The use of geosynthetics in mining operations grows annually, as mining companies focus on the technical and economic advantages of geosynthetics. These materials have enabled more efficient barriers, stronger access roads, space-saving and safety-enhancing retaining structures, and much more. Kent von Maubeuge and Raquel Ribera summarizing the use geosynthetics in mining. This publication is part of Geosynthetica’s GeoAmericas series. FEATURE IMAGE: NAUE.
1. INTRODUCTION
The daily mining rates, scale of single-site operations, and costs associated with mining increase every year. Advances in extraction technologies have greatly increased recovery rates from ore bodies. Mine designs previously thought to be too big to be possible are achieved every year or two so that an average mine today is significantly larger than an average mine just 10 years ago (Smith, M.E., 2013).
To construct on this scale, which is often necessitated by marketplace price points and competition for investor support, requires substantial engineering to make mines economically feasible and environmentally sound.
Geosynthetic materials are one part of how mining companies achieve their goals.
The heap leach projects are some of the largest users of geomembranes, by some estimates consuming 40% of all geomembrane produced, and continued uses are still being developed (Christie, M.A. 2013). Heap leaching has grown substantially as a technique for extracting valuable material from ore. Ore heaps of 200 m are being constructed.
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Mountaintop and “valley leach” designs are implemented. Geosynthetic lining systems contain the pregnant solution so that it is not lost in seepage into soils and does not flow into local waterways. Gold, copper, nickel, uranium, and even rare earths are being heap leached. Geosynthetics contain the valuable reserves and isolate the waste (e.g. the tailings), thus providing economic and environmental advantages to the site.
“Raincoat” liners keep storm water out of ore heaps so that the pregnant solution is not diluted. Processing is more efficient this way. Also, geosynthetic lining systems protect water resources on site. With water costs in some regions having increased by 300% in the past five years, conserving water on remote mining operations significantly reduces expenses (Smith, M.E., 2013).
Containment isn’t the only solution needed to keep mining operations competitive and viable. A vast range of geotechnical works are required for operational performance and environmental security. Geogrid reinforcement stabilizes berms, embankments, crusher walls, and other soil structures. They support access roads so that 100 ton payloads can pass daily for years on site without costly roadway failures. (A mine can lose millions of dollars, USD, per day if an access road fails.)
Geotextiles provide separation of granular layers, filter stability in geotechnical constructions, and protection of other geosynthetics. In combination, these materials improve the recovery of valuable materials, isolate contaminated waste, keep sites open, and make closure a more efficient and less costly endeavor.
Geosynthetic solutions for the mining industry are engineered for long-term performance in all environments and with the chemical compatibility necessary to meet the economic and environmental goals of today’s mining operations. Solutions include:
- Polyethylene (PE) geomembranes as barriers
- Geosynthetic clay liners (GCLs) as barriers
- Soil-reinforcing geogrids
- Nonwoven filter and separation geotextiles
- Geocomposite drainage/gas venting materials
2. HEAP LEACH
Without question, heap leach has become an enormous driver to the growth of mining operations around the world. Twenty-five years ago, only about 3% of copper and gold supplies were produced through heap leaching. Today, the volume is surpassing 30% annually (Smith, M.E., 2013). Valuable chalcopyrite copper, previously not considered economical in heap leach development, is now heap leached, as is nickel laterite, uranium, and even rare earths.
The growth of heap leaching is heavily tied to the massive scale on which mines are being built, with as much as USD $2 billion being invested in single sites. Heap leach stacks can near 200 m as operations look to more quickly prove site yield.
Heap leaching accomplishes this—but only with the containment support of geosynthetics. Geomembranes and geosynthetic clay liners (GCLs) are used in lining system solutions as heap leach pad liners, pregnant solution trench liners, processing pits, onsite water storage, “raincoat” covers over ore stacks to shed storm water (rather than dilute the leach heap solution), and onsite wastewater management.
Geosynthetic lining solutions enable steep slope (including mountaintop) developments. Pregnant solution flows more easily from heaped ore, and valuable material is not lost in seepage into soils or local waters. Onsite water is managed more efficiently, which also improves site costs, as water and wastewater management is a major cost in mining.
