Several months ago, a PAWA member asked me what kept the big stack of tailings (500 acres and 200 feet deep) proposed at Copper Flat Mine from sliding down the hill. If I were an engineer, I might have a quick answer. Since I’m not, I had to go slowly through an expanding sequence of imaginings which I will share with you because the thought sequence allowed me to understand that not only do I not have an answer to the question but the miners also don’t have an answer.
1. The tailings sit on an incline. The incline is not very steep, but 200 feet of tailings weigh a lot. If the tailings were one solid object, whether it slides or not depends on the degree of incline (the steeper the incline, the more tendency to slide), the weight (a balance between the increased force of gravity and the increased friction along the bottom), and what is called the coefficient of friction of the HDPE liner that the tailings sit on. The mine plans to use a textured liner to increase the coefficient of friction and prevent sliding.
2. But let’s get a bit more real. The tailings are not a solid object. It’s a mass of very fine powder that is held together by being moist. In fact, it is quite moist because there is seepage from it. That is what the liner is partly for, to catch the seepage, collect it, and reuse it in the processing. If there is seepage, then that seep water is coming out of pretty wet tailings. Imagine a coffee filter full of powdered coffee that is a lot finer than coffee grounds. Add water slowly until it seeps out the bottom. When water starts coming out the powder is saturated. That is what saturation means: it won’t take any more water. So, if there is seepage from the tailings there has to be saturation.
But the whole thing isn’t saturated because the filter presses have sucked a lot of water out. That means that the tailings stack is unsaturated generally, moist enough to compact and hold together, but there seem to be places in the stack that are saturated enough to seep water.
Now, when powder gets really saturated, it loses its cohesion. If your coffee filter weren’t holding the saturated powder together, some of the powder would flow off with the seepage. Too much water and a bit of shaking will get you liquifaction; buildings built on sand will just sink in an earthquake if there is a lot of water in the sand and the combination is shaken.
Go back to the tailings stacks where water content seems to be crucial to stack stability. If the water in the unsaturated tailings moves downwards pulled by gravity, the water will collect at the bottom, just above the liner, before seeping out along the liner. I imagine, then, a saturated situation along the liner. The texture in the liner may keep some wet tailings which lodge in the texture preventing them from moving sideways, but if the saturated area is thicker, extending further up, away from the textured surface, then the saturated tailings would have no rigidity, and the tailings there could move sideways, beginning a collapse. If you add the factor that there will be a great deal of weight on the tailings, that might keep the tailings compacted, but we just don’t know.
In this scenario, everything depends on how much water is in the tailings, where that water is gathering, the degree of incline, the height of the stacks, etc., etc. including molecular forces between water molecules and the molecules of powdered rock, the tailings, making water cling to the tailings rather than simply moving down with gravity (surface tension). In our non-technical experience of how water moves in fine grain medium, we might think of water in unsaturated soil. You see surface tension at action when you water your garden. Water out of the hose spreads out in the soil rather than just disappearing straight down into the ground. You see that the soil you water keeps the soil moist for some time. It doesn’t just sink and disappear into the ground on its way to the aquifer. Unsaturated tailings hold water in the same way.
Water movement in unsaturated soil is much more complicated than water movement in saturated soil such as in an aquifer below the water table. In unsaturated soil, water will cling to the soil particles and gather into a big saturated ball until the whole thing gets big enough for its weight downwards to pull the glob away from the particles it clings to. It’s one of the reasons rain does not just become groundwater. It stays in that unsaturated zone. The filtered tailings are like that, unsaturated but with areas of saturation.
Basically, I don’t know what keeps it all from sliding because it seems a balance of forces which I have no means of calculating.
3. If we turn to experts, we can understand their general warnings. Water in the stacks is a big problem. Filtered tailings are best suited to arid climates. Heavy precipitation can cause collapse. Miners need to be experienced with careful water management. But the details are still very technical.
Managing water content in the stacks begins by figuring out how to adjust the vacuum filtering process to leave the right amount of water in the tailings. That “right amount” has to be determined by how fast the tailings dry out in the process of being distributed and packed down and that, of course, depends not just on the weather (which changes) but on the manpower and machine power. The whole process seems rather finicky.
I read a very technical article from South America, where the researchers modeled the distribution of water in the unsaturated filtered tailings stacks of a large copper mine. See Ricardo Gallardo, et al., “Modeling self-compaction and static stability of a copper filtered tailings pile under unsaturated conditions,” Acta Geotechnica (2024) 19:4297–4312. Their modeling showed that like unsaturated soil, the water gathered in pockets in the tailings and these were located in places which threatened the stability of stack faces because water concentrated near the bottom of the stack faces. The tailing stacks that Gallardo’s team modeled sat on a level surface. At Copper Flat, they sit on an incline, so there is more chance that water would collect at the bottom of an open face.
The tailings they studied were filtered to have a water content by weight of 16%, “which corresponds to a saturation degree of about 60%” (p. 4298). That is pretty wet, but it follows normal industry standards; I understand from a Finnish maker of the filtering machines that normally tailings are filtered to between 12% and 20% by weight. I assume that in this range the tailings will compact well and have some rigidity. The Gallardo team also says, “[t]he potential failure surfaces appear in tailings when reaching a saturation of about 65–70%” (p. 4310). Failure seems very near: the tailings are laid out at 60% saturation and if the water in the tailings gathers anywhere near a surface to increase saturation by 5%, we have a potential breakdown of that surface.
The whole process is more than finicky and delicate. The margin of error or the margin of safety seems pretty small. Does this worry anyone? Tomorrow, in the second part of this article I will turn to what NMCC says about all this: to be continued.