Impermeable basal structure - with synthetic liners
Anna Tornivaara, Geological Survey of Finland, P.O. Box 1237, FI-70211 FINLAND, e-mail: anna.tornivaara(at)gtk.fi
When planning a mining waste facility the key issue is to determine the right combination of materials and structures to protect the environment and human health. The purpose of a bottom liner, dam structure, suitable soil layers and firm foundation is to reduce contaminant migration to the underlying vadose zone or aquifer and to minimize settlement under the load of waste material. Synthetic liners are especially useful when geotechnical properties of soil material under the designed waste area (such as non-existent tensile strength or significant permeability) limit its usage as a basal layer. Synthetic liners are also needed when environmental characteristic of waste or regulations and legislation require impermeable liner to seal the waste from the environment (Blight 2010, Bouazza et al. 2002).
Description of the technology
Geosynthetic liners such as geomembranes (GM) can be used in many ways as tailings impoundment liner systems, process solution pond liners, waste rock encapsulation, and heap leach facility liner systems (Fig. 1) whereas geosynthetic clay layer (GCL) can be used as a barrier layer (also below geomembrane liner) and geotextile can be used as cushion layers over geomembrane liners. Different geomembranes have been used as an impermeable basal structure for decades especially in landfills (Breitenbach & Smith 2006). Main purpose in the use of synthetic bottom liners is to get impermeable layer or at least provide a low permeable barrier between the waste material and the uncontaminated soils. GM and GCL used as liners prevent contaminants from penetrating through the basal structure and minimize effluent seepage into the groundwater system or rivers nearby. Water can seep through these liners by diffusion, but the water transmission rates are very low. In spite of that geomembranes have their own defects which may results in a large leakage and are thus rarely used alone as a bottom liner (Bouazza et al. 2002, Shackelford et al. 2010).
Figure 1. Heap leach facility under construction at the Talvivaara mine site, Finland (Photo © A. Tornivaara, GTK)
Geomembranes are planar, relatively impermeable and usually a combination of one or more plastic polymers with ingredients such as carbon black, pigments, processing acids, crosslinking chemicals, fillers, plasticizers, and biocides. Geomembrane should function as a barrier and not as a load bearing member of the lining system. Due to that a geomembrane liner should be fully supported and the structure underneath well designed (sand, clay, synthetic liner etc.). Liner design generally consists of a single liner, composite liner or a double (composite) liner system. There can be also hydraulic head control above the structure and/or leachate collection and removal system under the primary liner (Hutchison & Ellison 1992). Nowadays it is common to use composite liners which consist of both geomembrane liner and natural soil. Composite liners can be either a) single composite liners or b) double liner systems (Fig. 2).
Single composite liner which typically consists of a geomembrane liner placed over compacted soil bedding is commonly used in areas with low hydraulic head such as waste rock facilities and in some tailings impoundments (depending on the properties of the waste, site conditions, and environmental regulations). Geomembrane liner provides a highly impermeable layer to prevent leaching and the natural soil liner beneath serves as a safety back-up in the case of leakage. There should not be any porous material between layers, which could increase the amount of leakage.
Double liner system typically consists of two geomembrane liners and compacted fines (or a composite consisting of geomembrane and natural/synthetic clay liner) and it is recommended to construct a leakage detection, collection and removal layer system above of the primary liner or between the liners (or both). Lower geomembrane liner is placed over compacted soil bedding and it is used in mine facilities where high hydraulic heads may occur (e.g. tailings impoundments). Double lining system also decreases the stress geomembrane meets (EPA 2012, Lupo & Morrison 2007).
The performance of the geosynthetic liner (especially long-term) depends mainly on the type of polymer used in the manufacturing. The most common geomembrane liner materials of waste facilities in the mining industry include HDPE (high-density polyethylene), LLDPE (linear low-density polyethylene), PP (polypropylene), and PVC (polyvinyl chloride) (Lupo & Morrison 2007). HDPE liner has displaced PVC in the geomembranes because of its broad chemical resistance, low permeability, higher strength, and its bonding result by thermal seaming methods compared to the usage of solvents and adhesives.