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2.1 Geosynthetic Solutions
High-density polyethylene (HDPE) geomembranes feature exceptional chemical, stress crack, and UV resistance. They have the durability and chemical compatibility to withstand aggressive mining heap leach solutions in stacks and solution trenches. Available texturing can enhance the frictional characteristics necessary for lining system slope stability. And for onsite water management and processing fluid containment, geomembranes are an economical and efficient solution.
Geomembranes are not all that mining sites require. Nonwoven geotextiles provide long-term, robust protection of and frictional stability for geomembranes on difficult terrain and in tall ore stack scenarios. Additionally, composite lining solutions (geomembranes with geosynthetic clay liners, GCLs) provide dependable, efficient, long-term lining performance for improved heap leach economics and environmental performance if used in a mining operation.
3. TAILING MANGEMENT
When an ore’s valuable deposit is extracted, what remains of the ore is waste. Often, it is a high percentage of the ore handled at the mine. Potentially contaminated from the extraction process or containing environmentally harmful components, tailings must be isolated to prevent long-term environmental damage.
Design engineers working on mines must allot significant space for proper containment of tailings. All or much of this area must be sealed with an impermeable geosynthetic (e.g., geomembrane) or composite lining system (e.g., geomembrane/geosynthetic clay liner). These sealing systems might protect the base and walls of an impoundment. Often, the surface of the tailings will be covered by a geosynthetic system after cell or mine closure.
As mine sites increase in size, the engineering needed to properly contain the volume of tailings has intensified. This scaling up of containment frequently requires not just lining systems but reinforcement and sealing systems for perimeter berms on tailings pond. Weaker, earthen-only berms are at risk of saturation, erosion, and failure. Furthermore, the increasing depth of tailings storage ponds requires stronger containment engineering design. The geosynthetics used must be durable and proven in aggressive environments over the long term. The depth of a tailings pond might exceed 50 m, for example. In these cases, the contaminated, generally sludgy waste is too deep and hazardous for the lining and reinforcement system to be monitored. With the environmental security of the site relying on these environmental protection systems, the geosynthetics selected must be trusted.
Here, geosynthetic solutions include HDPE geomembranes, GCLs, geomembrane-GCL composite systems, nonwoven geotextiles, and geogrid reinforcement.
4. EVAPORATIONS PONDS
Evaporation is used in a variety of mining operations to separate valuable materials from water or brines. Diverse salts, for example, can be extracted by evaporation. Lithium-rich brines may be concentrated through evaporation. These materials, when harvested from solar ponds, are then able to be refined into items used across a wide variety of industries, in agriculture, in food products, etc.
For sites where remediation or isolation of a contaminant is a goal, the act of evaporation in an engineered pond can also be an effective solution, especially in the case of contaminated sediments.
The evaporation process generally requires a significant scale to be more efficient and economical.
Geosynthetic lining solutions are used to prevent the loss of valuable materials in seepage. They also provide strong environmental protection. The potentially aggressive nature of the material being mined by evaporation demands environmental care, especially with the concentrated masses that the evaporation process yields.
In many situations, pregnant solutions are pumped into the engineered pond for multiple cycles until the pond has been filled with a sizable enough harvest to economically justify collecting it. The system will likely be exposed to both the material of interest and difficult environmental conditions for a considerable period of time (e.g., years). As such, long-term performance and durability are essential for an evaporation pond lining system.
Geosynthetic solutions in evaporation ponds commonly include HDPE geomembranes and geosynthetic clay liners.
5. MINE CLOSURE
The life of a mine varies wildly. It could be shuttered after 6 months due to a swift decline in market prices for metals. That same site might be reopened 10 years later when a rise in prices makes the site economically viable again. A mine might operate for 20 years with little interruption. It might even change the type of ore it concentrates on multiple times over those 20 years. Ownership of the site can transfer. The development of new extraction technologies might cause some long-closed facilities to be reopened so that ore can be further exploited.
Whatever occurs during the active phase of a mine’s life, the need for responsible closure is always present. Mining activities involve significant disturbance of soils. Dangerous chemicals are used. Environmental threats will remain after operations cease.
