How many types of tailings facilities are there? How many stars are there in the night’s sky? How many licks does it take to get to the center of a Tootsie Pop? Okay, maybe not that many, but there are a lot, because each one should be designed based on site-specific and project-specific requirements, goals, opportunities, constraints and limitations. And it depends how you wish to categorize the type of facility. Come on along, and I’ll be your dam guide.
Let’s start with the very basics. There is the downstream-constructed type. In this type of design, a starter dam is constructed. When a facility expansion is needed, fill is placed on the downstream slope of the starter dam, and the new crest is “downstream” of the starter dam crest (the “crest” of the dam is where the road may be constructed, right at the top of the dam). That is the downstream-constructed type of dam. There may be internal zonation, like a clay core to reduce seepage, and filter drains to intercept seepage and safely carry it away. There could be transition zones, to transition from one zone to another. There may be several zones. Drains may be vertical, sloping, or horizontal. Likewise, clay cores may be oriented in a variety of ways. Or it may me a homogenous fill.
Is that enough types of dams? But, wait, there is more!
Let’s talk about the upstream-constructed tailings dam. Here, you also begin with a starter dam. When a facility expansion is needed, in essence, a new dam is constructed that is at least partially founded on the tailings beach, that is, it begins at the upstream slope of the starter dam. The crest of the expansion dam is upstream of the crest of the starter dam. Thus, it is an upstream-constructed dam. This type of design requires considerably more engineering effort than does a downstream-constructed facility. I’ve designed several upstream raises for tailings facilities, and if they are done right, they can work very well.
There is an intermediate type of structure, between the upstream-constructed tailings dam and the downstream-constructed tailings dam. It is called a centerline-raised facility. In this case, when a facility expansion is needed beyond a starter dam, the majority of the fill is placed on the downstream slope of the starter dam; however, the crest of the expanded facility is vertically above the crest of the starter dam, and only a relatively small portion of the expansion protrudes onto the tailings beach.
There is also an intermediate type of structure that is between an upstream-type facility and a centerline-type facility, and it is called a modified centerline-raised facility. In this arrangement, the crest of the facility moves somewhat in the upstream direction as the facility is expanded. I have worked on several tailings facilities that used the modified centerline technique.
And of course, there are facilities that have elements of more than one type of construction method. There. That’s all the types, right? Nope.
A dam that spans across a valley is called a cross-valley dam (what PR firm was hired to come up with that name?) If the land is relatively flat, then a “4-sided” facility is needed. This is sometimes referred to as a ring dam or a paddock dam. And it needn’t actually be 4-sided. It is usually designed to take advantage of what little relief there is in the topography. I know of one that was designed to be perfectly circular. And there is a dam-type that is intermediate between these two (did you see that coming?). This one is called a side-hill structure. When there is some relief (slope) to the land, a dam is constructed parallel to the orientation of the hill (or alternatively, parallel to the streambed that runs along the toe of the hill). Then the dam is angled and is terminated into the hill-side to form something of a u-shaped structure. And guess what? Some side-hill type tailings dams become ring dams as they are expanded.
Now what? Well, what is the embankment constructed with? Earth-fill? Rock-fill? A combination? How about tailings themselves? And if they are tailings, is it the “whole” tailings, or just the sand fraction? The sand fraction can be separated from the whole tailings using a simple gravity separation device called a cyclone. Because the sand settles at a different rate than the finer-grained silt and clay-sized particles, this gravity separation method, with no moving parts, can be used. The “sand dam” can be constructed with numerous smaller cyclones that are placed and moved around on the tailings dam, or larger cycloning station can be used with the sands either pumped to the dam, or mechanically place. I watched after one very large cycloned tailings facility in South Africa for several years. And, many years ago, I designed and watched over the conversion of a side-hill, cycloned TSF as it was converted into an earthfill dam, with an upstream raise. That’s it, in the photo. For that facility, I did the site investigation, developed the design, watched after the construction and I wrote the record-of-construction report, once it was completed. It was a great way to spend the summer, and I got to spend my days off being a rock hound, wandering through the numerous old trails left by prospectors of the olden days. Okay, what definitely wasn’t great was that my daughter was just born earlier that spring, and I had to be away from home for nearly the entire summer.
Where was I? Oh yes. Being the dam guide. For embankments that are constructed of whole tailings, there are still several types. The simplest to describe is a facility where the embankment is constructed of compacted, whole tailings. There are at least two other in this category, including the spigotted variety and the “daywall” variety. I don’t think I will go into the details of those two, except to say that I was once responsible for the design of a daywall TSF that was constructed in Saudi Arabia. It is likely the first daywall TSF ever to be constructed outside of southern Africa. The dam design was uniquely suited for that particular application.
What else? Well, we haven’t discussed the consistency of the tailings. A conventionally-prepared tailings slurry, is something like a muddy mixture. That is at one end of the “thickened tailings continuum”. The thickened tailings continuum depicts the consistency of the tailings. From conventionally-prepared tailings slurry, we have thickened tailings. The consistency of these tailings are slightly thicker, but still pretty dilute. From there we go the high-density, thickened tailings. At this consistency, the mixture would tend to show very little particle segregation while flowing, and while at rest, very little water would bleed from the slurry. Next up: paste. Paste tailings has the consistency of toothpaste, or wet concrete. It has particular uses for underground backfill, and on occasions, binders (like cement) are added to provide greater strength for added underground support. So far, all of these stages of thickening are accomplished using gravity separation methods, sometimes with the addition of chemicals to assist in the process. Also, tailings on this part of the continuum can be pumped and piped to the TSF, or sent via open channel. Finally, filtered tailings. Filtered tailings have the approximate consistency of used, wet coffee grounds from a drip coffee maker. Filtered tailings can be delivered to the TSF by conveyor or truck. Filtered tailings are made by either squeezing moisture out of the tailings, or by using a vacuum force. There are several different devices that can be used to accomplish this dewatering, and perhaps that will be the subject of a future article. Filtered tailings are often placed into a facility that is sometimes referred to as a “dry stack”, but that industry term is somewhat misleading to non-practitioners. As I said, the consistency is something akin to used, wet coffee grounds from a drip coffee maker, and that definitely isn’t dry. If filtered tailings are done really well, you can create a facility that is nearly any shape that you desire, as long as you have the right practices in place. Unfortunately, not all filtered tailings are produced at the most geotechnically optimal moisture content, and good engineering practices will need to be adopted in the design. I have been involved with several filtered tailings designs, and they can be just as challenging to develop as conventional tailings facilities.
Now, how many kinds of tailings facilities are there? All those combinations and permutations? I’ve lost track! And I’m the dam guide!