Site conditions, topography and environmental factors all play critical roles in determining the type and placement of a tailings storage facility. Considerations must be made for operational needs both while the mine is active and for continuing management after it’s closed. When weighing the pros and cons for a variety of potential solutions, studies are conducted to predict the cost, risks, and impact of each. Ultimately, a trade-off analysis allows engineers to determine which option represents the best overall plan for that site.
Configurations
Ring dikes
Ring dikes can also be referred to as paddocks or cells. Specifically, these are standalone impoundments built to enclose a storage area on all sides. Since this type of enclosure is independent of topography (i.e., it doesn’t need to be positioned near a natural depression), it represents a flexible option and can typically be sited closer to the processing plant. Another advantage to ring dikes is that the structure naturally protects the impoundment from surface water runoff, so the design needs only to account for the volume of precipitation and original process water. However, with constructed walls on all four sides, this type of enclosure requires much more material than other configurations, making it one of the most expensive types to build.
Valley Impoundments
Valley type impoundments make up most tailings ponds currently in operation. The cost of building an embankment or a dam is largely correlated to the volume and cost of material, and since valley impoundments employ the sides of a valley to help contain tailings, they can represent a significant savings during construction.
Cross Valley - In cross valley configurations, a single dam is constructed reaching from one side of the valley to the other. The most common type of tailings pond, these structures are intensely vulnerable to flooding from snowmelt or severe weather events. Thus, the impoundment area is usually located at the head of the drainage area to limit large incursions of water. Additional flood mitigation might include diversion ditches, spillways, or additional upstream dams.
Side Valley - Side valley systems are primarily employed when there is a relatively low grade and consist of a three-sided dam built against the hillside. They’re generally not well suited for steeper terrain, but management of surface runoff is fairly easy to accomplish through use of ditches and embankments.
Valley Bottom - Valley bottom impoundments are essentially a combination of cross and side valley designs. They are best suited when the valley sides are too steep, and the catchment area is too large to accommodate either of the first two options. A valley bottom design involves constructing a series of impoundments along the valley floor as it rises, and almost always requires building a diversion route for natural surface water flow in the area.
Valley type designs are popular because they are relatively cheap to construct, compared to ring dikes, and can contain more tailings in relation to the amount of material required. However, their initial economic advantage is tempered by elevated environmental risks. Specifically, valley impoundments are vulnerable to flood waters which can overtop the dam, carrying toxic tailings materials into the environment. Erosion where the dam structure meets the valley, and even changes to the internal stability of tailings incorporated into the walls themselves can significantly reduce the structural integrity of the entire dam, with devastating results.
Using Liners
Liners fulfill several different capacities in tailings management facilities, but virtually all capacities are protective: of the environment, of the extracted resources, and of associated structures. In tailings ponds, liners are generally used to contain the tailings and associated fluids or materials while preventing transference into neighboring soil and water. Effective design and installation of high-quality liners can help lower insurance premiums and cleanup costs, as well as reduce liability risk and associated environmental or reputational damage. Far from a quick matter of applying a simple layer of thick plastic, tailings liners require a carefully designed system that may include components to reduce hydrostatic pressure on the liner itself and to mitigate the accumulation of fluids under the liner. As technology advances, new liner types and configurations can be integrated to create a range of systems well suited to local requirements and conditions. Choice of a specific type and brand of liner involves a complex calculation that depends on the characteristics of the liner material and its suitability for the conditions at the mine site. Economic considerations also play a role, as the size of the impoundments, especially for cross-valley dams, can be enormous.
Compacted Clay
Not all liners are composed of manmade materials. Compacted clay liners use natural clay sourced from nearby deposits and are constructed rather than installed. The construction sequence of a CCL can be arduous and includes preparation of the foundation, optimizing the moisture content of the clay, placement of the materials and finally compaction to minimize any tiny air pockets, cracks or other potential faults in the seal. It’s also critical during construction to maintain the moisture content of the prepared clay until it has been covered with water, since drying out leads to cracking, shrinkage and other sources of leaks. CCLs are vulnerable to seismic activity and cannot withstand much differential settlement in the foundation without cracking. While most synthetic liners are resistant to acids, bases, and salts present in tailing dam seepage, clay may be vulnerable to interactions with certain leach solutions. In such a case, associated weakening of the seal could eventually lead to impoundment failure. Perhaps most importantly, even if faults are promptly discovered, repair of a damaged compacted clay liner is extremely difficult, if not impossible.
Geomembranes
Geomembrane liners encompass an assortment of manmade materials, which in turn are available in multiple configurations. Examples include high density polyethylene (HDPL), polypropylene (PPL), and polyvinyl chloride (PVC). Different liner products and combonations are typically evaluated based on price, performance, and availability, as well as their reputation for reliability over time. Since geomembranes can be custom fabricated and shipped directly to the site for quick installation, the savings can be substantial when compared to other liner options that must be constructed on-site.
Fabrication and Installation - Geomembrane liners are fabricated and shipped to a site in large rolls. They can be customized to fit the specific topography of a site, permitting rapid installation. Liners of this type are relatively thin, however, and care must be taken to select and properly install a high-quality liner, formulated for reliable containment. Rigorous quality control and assurance programs exercised during construction and installation should result in a well-engineered system which virtually eliminates the risk of environmental contamination through seepage or leaks.
Stretching and Distortion - Ores stored in ponds are especially heavy and tend to distort non-reinforced liner products, stretching them like a balloon, which increases wear and risk of tears or impacting the ability of liquids to flow within a collection layer. Reinforced polyethylene (RPE) is especially designed to resist this type of expansion under heavy loads. One excellent example is BTL’s innovative, industry leading ArmorPro liner, which combines a layer of woven reinforcement to limit stretching, bulging and tearing with specifically engineered coatings to maintain toughness, ease of installation and long term reliability.
Punctures and Holes - Punctures and increased wear from sharp materials such as crushed rock can impact the integrity and longevity of a liner. This can be mitigated by the addition of a small layer of sand between the rock and liner or the use of a thin cushioning underlayment, but the use of a reinforced liner specifically engineered to withstand abrasion and wear, such as BTL’s ArmorPro, is advised.
Chemicals and Temperature - Chemical and temperature effects on geomembranes can also alter their performance, and selection of liners that are inert to possible chemicals and chemical processes in a storage facility is critical. Consideration of possible (not simply typical) temperature extremes should also be considered.
Geosynthetic Clay
GCL is a hybrid product, composed of a layer of bentonite clay sandwiched between two geosynthetic layers. When this type of liner is exposed to water, the bentonite absorbs it and expands up to 30 times in volume until it forms a gel, which effectively prevents passage of liquids. The gel-like consistency of the clay in this liner means it’s less vulnerable to uneven settling or shifting and it’s fairly well suited to self-repair in certain situations. However, since the bentonite is held between layers of geomembranes, the longterm effect of abrasion on the liner and possibility of punctures should be considered as well.
Selecting a Liner
The use of a high-quality product and a team of experienced, professional installers can ensure that installation is faultless, all joints and seams are optimal and no potential wear points are overlooked. BTL Liners has been recognized as an industry leader in RPE liner innovations for 40 years. Our team of experts can help you select the best geomembrane for your containment specifications based on each section of treatment or processing step. No matter what kind of ponds and channels you need for your mine, we can help you select and install the right liner.
