The Critical Role of Engineered Covers
Engineered cover systems are designed to isolate contained mine waste, such as that found in waste rock dumps or tailings storage facilities (TSFs), from the surrounding environment. The primary function of the engineered cover is to minimise the potential for environmental contamination from waste by controlling infiltration of rainfall (percolation). A secondary, yet often equally critical objective, especially when dealing with potentially acid-forming (PAF) materials, is to limit the ingress of oxygen, thereby mitigating the chemical reactions that lead to acid and metalliferous drainage (AMD).
Effective cover design is important for ensuring the long-term physical and geochemical stability of contained mine waste facilitating successful rehabilitation. The choice of engineered cover strategy is highly site-specific, influenced by factors like the nature of the waste, the method of waste deposition, local climate conditions, and the availability of suitable construction materials. In situations where a mine has a history of poor segregation and stockpiling of materials suitable for engineered cover construction, options for rehabilitation using natural soil alone can become limited. This often results in the incorporation of engineered components, specifically artificial reduced permeability layers (RPLs), into engineered cover design to meet performance objectives.
Artificial Reduced Permeability Layers
When natural soils cannot achieve the required low permeability, or when regulatory limits on percolation or oxygen flux must be met, artificial RPLs are used. These are manufactured materials designed to act as hydraulic or oxygen barriers within the engineered cover. Two common types used in mine closure applications are geosynthetic clay liners (GCLs) and bituminous geomembranes (BGMs).
Geosynthetic clay liners are a hydraulic barrier. They consist of a thin layer of processed bentonite clay (typically sodium bentonite, known for its high swelling capacity) either bonded to a single geosynthetic layer or, more commonly, encapsulated between two geotextiles (woven, non-woven, or a combination), which are then needle-punched or stitched together. The geotextiles provide tensile strength and protection for the bentonite. When confined and exposed to water, the bentonite swells significantly, filling void spaces and creating a continuous layer with very low hydraulic conductivity, often equivalent to much thicker layers of compacted clay.
Bituminous geomembranes are robust composite geomembranes. A typical structure involves a non-woven polyester or polypropylene geotextile heavily impregnated with an elastomeric or polymer-modified bitumen binder. Some designs incorporate additional layers, such as a glass fleece for stability or a thin surface film. Their construction results in a flexible, tough, and impermeable barrier with distinct properties, including puncture resistance and the ability to be installed under a wide range of temperatures compared to some other geomembranes.
Case Study: Field Trials
An industry example incorporating artificial RPLs into engineered cover design comes from SGME, who have been assessing the performance of engineered covers on a TSF in Tasmania. The mine site is located in a temperate, high-rainfall region, and has a TSF containing PAF tailings. Closure planning requires the design of an engineered cover primarily aimed at limiting percolation, with the secondary goal of limiting oxygen ingress to prevent AMD.
Recognising the site-specific challenges, particularly climate, large-scale field trials commenced in 2021 to compare the performance of two barrier cover designs before selecting a final option for full-scale implementation. Option 1 is a multi-layered cover comprising (from bottom up) 0.5 m glacial till and 0.2 m Moorland peat. Option 2 is similar to Option 1, but incorporates a GCL placed directly on the tailings surface, beneath the 0.5 m glacial till layer. Both covers are instrumented, including lysimeters to continuously monitor key performance indicators including percolation, volumetric water content, matric suction, temperature, and oxygen concentration within the covers and underlying tailings.
Key findings from the trial regarding percolation showed the GCL option demonstrated superior performance in limiting percolation. After an initial period allowing for consolidation and water redistribution, percolation beneath the GCL layer is effectively near zero. In contrast, the non-GCL option showed measurable percolation, indicating that the glacial till layer alone, while providing resistance, is more permeable than the GCL.