One of the most effective ways to improve the long-term safety of the site is to isolate what had been the mining zones (e.g., former heap leach or tailings storage facility) by installing a geosynthetic capping system.
Geomembranes, geosynthetic clay liners, geotextiles, and geocomposite drainage materials are used to cover, encapsulate, and cleanly isolate contaminated soils. These systems eliminate infiltration of precipitation, prevent polluted runoff, allow clean soil to be installed on top to support healthy vegetation re-establishment, and much more.
6. ACCESS ROADS AND OTHER GEOTECHNICAL STRUCTURES
Mines are, in many respects, small cities. They require roads, water, power, waste management, food, safety, housing, etc.
Access roads are especially integral to a mine’s viability. Ore must move around and away from the site. Shipments of supplies must not be impaired. Site access delays of a single day can cost millions of dollars (USD). Extended interruption in access to the site can threaten the mine’s continued operation, as investors and mine owners might no longer consider it economically viable (Smith, M.E., 2013).
The massive vehicles used in mining today require extremely strong roads. Haulers carry payloads of more than 100 tons. For ore, oil sands, rock, and coal operations, the roads must sustain repeated passes of these vehicles over years of mine activities.
Geogrid reinforcement materials and separation geotextiles are used to redistribute the tensile forces within the road and prevent the mixing of fines and coarse aggregate. The increased road strength mitigates the risk of road erosion and rutting in wet or arid mining environments.
These same reinforcement, separation, and drainage control materials are used in various other geotechnical applications in mining. The difficult terrain that characterizes many sites requires a number of vertical or near vertical constructions to be built, such as to support crusher walls. Mechanically stabilized earth (MSE) walls, reinforced with geosynthetics, are a common and effective strategy. Also, there are embankments, abutments, operating pads beneath heavy equipment and cranes, and many other points at which soils must be reinforced to enable the little city that a mine is to function as designed.
7. GEOSYNTHETIC COMPONENTS AND SELECTION
7.1 Geomembranes
Geomembrane liner materials belong to the group of geosynthetic polymeric barriers:
- Polymeric geosynthetic barrier (GBR-P): Factory-assembled structure of geosynthetic materials in the form of a sheet in which the barrier function is fulfilled by polymers other than bitumen.
- Polymeric geomembrane: Factory-assembled geosynthetic barrier consisting of one single flat polymeric core of thickness greater or equal to 0.75mm (30 mils). However, France and Germany consider such a polymeric geomembrane if the thickness is equal or greater than 1mm (40 mils).
In mining applications, such as heap leach facilities, evaporation ponds or tailings impoundments, where typically very high loads occur, geomembranes are more commonly used. Typical raw materials for geomembranes are: Linear Low Density Polyethylene (LLDPE), High Density Polyethylene (HDPE), Polyvinyl Chloride (PVC), Polypropylene (PP) and Ethylene Propylene Diene Terpolymer (EPDM). However, due to their high chemical resistance and physical properties mainly HDPE geomembranes are used. Additionally to the geomembrane properties other design issues must be taken into account, such as the effect of high stresses, the type of foundation and placed material under and on top of the geomembrane.
The foundation conditions should be firm to minimize settlements during the service life of the facility. Otherwise stress and over-elongation of the geomembrane could occur, resulting into damage of the geomembrane. Subgrade surfaces should provide a smooth, flat, firm, unyielding foundation for the geomembrane with no sudden, sharp or abrupt changes or break in grade that can tear or damage the geomembrane and additionally be free of loose rock fragments (>10 mm or 0.4 inches), sticks, sharp objects, or debris of any kind. Protection nonwovens can be used to protect against puncturing from soils.
The liner systems shown in Figure 1 to 3 can be either a single geomembrane, a composite lining system with a GCL or a double lining system with a geosynthetic drainage mat in between as a leak detection system. In many countries, landfills are first regulated by federal agencies through a rulemaking process. Typically, in the US geomembranes are made of HDPE and have a thickness of 1.5mm (60mils) in thickness and follow the GRI-GM13 specification. However, other countries have higher requirements, e.g. Germany requires an HDPE geomembrane for landfills with a thickness of ≥ 2.5mm (100mils).
