions with the natural groundwater level located below the landfill liner system. This requires that most depleted mines be backfilled with waste rock or other materials to near the open pit rim level to stay above the fluctuating wet and dry season groundwater level.

However, there is a possibility that “other materials” could be a completely backfilled pit with lined or unlined inert tailings covered by a thick granular cap barrier. The cap barrier thickness would need to account for typical 5–6% tailings beach fill self-weight consolidation at closure in addition to solid waste landfill material loading with sufficient granular thickness to maintain a long-term dry capillary barrier zone beneath the landfill liner.

Hope this answers the questions.

Allan J. Breitenbach, P.E.
Principal Geotechnical Engineer,
Environment & Sustainability
Ausenco Vector

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