Synthetic multi-layer cover
Henna Punkkinen, Markku Juvankoski, Tommi Kaartinen, Jutta Laine-Ylijoki, Elina Merta, Ulla-Maija Mroueh, Jarno Mäkinen, Emma Niemeläinen & Margareta Wahlström, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland.
Use of synthetic cover material, a geomembrane, can prevent or significantly reduce water and oxygen transportation into the waste (INAP 2009, Kauppila et al. 2013), and thus minimise acid generation and metal leaching formation of sulphide bearing mine wastes. Synthetic cover material is often a part of a multi-layer cover structure (INAP 2009), which is usually a compilation of a bedding layer, a synthetic layer, a protective layer, and a growth medium layer. However, cover design is always a site-specific process, which depends on the climate conditions, availability of materials, costs, tailings characteristics, sensitivity of the surrounding environment etc. (Mylona et al. 2007) and on this account the structure of synthetic multi-layer cover can vary site-specifically.
Description of the methodology
Many different types of synthetic cover materials have emerged during the last decades (Haug & Pauls 2002), for example plastic geomembranes (LLDPE, PE, HDPE, PVC, CPE, DuPontTM HYPALON®), geosynthetic clay liners (GCLs), and bituminous geomembranes (BGMs) can be used (INAP 2009, Kauppila et al. 2013). Due to its flexibility, LLDPE is probably the most commonly used plastic material. The use of synthetic material requires a careful seaming (Kauppila et al. 2013), and the layer is recommended to be placed between bedding and protective layers (INAP 2009). This chapter presents an example of a multi-layer cover structure in which synthetic material is used. The layers and their primary functions are described more specifically in Figure 1 and Table 1. The structure illustrated here is a modification of the layer structure model of non-hazardous landfills presented by Wahlström et al. (2009).
Figure 1. A cross-section of synthetic multi-layer cover (Wahlström et al. 2009).
Table 1. Cover layers and their functions in synthetic multi-layer cover structure (Wahlström et al. 2009).
|Layer||Purpose||Material (traditional)||Thickness*||Required material properties|
|Top soil cover||Frost protection of mineral layer, protects mineral layer from drying
Secures water supply for vegetation
Protects lower layers from roots
Biological oxidation of odorous gases
Reduces rainwater infiltration into the waste fill
Prevents spreading of dust
Promotes reclamation of the area
Prevents water and wind erosion
|Vegetation layer, humus soil||≥ 1 m||Surface erosion resistant
Sufficient protection against frost action in Nordic countries
|Filter-bed||Prevents the clogging of drainage layer||Sand, gravel, geotextiles||≥ 0.1 m||Filter criteria, strength|
|Drainage layer||Reduces hydraulic gradient towards mineral layer
Conducts infiltrated water out of the structure
|≥ 0.5 m||Water permeability, k> 10-4 m/s
Minimum slope gradient 5 %
Fine-grained material content <5%
|Blocking layer||Prevents the mixing of drainage layer material with mineral layer material||Sand, geosynthetic materials||Grain size distribution and grain shape
|Artificial sealing liner||Prevents the infiltration of rain water into the waste fill||Plastic geomembranes, geosynthetic clay liners, bituminous geomembranes||≥ 0.002 m||Resistant to strain caused by differential settlements|
|Impermeable mineral layer||Reduces rainwater infiltration into the waste
Protects and supports artificial sealing liner (downward)
|Clay, silt, moraine, sand-bentonite, (mixture of sodium silicate and sludge or fly ash, foundry sand)||≥ 0.5 m||Water permeability, k< 10-9 m/s (infiltration rate 5 %), exception: k< 10-8 m/s (infiltration rate 20-25 %)
Risk of cracking caused by drying and chemical changes of the material and settlement of the fill
Dissolution of carbonate minerals and other substances
Functionality with artificial sealing
|Filter-bed (when needed)||Prevents the mixing of mineral layer||Sand, gravel, geotextiles||≥ 0.1 m||Filter criteria|
|Primary cover||Prevents the mixing of top layer with other layers and waste
Balances the pressure towards other layers when compacting top layer
|Natural soil, (contaminated soil)||≥ 0.3 m|
A slope structure requires a drainage layer for the conduction of infiltrated water out of the structure, but in certain cases it may be possible to leave out the drainage layer from the waste rock heaps’ top cover structure.
Case studies are presented by:
- INAP (2009) (Upshur Mining Complex, Poirier Mine Normetal, Mount Washington, Kam Kotia).
