Almost every day I am asked or confronted by this question: what is the best cover for a tailings facility?
After twenty years of deliberation, I give this simple answer.
The only cover that is cost-effective and which will last indefinitely is this—from the top down:
- First a layer of durable rock.
- Next a layer of soil.
- Finally a layer of clay.
Here are the design criteria for each layer:
Rock. Calculate the size (gradation) of rock that resists erosion by the probable maximize precipitation and resulting runoff. Make the layer thickness at least 1.5 times the maximum rock size.
Soil. Make it as thick as the rooting depth of the climax vegetation.
Clay. Make is as thick as needs be to resist degradation by freeze-thaw and desiccation by the roots in the soil. Make its permeability that which is needed to limit infiltration to an acceptable quantity.
On the basis of logic, case histories, and calculations, this is the only way to go. If they tell you otherwise, they are lying, deluded, or incompetent.
The point is this cover will function as follows re the forces that nature imposes on such things.
Erosion: There will be none of significance. The large rock resists erosion and thus protects the underlying soil and clay.
Vegetation: With time vegetation will establish. Seeds will fall or be washed down through the rock and take root in the soil. Only the best will grow up through the rock. And their roots will go down into the soil and help extract water to evapotranspirate it and limit the quantity of water available to potentially infiltrate.
Infiltration. Limited or controlled by the low permeability clay. Additional evapotranspiration via vegetation will, as noted above, limit the quantity of water available to potentially infiltrate.
Long-Term Performance. With time, the rock will commingle with the soil. A rocky soil layer with vigorous vegetation will develop at the surface. The rock and the vegetation will resist erosion in a natural way. This is the way nature does it. We call it a desert pavement.
Now this cover may not be cheap. But it is cost-effective. It will last for a thousand years or more. There are plenty of natural case histories in the vicinity of your mine.
If your consultant blathers on about geomembranes, fire them. For they (both geomembranes & consultant) are mere placebos.
If they deny the need for rock, fire them. For they understand neither erosion nor geomorphology.
If they go on about the great vegetation they can plant, send them away, for they understand neither climax vegetation nor the processes of nature.
If they try to persuade you to a store-and-release cover, laugh at them. This a figment & myth of calculation. It does not happen thus in nature. There would be no groundwater anywhere if they were correct.
I recognize that what I write above is controversial. It deflates most current practice and conviction. It denies what most consultants recommend.
So be it. I am old, crusty, and know I am right. I have done the calculations, built the prototypes, and seen the results. The rest are crass youth yet to learn by real-time, in-field experience.
To prove my point, here is my offer: if you get a report or recommendation that is not in-line with what I write above, send it to me, and I will, free of charge, review it and comment in accordance with professional practice.