There are many liner suppliers and material types available on the market, the common synthetic liners including (e.g. Scheirs 2009):
- Geomembranes and flexible membrane liners
- HDPE (high-density polyethylene)
- LLDPE (linear low-density polyethylene)
- LDLPE (low density polyethylene)
- VLDPE (very low density polyethylene)
- VFPE (very flexible polyethylene)
- PVC (polyvinyl chloride)
- PVDF (polyvinylidene fluoride)
- EIA (ethylene interpolymer alloy)
- TPU (thermoplastic polyurethane)
- PP (polypropylene), fPP (flexible)
- CSPE (chlorosulfonated polyethylene), CSPE-R (reinforced)
- EPDM (ethylene propylene diene terpolymer)
- Nitrile rubber
- Butyl rubber
- Polychlorophene (neoprene)
- RPE (reinforced polyethylene)
- BGM (bituminous geomembrane)
- GCL (geosynthetic clay layer), sodium bentonite
- Geotextile (non woven and woven)
Different products have different specifications which should be taken under consideration already in the planning stage, for example HDPE geomembrane liner are packed in a roll, while PVC can be packed in panels. Seaming is needed between two liner sheets or panels, because the synthetic liners have their maximum width depending on manufacturer and/or transport equipments. For example, in the case of geomembranes two sheets (rolls or panels) are bonded together by melting or softening depending of the chemical composition of the liner to prevent leakage of the liner. Proper seaming methods are usually recommended by the manufacturer and the joined sheets should perform as a one single layer.
Factory-manufactured geosynthetic clay liner (GCL) is another commonly used option in basal structure of waste facilities. In GCL hydraulic barrier is typically based on bentonite clay and its long-term performance depends largely on the mineralogy and chemical combination of the bentonite. Dissolution reactions at extreme pH, pore-structure and loss of gel at elevated salinity, and shrinkage at elevated temperatures affect the performance of GCL (Hornsey et al. 2010). Bentonite is supported by geomembranes or geotextiles and held together by needling, stitching, or chemical adhesives. GCL is typically used in areas where natural clay is absent or where small thickness is an important factor. It can be replaced or reinforced by a compacted natural soil layer or geomembranes (EPA 2012). Geotextiles are commonly used in separation and filtration systems for tailings or waste rock dumps or for erosion control (Hornsey et al. 2010).
Figure 2. Typical liner designs for mining applications a) single composite liner and b) double composite liner (modified after Lupo & Morrison 2007).
Development stage, links to cases
Common issues under the development of geosynthetics in waste disposal industry are strength, durability, deformation resistance, and response to the chemicals. Construction test strips are advisable, helping in evaluation of process and quality control. Long-term field performance data is not easily available which means predictions about the material durability are based on a variety of laboratory tests and date modelling (Rowe & Rimal 2008). Laboratory tests are a key factor also when determining the differences in hydraulic conductivity (e.g. Shackelford et al. 2010). It is obvious that higher heap heights contribute to the development of new design, laboratory testing, and construction methods. The main concerns using geosynthetic as a bottom liners are duration of the service life and their cost-effectiveness compared to other alternatives. Service life is predicted to increase when the material is subjected to less stress, lower temperatures, and less oxidizing agents (Renken et al. 2005, Rowe et al. 2009).
Geotextiles and Geomembranes (Elsevier Ltd.) journal has published studies and articles about the geosynthetics since 1984. A case study from the Kittilä Gold Mine on the use of geomembrane liner in a tailings pond (Fig. 3) is presented by Breul et al. (2009):
- At the Kittilä Gold Mine in northern Finland features bituminous geomembrane liner (BGM) (Breul et al. 2009)
Figure 3. Basal structure of the tailings facility at the Kittilä mine site, Finland (Photo © A. Tornivaara, GTK)
Advantages and disadvantages of the liners depend on the chosen synthetic liner material. List below presents some advantages and disadvantages of synthetic materials for the design of a mine waste facility and evaluating the most suitable basal structure (synthetic, soil or combination).