- Haug & Pauls (2002) (Kjøli Mine, University of Saskatchewan Chemical Landfill Cover, Halifax International Airport, Waite Amulet, Iron Mountain Mine, P.T. Kelian Equatorial Mining Gold Mine, SPPA Potash Tailings, Ste-Gertrude Landfill, Trail Road Landfill, Dyer Boulevard Landfill, Spoil Heap in Northern France, Mount Washington, Poirier Mine Site, Weedon Mine Site, Munich Airport, Swedish Sludge Basin, Somex Mine Site, Memorial Gardens Fresh Water Pond, Summitville Mine).
- Also Talvivaara mine in Sotkamo, Finland, has a preliminary mine closure plan in which synthetic multi-layer cover structures are presented to be used in closure of mine wastes (Pöyry, 2009).
The synthetic multi-layer cover structure is suitable for covering acid-forming tailings, mineral precipitate sludge ponds (Kauppila et al. 2013), and waste rocks areas. Table 2 presents the most important advantages and disadvantages that are linked to the use of synthetic cover materials.
Table 2. Advantages and disadvantages of synthetic covers (Rathmayer & Juvankoski 1992, Mylona et al. 2007, Renken et al. 2007, INAP 2009, Kauppila et al. 2013).
|Impermeable / low permeability: Properly seamed plastic liner prevents water infiltration / low permeability of bentonite materials.||High costs (also transportation costs must be taken into consideration).|
|Resistant to chemical and bacterial activity.||Possible limited design life – between 50 and 100 years: ageing of plastic liners.|
|Synthetic liners are easy to install.||Needs also other layers above and below; proper bedding and protective covers required.|
|GCLs (geosynthetic clay liners) have ability to self-repair holes and rips due to the swelling properties of bentonite.||Geotechnical stability concerns for steep slope applications: mass slides are possible.|
|Also reinforcive and filtrative characteristics depending on the material used.||Gas formation and discharging may stretch plastic liners.|
|Possible vulnerability issues: puncture by surface traffic, sun light, cracking and creasing, improper seaming, degradation due to low acidity conditions / cation exchange (GCLs), contained fluid or gases may cause uplift pressure, differential settlement of underlying materials, thermal expansion and contraction (high thermal coefficient).|
|GCL has low shear strength at midplane, and this material type also requires additional laboratory and field testing to assess its effectiveness. The suitability of GCLs to act as oxygen diffusion barriers is uncertain.|
In the long run, all dry cover systems must endure the effects of climate, hydrology, human activity, animals, vegetation, and the settlement of underlying material (INAP 2009). The key constraints for the use of synthetic materials in mine covers are the duration of the service life and their cost-effectiveness. Although the long-term performance data of mitigating AMD (acid mine drainage) does not exist, polymer-based geomembranes such as HDPE and LLDPE, are applicable at least in the short and medium term. However, their service life is very temperature dependent. Also GCL may be suitable to act as a hydraulic barrier, but is probably not effective as an oxygen diffusion barrier. (Renken et al. 2007) Finally, as the synthetic materials have only a limited lifetime, replacing the cover may be necessary at specific intervals (Rykaart & Hockley 2009).
As mentioned above, the function of the synthetic liner is to prevent or reduce water and oxygen transportation into the waste material (INAP 2009, Kauppila et al. 2013), and thus the cover structure isolates harmful substances from the surrounding environment and prevents seepage water formation. Synthetic materials are often necessary components of the cover structure, if low net percolation rates (e.g. <5 % of precipitation) are included in the performance objectives of a cover system (MEND 2012). No regular maintenance is normally required after the cover installation, monitoring needs are discussed in the next chapter. Poorly installed or seamed synthetic layers do not retain water and oxygen diffusion into the waste material and this may lead to the formation of acid mine drainage.
Cover structures for mine wastes are often expensive. The costs are heavily dependent upon the availability of the cover materials (Rykaart et al. 2006, INAP 2009), the complexity level of the cover, the size of the construction fleet, and whether progressive reclamation is done during mining or if the reclamation is done after the mining has stopped (Rykaart et al. 2006). Due to the complexity of the cover structure and the use of synthetic materials, synthetic multi-layer covers belong to the group of the most expensive cover structure alternatives. Although actual costs are always site-specific, some estimates can be found in the literature. However, these cost estimates vary widely. For example, it is assessed that multi-layer cover containing synthetic materials can easily cost twice as much as soil cover structure, from around $50,000 to $200,000 / hectare (INAP 2009) (approximately €37,000-147,000 / hectare), while another study estimates the costs for complex multi-layer covers to be up to $500,000 / hectare (Rykaart et al. 2006) (around €370,000 / hectare). The most expensive cost estimate found, a case study in which LLDPE membranes were used to cover acid generating waste rocks, demonstrated the final costs for the installation (including resloping) to be around $807,500 / hectare (Renken et al. 2007) (~€594,900 / hectare).
