From the April 2007 issue of Mining Engineering, here is a summary of the description of the proposed Pebble Mine’s proposed tailings impoundment. Recall that Pebble Mine is a large copper-moly-gold deposit in Alaska that Northern Dynasty Minerals is seeking to develop. The project has been controversial, so here is a description of one of the parts thereof that will remain at the site long after mining is done.

The design objectives are:

  • Render the potentially reactive material inert and protect water quality in the impoundment.
  • Safely impound water, minimize the quantity of water used, and protect downstream water quality.
  • Minimize water seepage from the impoundment and capture seepage that does occur.
  • Meet of exceed the seismic design parameters set by the State of Alaska.

To render potentially reactive material inert, the material will be placed under water and entombed in non-reactive tailings. The five to ten percent of tailings that contain a high percent of sulfides will be separately discharged so that they are below the pond. Potentially reactive waste rock will be co-disposed in the tailings impoundment.

To control seepage, a trench will be excavated through alluvium to bedrock. A grout curtain is proposed into the bedrock. During the initial phases of embankment construction, a high density polyethylene geomembrane (HDPE) will be placed on the face of the embankment. Thereafter when the embankment construction converts to centerline construction, a compacted glacial till core will be placed. Drains will collect embankment seepage and direct it to sumps.

I would like to see a cross section through this proposed embankment: I cannot understand how you can use HDPE in a long-term structure subject to time and earthquakes. Why not use clay all the way?

Earthquake stability will be achieved by design for what sounds like the probable maximum earthquake that would result in a “maximum possible megathrust earthquake with a value of 0.3g.” I apologize to my earthquake-specialist friends if they quake at this seismic description. So do I.

The article gives no information about the post-closure geomorphic performance of the impoundment during the period of maintenance and thereafter.