- Not dependent on the availability of local soils
- Possible cost effectiveness if there is a lack of suitable natural soils in the designed area
- Versatility (each material has a specific function, demonstrated by manufacturers)
- Broad chemical resistance (especially polyethylene geosynthetics)
- High impermeability and other hydraulic properties
- Good gas barrier
- High performance compared to the thickness of the layer (more airspace available if some limitations)
- Degrades slowly, is resistant to weathering (HDPE seems to be more durable than PVC)
- High strength (very type specific)
- Drainage function if needed (transmits flow within the plane of their structure)
- Relatively easily transported to the mining site by vehicles
- Fragility (type specific), physical and mechanical compatibility with site conditions and adjacent material
- Seaming problems can cause leaks (seaming techniques may result e.g. in loss in tensile strength)
- Cracking (e.g. at low temperatures the tensile stresses in the HDPE liner can generate brittle stress cracking)
- Some materials are UV sensitive
- Often high coefficient of thermal expansion, which has to be taken into account in planning and construction
- Slope instability
- Unintentional wrinkles can cause defects
- Cost-effectiveness compared to other available alternatives (especially when suitable natural soil material is available in-situ)
- Low shear strength of hydrated bentonite, GCL has also smaller leachate attenuation capacity and potential strength problems at interfaces with other materials
- Is not sufficient as an only layer, geosynthetic liner needs to supplemented with earthen material components (protective soils above or supported layers beneath etc.)
- Lack of long-term geotechnical and environmental control, there is not much data available about the field performance, and the life time prediction is usually based only on material tests of synthetics (incl. high stress, aggressive fluids and elevated temperatures), which means no guarantee about the duration of the service life, causing too often unsatisfied lifetime prediction.
Properties and capacity
It should be noted that the decision between the liner types have to be based on the characters of the mine waste and site conditions. Chosen polymer of the geosynthetic materials or a clay component of GCL has to be strong enough to resist the distinct leachates produced by different ore types and mining processes (Hornsey et al. 2010). Before placing a liner, the material should be tested that it meets the requested specifications provided by manufacturer. Manufacturers should have their own quality assurances and manufacturing quality controls. Construction of a liner can only be started after acceptance and conformance testing. Geosynthetics have many measurable physical and mechanical properties, which directly or indirectly affect to the capacity and performance of the material. The main properties/parameters, which can be tested when evaluating adequate materials for certain waste facility, are (e.g. Scheirs 2009):
- thickness (smooth sheet, textured, asperity height),
- density, mass per unit area (weight),
- field hydraulic permittivity and diffusivity (GCL: K < 10-10 m/s; GM: K < 10-14 m/s),
- melt flow index ,
- vapour transmission (water and solvent),
- tensile strength and elongation,
- impact, puncture and tear resistance,
- anchorage and interface shear strength,
- UV resistance,
- carbon black content,
- susceptibility to stress cracking (constant load and single point),
- compatibility with other materials, and
- service life.
Construction quality control should be made by the contractor or installer while construction process is ongoing to maintain quality and make sure the product meets demanded specifications. After placement of the basal structures, construction quality assurance should be done by independent third party to get verification of quality (EPA 2012).
Leak detection, collection and treatment system (see Monitoring), if needed.
Environmental cost aspects
Liners cost often more than soil material, especially if suitable natural soils are available on site, but the properties of geosynthetics can often be more reliable.
Site specific data needs
Waste properties affect the selection of the waste area’s placement, structure, and disposal techniques. Soil investigation in the planned waste disposal area plays also an important role. The content of the investigations depends on the extent of the disposal area, quantity of waste fractions, and on the waste class (EC 2009). Important local factors are: foundation settlements, dam construction, and soil investigations (deposit types and formations, details about hydrology in the waste area e.g. catchment area boundaries, surface water flow directions) (Kauppila et al. 2013).
Requirements for the materials and appliances
The liner material has to face extreme conditions often beyond the recommended general design limits, such as over 150 m high heaps, heavy equipment loadings, coarse rock overliner, concentrated acid exposure, deep hydraulic heads, and high temperatures (Thiel & Smith 2004). Manufacturers provide specific material property documentations, suitability, requirements and specifications for proper installation (e.g. seaming). There are different methods available for testing geomembranes: ASTM (International American Society of Testing and Materials), ISO (International Organization for Standardization), and GRI (Geosynthetic Research Institute). Different countries and manufacturers can also have specific test methods. For example geosynthetic clay liners should design for a minimum of 3.7 kg/m3 dry weight of bentonite clay with hydrated hydraulic conductivity of no more than 5 x 10-9 cm/sec. Bentonite has low shear strength when hydrated. After the placement liners should be covered by soil layer or geosynthetic layer as soon as possible to prevent defects (EPA 2012).
