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.
why not sea disposal, or find a new way to dewater? in the 5 years that the project has been in serious study, how much effort and money has gone into researching and developing fundamentally new tailings technology? perhaps you can discuss these issues in the forthcoming Continuous Conference website to be launched by Infomine soon? see you at the pub next week – perhaps we should have a tailings night?
What an insightful post! Wow. I learned more in the last 5 minutes than in the last week. Wouldn’t it be nice if we reflected on everything we built these days with the future in sight. Then in 10,000 years our ancestors could reflect on our ingenuity – not our lack of foresight!
This has a lot of good information! Well done =) But 1 comment… this isn’t a short story lol
[...] Jack Caldwell from I THINK MINING has written about his concerns about the tailings dam. [...]
[...] Jack Caldwell from I THINK MINING has written about his concerns about the tailings dam. [...]
[...] Jack Caldwell from I THINK MINING has written about his concerns about the tailings dam. [...]
A responder asks “why not sea disposal”. No offense, but what planet are you from? Perhaps you underestimate the volume and toxicity of tailings and overburden that will need to be disposed of. Take gold, for instance. Twenty-four tons of tundra sacrificed for every ounce of gold. And that’s just the start. Transforming the oceans into toxic waste dumps, as we are now doing, is suicidal. When we kill the oceans, we, as a species, are done.