Mine sites: some guidance notes on impermeable/low permeability systems and membranes

Impermeable/low permeability systems are needed on mine sites to prevent the unwanted transmission, loss, or release of materials (such as lixiviating fluids, leachate, contaminant or precious metals laden fluids, water, and more) into the environment. Impermeable/low permeability systems are also needed to prevent the infiltration of fluids into materials that need to be covered from the elements – for example waste rock piles that could leach, and create acid mine drainage, when exposed to water and other atmospheric conditions.

Impermeable/low permeability bases, side slopes and covers may be constructed for impoundments, ponds, and pads on mine sites using locally available native materials, synthetic membranes, or a combination of both native and synthetic materials (in a composite system). The use of native materials can significantly reduce capital costs for construction works, if suitable natural earth materials such as clay or amended soil with bentonite, are locally available to construct compacted low permeability systems. Nonetheless, the lower permeability to fluid flow offered by synthetic membranes and composite systems (compacted clay and synthetic membranes) provides an advantage over the use of compacted clay systems alone. For comparison, the permeability of synthetic membranes may be over six orders of magnitude lower than that of compacted clay – at up to 10^-7m/s for compacted clay vs. 10^-13 m/s for synthetic membranes.

Although the capital costs of constructing synthetic liners may be higher than those for compacted clay materials, especially if good quality clay is locally available, the lower permeability provided by synthetic membranes, and other added benefits (see Table 1) – (such as ease of installation and quality assurance, leak location, repairs, maintenance, and more) provide good considerations for any added costs.

Table 1: Compacted Clay Liners (CCL) vs. synthetic membranes as barrier materials (Reference: Daniel and Koerner, 1993 – Cover systems. In: Daniel D.E. (eds) Geotechnical Practice for Waste Disposal)

Aspects considered	Compacted Clay Liners (CCL)	Synthetic membranes
Desiccation	Susceptible to desiccation from above and below unless protected properly. Designers usually consider drying of the clay layer from the top; however gases and materials in underlying materials may dry out the clay layer from below as well.	Do not desiccate
Cyclical freezing and thawing, wet and dry cycles	Susceptible to damage from freezing and thawing, wet and dry cycles unless suitably protected	Freezing and thawing, wet and dry cycles do not affect synthetic membranes
Compaction	Difficult to compact properly on gas collection layers and over soft materials e.g. waste	No need for compaction
Differential settlement	Differential settlement in the underlying materials may create excessive tensile strains that may cause the CCL to crack	Can withstand large differential settlements. Some are better at it than others
Repairs	Difficult to repair if they crack	Easier to repair
Permeability	Several orders of magnitude higher than synthetic membranes (at about 10^-7m/s for CCL vs. 10^-13 m/s for synthetic membranes)	Lower permeability to liquids and gases than CCL (permeability can be up to six orders of magnitude lower than CCL – 10^-13 m/s for synthetic membranes vs. 10^-7m/s for CCL
Long term effectiveness as a barrier layer	Because of the higher permeability, potential desiccation, and cracking issues, CCL is considerably not as effective as synthetic membranes for impeding fluid flow long term	Virtually impermeable unless they are damaged. If practices for preventing and finding damages and leaks are incorporated, synthetic membranes are very effective as long term barrier layers
Construction	Convenient if clay is available locally, may be challenging if clay supply is limited	Readily available and can be shipped to locations. Straight forward and rapid installation. Gas collection penetrations and boots are easy to construct
Slope stability	May be a concern for steep slopes	Comparatively less concern as they can be anchored and textured for improved slope stability and increased interface friction and shear strength

The following are some commonly used synthetic membranes, and their suitability and limitations for various mine site conditions and applications

Table 2: Some synthetic membranes for heap leach mining pads, ancillary ponds, processing and storage units (Scheirs, 2009)

Synthetic membrane type	Reasons for suitability	Limitations
HDPE – High Density Polyethylene	Broad chemical resistance Good abrasion resistance Good tensile strength Good UV resistance for exposed applications Broad temperature window for field deployment, welding and installation (< -22 F (-30 C) to 122 F (>+50 C)	High susceptibility to stress cracking – a brittle failure below the yield stress of the material. Stress cracking may intensify in certain chemical environments and site conditions (environmental stress cracking) Stiffness may increase significantly in low (sub-zero) temperatures and making it challenging to weld HDPE is stiff and this may be limiting for differential settlements and in undulating terrains Poor multi-axial strain resistance – resulting in lower resistance to localized strains and lower puncture resistance. Puncture resistance may need to be enhanced with cushioning materials Recommended operating temperatures are ≤ 140 F (60 C), unless “hot solutions rated” version is used
LLDPE – Linear Low Density Polyethylene	Broad chemical resistance Good abrasion resistance Good puncture resistance Good UV resistance for exposed applications Little to no susceptibility to stress cracking Easier to weld and install in low (sub-zero) temperatures Good flexibility – provides good adhesion to materials for intimate contact and press fit Good tensile strength and elongation capabilities Good resistance to multi axial strains, giving better performance for differential settlements and localized strains Can be folded and prefabricated for fewer field seaming Broad temperature window for field deployment, welding and installation (< -22 F (-30 C) to 122 F (>+50 C)	May be challenging to weld for installers who are more used to welding stiffer materials such as HDPE Recommended operating temperatures are ≤ 140 F (60 C)
PVC – Polyvinylchloride	Good flexibility – provides good adhesion to materials for intimate contact and press fit Good tensile strength and elongation capabilities Good resistance to multi axial strains, giving better performance for differential settlements and localized strains Can be folded and prefabricated for fewer field seaming Good puncture resistance	Poor UV resistance – the material has to be covered up promptly unless specially formulated with additives for UV resistance Reduced temperature window for field deployment, welding and installation – ambient temperature and membrane sheet temperature for deployment are between 60 F (15 C) and 105 F (41 C)
Hot Solutions Rated Membranes	Good resistance to elevated temperatures up to 212 F (100 C) Chemical resistance, abrasion resistance, puncture resistance, tensile strength and elongation properties similar to those of the base polymer/resin	Relatively new series of synthetic membranes and fewer years of experience with long term exposure to elevated temperatures and other service conditions