In the mining industry there are no specific regulations for barrier applications, so that the liner thickness is generally selected based on experience, anticipating ore loads, the grain size of the material placed on top of the geomembrane and the material underneath. Due to the typical required chemical resistance required for the geomembrane HDPE is used in most cases. HDPE should be used where:
- An exposure to ultraviolet radiation occurs
- High chemical resistance is required
- Long-term service life is expected
- High stress crack resistance is important (typically important for HDPE)
- Good performance against thermo-oxidation is needed
- High puncture resistance is requested
- High mechanical properties are important.
7.2 Geosynthetic clay liner
Geosynthetic clay liners and multi-component clay geosynthetic barriers belong to the group of geosynthetic clay barriers and are defined as such:
Geosynthetic clay barriers (GBR-C): Factory-assembled structure of geosynthetic materials in the form of a sheet in which the barrier function is fulfilled by clay. [Current ASTM terminology discussed definition – similar to ISO 10318]
Geosynthetic clay liners (GCL): Factory-assembled geosynthetic barrier consisting of clay supported by geotextiles that are held together by needling, stitching, or a chemical adhesive. [Current ASTM terminology discussed definition]
Multi component Clay geosynthetic barrier (MGCL): A Clay or Geosynthetic Clay Liner (GCL) with an attached bituminous, polymeric or metallic barrier decreasing the hydraulic conductivity or protecting the clay core, or both. [Current ASTM terminology discussed definition]
GBR-Cs are used in mining applications, such as heap leach facilities, evaporation ponds or tailings impoundments, process solution containment, storm water containment, wastewater treatment ponds, closures and reclamation.
Harsh environmental conditions challenge the engineers designing these types of projects. In some applications the lining system can request a composite lining system with a geomembrane or a multi-component GCL. Due to the benefits GCL provide, they are more and more seen as an alternative to compacted clay liners in mining applications and in some cases an MGCL can be an alternative to a geomembrane. Some GCL benefits are:
- Cost effective liner and installation
- Easy to install under most weather conditions
- Effective barrier, especially under high normal loads
However, the designer should consider site specific conditions (soil material, slope angle, interface friction) and specify relevant characteristics to ensure a long-term and safe design. Current standard GCL properties could be on the lower limit (e.g. GRI-GCL-3), so that increasing some GCL properties (e.g. mass per unit area of the geotextile and bentonite component) are in some cases recommended.
The Geosynthetic Research Institute has published a White paper # 5 (GSI 2005b) and a GRI-GCL3 (GSI 2005) standard and has made aware the necessity to consider several important topics, especially overlap separation under certain conditions of pre-hydrated GCLs. However, this topic can be solved by means of immediate soil coverage or an increasing overlap for these types of products.
An interesting alternative for mining applications are multicomponent GCLs. By adding an extruded polymer coating to the needle-punched GCL this product type is suitable to more mining applications, especially in presence of aggressive liquids which might influence the performance of the bentonite, especially if not hydrated.
Further advantages of extruded polymer coated barriers are: Prevention of Root Penetration; Increasing Resistance against Desiccation; Bentonite Piping Resistance under High Water Gradients; Lower Permeability; Barrier against Ion Exchange; and Gas Barrier.
To ensure the long-term performance of extruded polymer coated GCLs other design issues might be of concern and should be considered prior to the installation: Durability of the Coating; Resistance against Installation Stress; Overlapping of Polymer Coated GCLs; Transmissivity between coating and GCL; Interface and internal Shear; Peel Value of Coating.
7.3 Nonwoven Geotextiles
As a separation layer, geotextiles are used to prevent adjacent soil layers or fill materials from intermixing. In filtration applications, nonwoven geotextiles are used to retain soil particles while allowing the passage of liquids through the filter media.
Needle-punched (mechanically bonded) nonwovens are robust geotextiles capable of withstanding harsh installation conditions and challenging construction loads. Their unique flexibility and elongation properties combine to provide high puncture resistance without sacrificing frictional of filtration properties. When properly selected, needle-punched nonwovens can provide superior long-term filtration and achieve high interface friction angles.