Both covers successfully reduced oxygen concentrations within the profile compared to ambient atmospheric levels. However, the GCL option provides an enhanced barrier to oxygen diffusion. Oxygen concentrations beneath the GCL are consistently lower than at equivalent depths in the non-GCL cover. This is particularly evident when the glacial till layer has a high degree of saturation, which is more noticeable in the GCL option due to the underlying low-permeability layer impeding percolation. The near-saturated Moorland peat in both options also contributed significantly to limiting oxygen ingress.
The presence of the GCL significantly influences water dynamics in the overlying layers. Glacial till above the GCL maintains a higher degree of saturation for longer periods compared to the non-GCL option, reflecting the GCL’s effectiveness in limiting percolation. The Moorland peat layer remains near-saturated in both options, functioning as intended to create a perched, largely anoxic zone.
The conclusion from these trials demonstrates that incorporating a GCL into the engineered cover significantly enhances its performance by reducing percolation and providing a more effective barrier to oxygen ingress compared to an engineered cover relying solely on glacial till.
Advantages and Limitations of Artificial RPLs
The selection of an appropriate artificial RPL involves weighing its advantages and disadvantages in the context of specific project requirements.
For geosynthetic clay liners, advantages include lower hydraulic conductivity than compacted clay liners when properly hydrated. As factory-manufactured products, they offer greater consistency compared to field-compacted clay. Installation is typically faster, less weather-dependent, and requires less heavy equipment than constructing thick compacted clay liners. They achieve low permeability with minimal thickness, preserving valuable airspace/storage volume and reducing material transport costs. GCLs are better able to accommodate differential settlement without cracking compared to compacted clay liners. Furthermore, bentonite’s swelling nature can allow minor punctures to self-seal under favourable conditions, and they generally perform better than compacted clay liner under freeze-thaw cycling.
GCLs also have limitations. Their performance relies critically on achieving and maintaining adequate hydration; desiccation can increase permeability. They can be damaged by sharp objects during handling, placement, or by overlying materials if not adequately protected. Performance can degrade in contact with aggressive leachates (eg very high or low pH, high salinity) that inhibit bentonite swell. The internal shear strength can be a limiting factor on steep slopes, requiring careful design and potentially reinforcement. While effective water barriers, they may only act as effective gas (oxygen) barriers when fully saturated. Finally, as with all geosynthetics, long-term degradation is a factor, although design lives can be extensive.
Advantages of BGMs include high puncture and tear resistance, which allows for potentially less stringent subgrade preparation and direct contact with certain cover materials or light traffic. They can be installed across a broad range of temperatures, including sub-zero, and are less sensitive to light rain or wind compared to some other liner types. Their low coefficient of thermal expansion minimises wrinkling issues common with high-density polyethylene in hot conditions. BGMs drape well over subgrades and accommodate settlement. They generally exhibit good resistance to a range of chemicals found in mining environments, and certain grades can be left exposed to UV radiation for extended periods, potentially deferring costs.
Limitations of BGMs include the fact that while field seams are robust, they rely on thermal bonding (torch or welder) and require rigorous quality assurance and control. Despite overall toughness, they are not immune to damage from poor handling or installation practices. The bitumen binder can soften under intense direct sunlight and heat, potentially increasing susceptibility to damage during that time. They can also have a higher material cost compared to some other geosynthetic options, although installation efficiencies can sometimes offset this.
Conclusion
Engineered covers are indispensable components of responsible mine closure, designed to ensure long-term environmental protection. Where site conditions or performance requirements preclude the sole use of natural soils, artificial RPLs such as GCLs and BGMs provide robust engineering solutions to control percolation and, where necessary, oxygen flux. The field trial discussed in this white paper exemplify the significant performance benefits a GCL can offer in reducing percolation and oxygen ingress in a challenging, high-rainfall environment.
The optimal choice between GCLs, BGMs, other geomembranes, or composite systems hinges on a thorough evaluation of site-specific conditions, including climate, waste properties, available materials, topography, regulatory requirements, cost-benefit analysis, and desired long-term performance longevity. Successful implementation requires careful design and attention to installation quality assurance, and an understanding of the inherent limitations and vulnerabilities of the chosen materials.