Table 2b presents some surface treatments that may be applied to the surfaces of synthetic membranes to enhance their performance for installation, quality assurance and leak location, interface shear friction and strength.

Table 2b: Some surface treatments that may be applied to synthetic membranes to enhance long term performance

Surface treatments	Reasons for suitability	Limitations
Conductive coating	Can be used for detecting fully penetrating leaks such as holes and cuts and non-penetrating leaks such as thinning in the membrane from occurrences such as localized strains Can be used to find small sized holes (smaller than pin hole size) in the membrane after installation and larger sized holes (such as from equipment damage or weld opening) after placing cover material on the membranes Leak detection abilities of the liner help to prevent leakage from mine sites, protect mine assets, protect the environment and meet regulations	Welding, installation and leak location may require specialized training
Light reflective coating to reflect UV light	Light coloured coating on the surface of a membrane reflects UV light. This reduces the temperature of membrane, ultimately helping to prolong the membrane’s lifespan in exposed applications The reduced temperature of the membrane also helps with installation, reduces wrinkles, and improves lay flat and intimate contact with underlying materials thus minimising leakage through the membrane Light coloured coating on the surface of a membrane reflects UV light and prevents underlying clay materials from desiccation. It also improves construction quality assurance for spotting damages and leaks	The light coloured pigmentations that are used may not be resistant to UV light, and may crack and leave the membrane exposed to UV degradation if not formulated with high quality UV stabilizers and additives
Texturing	Prevents slippage along the interfaces in contact with the membrane surface Can be utilized to maximize mine air space by making it possible to increase the steepness of slopes without the risk of down drag or materials sliding off Suitable for minimizing sliding along the surface and interface of the synthetic membrane (such as in unstable ground conditions or seismic prone areas) Good for mine health and safety for preventing slips and falls on the synthetic membrane surfaces or entrapment of human or wildlife in exposed ponds and impoundments	Challenges may occur with selecting the required surface texturing that will achieve a desired friction angle while keeping costs low. The material cost increases with the asperity height of the textured surface

Table 3 presents some budgetary estimation of costs for the synthetic membranes discussed, including installation costs.

Table 3: Some budgetary costs for synthetic membranes (inclusive of the cost of the synthetic membranes, installation and ancillary costs)

Synthetic membrane type¹	Cost (US$/ft²) (materials and installation)
HDPE – black – smooth	1.80 – 1.85
LLDPE – black – smooth	1.80 – 1.85
HDPE – Hot Solution Rated	2.10 – 2.15
Conductive – HDPE	1.85 – 1.90
Conductive – LLDPE	1.85 – 1.90
Light reflective surface – HDPE	1.80 – 1.85
Light reflective surface – LLDPE	1.80 – 1.85
Textured – HDPE (medium asperity)²	1.85 – 1.90
Textured – LLDPE (medium asperity)²	1.85 – 1.90
PVC	1.80 – 1.85

¹ A thickness of 80 mil was used. The cost of the synthetic membranes will increase with the material thickness, since more resin is needed as the thickness increases. The cost of the materials will also vary with the cost of resins and other market conditions (2018 values shown). ²The cost of textured surfaces is dependent on the asperity height following texturing. The cost will increase as the asperity height increases, since more resin is needed for higher asperities.

Terrasyntec will work with you throughout the planning, preparation, and decision making stages – to the construction and post construction after care, to help you ensure the success of your project. Please forward your enquiries and questions to technicalservices@terrasyntec.com or fill out the form on our Contact page to engage with us.

References

Daniel D.E., Koerner R.M. (1993) Cover systems. In: Daniel D.E. (eds) Geotechnical Practice for Waste Disposal. Springer, Boston, MA. https://doi.org/10.1007/978-1-4615-3070-1_18

Scheirs, J. 2009. A Guide to Polymeric Geomembranes, 1st Edition, Wiley and Sons, West Sussex, UK

Mine sites: some guidance notes on impermeable/low permeability systems and membranes

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