In mining applications, geotextiles are widely used for protection of geomembrane barriers against puncturing and unacceptable deformations.
The aim of testing the protection behavior of a nonwoven geotextile for a geomembranes is to help ensure long-term, effective protection. To simulate site conditions the mechanical protection is examined using a modified long-term plate-loading test (EN 13719).
A brief description is given in the following:
An elastomer disk with a Shore A hardness of 45 – 50 is installed as the base layer in a cylinder with a diameter of 30 – 50 cm. A soft metal sheet is placed on this, followed by the geomembrane and the protection layer, and finally by the site material which is supposed to be laid on top. The calculated load is then applied by a pressure foot and regulated using a load cell device underneath the elastomer disk.
Depending on site temperature conditions and on the load applied on top of the sealing system, EN 13719 gives different approaches for calculating the testing load, temperature and time.
The principle of the test set-up is show in Figure 4.
The deformations of the geomembrane are visible as permanent deformations in the soft metal sheet. After the specified loading period the metal sheet is removed and the indentations/deformations are measured.
In nearly all protection layer systems, deformations occur in the geomembrane which need to be quantitatively assessed by reference to the indentations in the soft metal sheet. This also applies to the joints and at overlaps.
According to various publications protection layers are suitable if the indentations conserved in the soft metal sheet after the mechanical protection efficiency test with a particular applied load show bulge elongations less than 0.25 % and no damage has occurred which might have an adverse effect on the functionality.
Due to the expected service life (>>100 years) of a geomembrane, for landfill base lining system requirements request often a maximum deformation of 0.25%. In mining applications a shorter service life may occur, so that higher deformations, however lower than 1.5%, may be acceptable. A critical aspect for determining the long-term performance is also the temperature of the liquid over the geomembrane.
7.4 Geosynthetic Drainage System
Drainage in heap leach pads is important to metal recovery, stability, and leakage control. Regardless what type of drainage material is selected (aggregate or geosynthetic) the liquid drainage layer at the base of heap leach pads should fulfill the following requirements:
- The liquid should be able to flow into the drainage layer, without building up a water head in the heap leach pad
- Sufficient long-term transmissivity within the drainage layer with a lowest possible gradient over the lining system
- Durable system (chemical compatibility) of the drainage system for the heap leach pad service life
- Withstand compressive loads (long-term and short-term)
- Meet shear stability requirements
- Avoid damage to the lining system
While most heap leach pads are covered with aggregate as drainage material (typical more than 0.5m crushed gravel (10 mm to 50 mm) geosynthetic drainage layers are now more and more used as an alternative to the conventional gravel drainage system.
A geosynthetic drainage system is defined as: Three-dimensional prefabricated product manufactured from synthetic raw materials, consisting of a drainage layer (core) which is in most cases covered with at least one geotextile filter, for liquid and/or vapor transportation.
A further application of geosynthetic drainage systems is the use as a leachate detection system between two barrier liners, e.g. between two polymeric geosynthetic barriers.
In order for a geosynthetic drainage system to perform equivalently to a mineral drainage layer in e.g. heap leach pads or to out-perform it, performance tests must be sufficient to demonstrate its long-term performance. These should include the filter performance of the geotextile filter, the long-term compression behavior of the geosynthetic drainage system under the site loads, the long-term horizontal (in-plane flow/transmissivity), as well as other site specific requirements, such as interface shear behavior or puncture resistance.
The design engineer typically will have the option between a mineral drainage layer and a geosynthetic drainage system during the evaluation and selection process. Engineers are more familiar to mineral materials and oversee the potential in geosynthetic drainage systems. However, it is often overseen what disadvantages can occur by using a mineral drainage layer. Placing this type of material directly on top of a geomembrane causes puncture stresses and can damage the geomembrane already during the placement process. Fur stresses occur during the loading of the heap leach pad, especially if no protection layer or an insufficient protection layer is used. The placement of the mineral drainage layer is also time consuming and can slow down the entire mining operation. Geosynthetic drainage systems on the other hand offer many advantages. Ease of installation, especially on the slopes, consistency in material properties, quicker installation, combined puncture protection and drainage layer, and in many cases cost savings.
