Whether kept relatively dry and heaped or mixed as a sludge and pumped into a pond, tailings are generally contained by impoundments. Impoundments are any man-made or natural structures that have high enough walls and enough seepage control to keep the tailings from escaping. Since the sheer volume of the production of most mines can lead to large scale storage areas, dams are usually built to help contain the material. It is simply unfeasible on most sites to try and excavate an area large enough to keep the material below grade, unless the pit of the mine is available for storage from the start. Building berms, dikes, dams, and other raised impoundment features is the standard way of containing tailings. Of course, none of these structures will truly contain the material and its runoff unless they are lined with a geomembrane like the RPE available from BTL Liners.
Valley Impoundments
If there is an available valley near the mine area, the natural depression is usually dammed, and the resulting space is used for the impoundment. These methods work for both wet and dry tailings mixtures, but the weight of wet tailings can increase over time and eventually weaken the dam. These dams must be built with care to prevent seepage through it and eventual rupture. The majority of costly and impactful tailings emergencies in the past were caused by poorly designed impoundment dams for valleys. Using a flat area with raised embankments or excavated pits can prevent a lot of the risk of flooding and slumping that comes from using the traditional valley impoundment method. Valleys remain popular where they are available because they are the least expensive to construct. With little to no excavation needed, only the dam building and lining costs contribute to the final price of impoundment. Unlike other containment designs, these impoundments are generally built and finished before any tailings or water is directed into it. This can be a hassle if the impoundment is large and the dam must reach hundreds of feet in height to completely seal off the valley. Choosing another method that is constructed as the tailings volume grows is often considered a faster way to get started on a mining project.
Ring-Dikes
As the name suggests, ring-dike impoundments are built over flat or level ground and involve raising embankments on a ring around all the sides of the pond. The height of the embankments and their recommended amount of slope depends on whether mostly dry or wet tailings are being stored within. The dikes must be built high enough to contain any runoff or rising water levels during the worst flooding events the area might experience. The dikes are generally constructed with a combination of the overburden removed during mine preparations and the material excavated from the impoundment area to lower the surface and increase storage area. If this is not enough material to create the desired height of embankment on all four sides, other fill material may need to be brought in. However, these dikes can be added onto as the tailings pile grows. It does not have to be built to full height, only full surface area and depth, at the beginning of the mine’s operation. This means that future additions to the embankments can be added after the mine begins producing tailings, allowing for the new additions to be made with the waste material. These designs also have the greatest flexibility of placement and overall size since they can be built almost anywhere the ground can support the weight, not just where there’s an existing depression or valley. Finally, the ring design limits the amount of wastewater that enters the impoundment. This reduces the total storage volume needed and can prevent runoff issues.
In-Pit Designs
Depending on how long the tailings will spend in processing, they may be able to be moved back into the pit excavated for the original mine at the end of its operation. Surface and strip-mining projects tend to offer the most opportunities for in-pit tailings storage since they generally excavate large and deep areas that become closed long before processing completes on the tailings. These kinds of pits generally do not need lining because they are usually already contaminated with anything found in the mine, in addition to being naturally semi-impermeable. However, unlined in-pit tailings containment can put groundwater at risk as the liquids slowly seep down through the ground. Even previously mined pits should be lined before they are filled with tailings. Unlike impoundments built with raised embankments over level areas, these storage areas can’t be expanded over time. They are also prone to accumulate runoff from the surface unless berms are built around the edges of the pit to prevent this issue.
Specially Dug Pit and Pond Designs
While the majority of pit tailings storage systems are based around existing or new mines, specially dug pits can also serve as an impoundment. However, they are usually considered too high cost to invest in just for the storage and containment of a worthless by-product like tailings. One option is to turn the tailings into a high density paste and use it to fill voids around other excavations, lowering the risk of collapses underground. Ponds are the most commonly used pit design for open ground storage of tailings because these features can double as processing and storage areas at once. Most follow the general pattern of a ring-dike impoundment, but some feature more unique designs that address the risks of tailings loss with covers or rigid reinforcements.
All of these impoundments require liner of some kind. Some of the designs call for installing liners in between layers of embankment as well to control lateral seepage and prevent slumping that can release the contained tailings. Trying to get around the need to line the containment area by choosing a different type of impoundment generally won’t work. The best options is to use an affordable geomembrane liner you can count on lasting for decades, such as the products available from us here at BTL Liners.