Site specific data needs to be collected when selecting and designing the cover structure materials to ensure that the chosen membrane material will be chemically applicable with the waste material and the cover will function as desired. The dimensions of the waste heap such as settlements, inclinations, gradients, and the water conveying actions of the mine site may have effects to the selection of the material used and should be observed. When the structure is placed on a relatively steep slope, a slope stability analysis is required, possibly resulting into a reduction in slope gradient. Any possible gases and/or heat reactions (expansion) formed by the long term chemical alteration of the waste material must also be taken into consideration. (Kauppila et al. 2013) Ground temperature controlling may be useful in certain cover construction projects. (Rykaart & Hockley 2009)
Nowadays, many different kind of synthetic materials with variations of characteristics exist. Some important membrane characteristics that must be taken into consideration when designing and installing cover structures that contain synthetic materials include (Leppänen 1998):
- An adequate chemical and mechanical stability of the liner (Leppänen 1998; Suomen ympäristökeskus 2008): A chemical applicability between the chosen membrane and waste material. The usage of bentonite materials sandwiched between geotextiles, or bonded onto a geomembrane (GCLs) requires that the structure will not dry (desiccation cracks formation) (Kauppila et al. 2013) and no cation exchange reactions will occur (INAP 2009). The protective layer below (and also above in case of waste rock piling) the synthetics prevents breaking of the liner (by sharp rocks/sand particles) and point loads (Kauppila et al. 2013).
- Environmental stress cracking risk (Leppänen 1998).
- Installation and weldability (Leppänen 1998): A careful joining of the sections should be performed in order to avoid the breaking of the synthetic liners (Kauppila et al., 2013). Quality controls are needed during the installation, especially the membrane welding requires proficiency. The pile traffic during the rehabilitation should be taken into account when selecting upper and lower protective layers (Kauppila et al. 2013).
- Friction factors (in slopes) (Leppänen 1998): A friction surface between synthetic material and soil layers, and its effect to the cover structure (Suomen ympäristökeskus, 2008). In some cases (in slopes), the use of textured plastic liners may be useful.
- A resistance to UV-radiation (during installation, in slope top) and frost-proofness (Leppänen 1998): The liners are sensitive to degenerate if exposed to sunlight, and should be covered with soil (INAP 2009, Kauppila et al. 2013). If synthetics cannot be immediately protected with an earthen cover material their resistance to UV-radiation and frost must be confirmed (Suomen ympäristökeskus 2008).
In general, handling, transporting and storing synthetic materials is strictly specified. Especially in cold regions, such as in Nordic countries, these specifications have a very important role, because the wetting and freezing of synthetics prior to the construction prevents their deployment during the winter time and some materials may also have minimum temperatures below which they cannot be installed (Rykaart & Hockley 2009).
There are two options for the use of plastic geomembranes in cold regions; regular liners have to be installed during summer time, or liners which are specifically developed for the use in cold regions have to be used. Weather conditions (frost) must also be taken into consideration when seaming these liners. GCL materials can be placed at any temperature. However, a significant moisturation in the GCL before or during installation increases the risk of puncturing, (Rykaart & Hockley 2009) although usually GCLs are not allowed to get soaked before there is enough pressure on the GCL (at minimum 0.3-0.5 m soil cover). It has been shown that CGLs are able to withstand freeze-thaw cycles without any notable increase in the hydraulic conductivity. Bituminous liners have not been used in large scale cover applications for mine waste areas in cold climates. Generally, bituminous liners that are used in cold regions are especially designed for the use in this kind of areas. (Rykaart & Hockley 2009) The installation of bituminous liners is straight forward and can be done under unsettled weather conditions (Renken et al. 2007). However, also the other layers in multi-layer cover structure have certain requirements for the weather conditions during their installation. For example, the processing of dense mineral materials should be done in temperatures above +5 °C. The compacted material must be unfrozen and free of snow and ice. Also rain, wind or sun light may complicate the installation process. (Leppänen 1998)
After the installation work is finished, some monitoring may be required from time to time to ensure that the cover structure is working properly. Synthetic multi-layer cover structures may require settlement controlling, whose results may indicate for puncturing and transitioning. Also water controlling may be used for estimating the condition of synthetic liners (a formation of acid mine drainage may be a sign of puncturing). It may also be necessary to control the slope stability of the cover structure to observe or prevent possible slope failures.
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