Further benefits of using geosynthetic drainage systems are:
- High-volume flow paths for fluids
- Typically lower installation and material costs and therefore cost-effective alternative to mineral drainage materials
- Easy and quick to install due to their light weight
7.5 Soil Reinforcing Geogrids
In mining, geogrid applications include base course reinforcement and stabilization, reinforcement of slopes and retaining walls, and reinforcement of tailings ponds cover layers. Where the bearing capacity of soils is insufficient or shear characteristics too low to be stable for planned slope inclination or loadings, the geogrid reinforcement helps to bridge the gap to reach sufficient stability and safety.
The geogrid structure should provide stiff apertures. This influences the ability for lateral confinement of the aggregate which interlocks in the apertures. The greater the aperture stability of the geogrid the better is the lateral restraint provided for the granular material. The interaction with the aggregate is one of the main principles for geogrid reinforcement. As a result of the interlocking mechanism the geogrid absorbs stresses from the soil and increases safety and serviceability.
For optimal absorption of the stresses the geogrid needs to provide high strength at low strain. The greater the tensile modulus at low strain, the lower the resulting strain and finally deformation in the structure. The ultimate tensile strength is affecting the level of available tensile strength at low strain as well as the increase in ultimate strength results in the same rate of increase at low strain.
In structures where the geogrid is utilized to provide sufficient stability and safety as determined by a structural analysis, the long-term behavior of the product becomes decisive. Different raw materials and manufacturing processes influence characteristics like creep behavior, robustness against installation damage, and chemical/biological influences. Those values directly influence the long-term design strength of a product which is considered in the stability analysis. Products with equivalent ultimate strength will usually differ in their resulting long-term design strength.
8. ADDITIONAL DESIGN CONSIDERATIONS
8.1 Shear behavior
In order to ensure a safe and reliable stability analysis, dimensioning and design, it is important to have detailed information on each single shear plane. Safety is therefore top priority for all applications, especially on slopes. Geomembranes are therefore available with smooth or structured surfaces.
However, it is necessary that a project specific analysis should be performed, including direct shear testing, to confirm slope stability calculations. In cases where slope stability is not ensured with an accepted safety factor, geogrids can be used to improve the stability of soil veneers or entire lining systems.
8.2 Climate Conditions
Climate conditions might also be important to consider, especially if exposed for a longer period. Higher elevations can increase heating by solar radiation, exposure to UV but also temperature changes. Additionally construction considerations can include:
- Effect of strong wind
- High rain fall
- Snowfall
- Variable temperatures
- CONCLUSION
This paper presents an overview about common heap leach pad liner systems and their design requirements. Especially the geosynthetic components, the geomembrane and, if used, the geosynthetic clay liner have to be chosen in consideration of the harsh conditions of a heap leach pad. Therewith the requirement of well-designed and qualitative geosynthetic components comes up which achieve their function over a sufficient period of time.
The long-term performance of the geosynthetic performance can be influenced by using appropriate values in the specification text together with proper design, installation and site quality control.
Geosynthetic materials have been proven effective on various mining applications. Manufacturers do offer support for designers in questions of applicability and product choice of any geosynthetic material with respect to the project specific boundary conditions.
ABOUT THE AUTHORS
Kent von Maubeuge and Raquel Ribera work with NAUE.
REFERENCES
Christie, M.A. (2013). A Brief History of Heap Leaching, Retrospective of GRI-25’s retrospective conference; Long Beach, California, USA.
GRI-GCL3 (2005). Standard Specification for “Test Methods, Required Properties, and Testing Frequencies of Geosynthetic Clay Liners (GCLs), Geosynthetic Institute, Folsom, PA, USA.
GRI White paper # 5 (2005b). In-Situ Separation of GCL Panels Beneath Exposed Geomembranes, Geosynthetic Institute, Folsom, PA, USA.
Smith, M.E. (2013). Emerging Issues in Mining Containment, keynote lecture from Geosynthetics 2013, IFAI, Long Beach, California, USA.
Wikipedia; Internet Website: en.wikipedia.org/wiki
