A slow morning and an even slower computer that took its sweet time downloading files. During the enforced no-compute period, I pulled out the proceedings of the Mine Closure 2010 Conference held in Chile in November 2010. I read a paper that had not previously caught my attention. I could not but stop to wonder how this will be dealt with at the Pebble Mine when it closes in a hundred years time.
The paper is “Critical hydraulic gradient in tailings dams in long-term perspective” and is written by I. Jantzer and S. Knutson, both of the Lulea University of Technology, Sweden. (Hence the absence of the definite article you would expect in the title.)
In the paper, the authors report on a research project to determine the safe hydraulic gradient in a closed mine tailings dam. They look at internal erosion and hence potential dam failure in the 1,000 years or so following closure. The 1,000 year period is said to be the Swedish mine closure criterion. The authors look at natural analogues in the landscape where seepage through naturally deposited earth structures has gone on for a long time, in some case up to 6,000 years.
Then the going gets tough, if you are not a geotechnical engineer and comfortable with the concept of a critical hydraulic gradient. For those who are not knowledgable about critical hydraulic gradients, believe me when I tell you the critical hydraulic gradient is a measure or index of the speed with which water seeps through the tailings and may cause failure of the dam and a nasty mess as the tailings flow out and far downstream.
The scary conclusion from the authors’ research is that current Swedish guidelines and standard geotechnical practice set far higher permissible critical hydraulic gradients than it appears safe in natural analogues. Put another way: if you want your tailings dam to have a chance of remaining stable for 1,000 years after closure, you had better not design in accordance with current practice. For if you do, there is a good chance your dam will fail well before the 1,000 years have posited. Put in another way: you had better be a very conservative in evaluating long-term seepage though the dams or embankments of your closed mine waste facilities—you had better replicate natural analogues, and not pay heed to codes or theory or the writings of the so-called experts.
Which leaves me wondering: are there natural analogues in Alaska for 700-ft high earth structures that have remained stable for more than 1,000 years and maybe even 10,000 years? I would certainly be looking for some now, if it were my job to design the Pebble Mine tailings and waste rock facilities. Maybe somebody from the EPA can find one to convince us all of the long-term performance of these proposed mighty structures.
With which I leave you to a good weekend and some interesting reading.