Synthetic liners should also be protected from physical damage and deterioration by ultraviolet (UV) attack by providing a protection layer. Liners should be wrapped and protected well already during shipments and site storage. Usually there are specified requirements for granular protective soil layer (beneath and above the liner), such as specified grading, moisture content, and density. Other geosynthetic material such as geonet or geotextile can also be used under the geomembrane. To avoid further defects, it is important to remove all folds or wrinkles before the liner is place.
Design criteria must adequately assess and address the potential for chemical and thermal degradation. Some important issues and site-specific factors to consider in a liner system design have been listed below (after Bouazza et al. 2002, Hutchison & Ellison1992, Lupo 2009, EPA 2012):
Site characteristics and hydraulic conditions
- Location to beneficial water resources (including depth)
- Geology and topography
- Unsaturated zone conditions
- Climate conditions
- Potential for advective transport
- Potential for natural attenuation (sorption, biodegradation, and dilution)
- Drainage layer or solution/leakage collection layer
- Grain size distribution
- Chemical composition and properties
- Physical changes resulting from the extraction or mill processes
- Net Acid Generation potential
- Permeability (liquid, gas), thickness and breakthrough time
- Tailings and solution chemistry which could affect the selection of most suitable geomembrane (interaction with waste)
- Load/bearing capacity, thickness of the waste material in closure
- Puncture resistance
- Suitable foundation materials and firm foundation, compaction and liner bedding soil
- Type of tailings to be stored (slurry, thickened, paste, dry stack)
- Spigot/central discharge/truck haul or conveyor (protection layer)
- Friction between the geomembrane liner and the soil subgrade (or between any geosynthetic components)
- Slope stability and erosion
- Freeze-thaw, wet-dry
- Facility type and waste placement method
- Reclaim/decant configuration
- Weather conditions, especially in the construction stage, material properties can be altered by extreme temperatures (high temperature can cause blocking or expanding and low cracking or contracting), heavy winds can be also very problematic in construction resulting in uplifting and tearing
- Material availability
- Solution recovery (piping and drainage layers)
- Leak detection system
- Shipment, handling and site storage
- Water requirements and hydraulic head controls
- Duration of operation
- Quality assurance and monitoring
Minimisation / treatment of potential discharges
It is important that the subgrade (usually native soil sometimes even bedrock) is compacted accurately to remove any soft spots and to reach the right water content to obtain a specified firmness. Layers above the liner can be drainage materials like gravel or sand or other geosynthetics, typically geonet or geocomposite drain. It is critical to avoid puncturing or tearing of the geomembrane or forming of waves, wrinkles or blisters. Careful placement of the protective material should occur soon after the installation of geomembrane and driving directly on the liner should be avoided. Problems with the joints are a typical problem when using geomembranes. The factors influencing the seaming process should be taken into account: weather (temperature both in air and in geomembrane, humidity, wind), subsurface water content, experience of seaming crew, quality of chemical or welding materials, cleanliness of the seam interface etc. Geoelectrical surveys and ultrasound testing probe can be used to check and ensure the quality of the seams. The ultrasound waves are able to detect imperfections in the seam and to measure the width of the defect (Breitenbach & Smith 2006, Breul et al. 2009). Majority of the damages in geomembrane liner is caused by stones (71%), followed by damages caused by heavy equipments (16%), welds and workers (both 6%) (Nosko & Touze-Foltz 2000). Majority of defects are formed during the placement of cover materials. Different stages has their typical defects and especially placement, welding and covering can cause scratches, cuts, holes, tears, overheating, crimping and stress in geomembrane (McQuade & Needham 1999).
Monitoring / control needs
The performance of a liner is controlled by monitoring. Plastic pipe system constructed inside the waste facility is needed to collect and transport the process solution and to detect leakages. In addition, failures in a liner system can be detected much faster using a plastic pipe system than groundwater pipes in the surrounding area. In the latter case, effluent transfer to the aquifer has already occurred when spotted in the groundwater pipes. Proper leakage detection and collection system is effective to intercept, collect and even remove seepage and can be directed to a separate leakage collection sump. In that basin the effluent quality and quantity can be determined and redirected or processed further. It is less time consuming if the detection system is divided into zones, which means the source of any collected leakage can be identified.
Monitoring of the following aspects on weekly/monthly basis is recommended:
- Leakage detection system; Piezometers -> Defined alarm level, and passing the level causes an investigation to detect the reason why an abnormal value was obtained.
- Water level detector (inside the facility and also controlling the water level in the pond/tailings).
- Dam inspections
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