A geomembrane is very low permeability synthetic membrane liner or barrier used with any geotechnical engineering related material so as to control fluid (liquid or gas) migration in a human-made project, structure, or system. Geomembranes are made from relatively thin continuous polymeric sheets, but they can also be made from the impregnation of geotextiles with asphalt, elastomer or polymer sprays, or as multilayered bitumen geocomposites. Continuous polymer sheet geomembranes are, by far, the most common.
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Manufacturing
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The manufacturing of geomembranes begins with the production of the raw materials, which include the polymer resin, and various additives such as antioxidants, plasticizers, fillers, carbon black, and lubricants (as a processing aid). These raw materials (i.e., the "formulation") are then processed into sheets of various widths and thickness by extrusion, calendering, and/or spread coating.
Three methods used to manufacture geomembranes[1][self-published source
]A 2010 estimate cited geomembranes as the largest geosynthetic material in dollar terms at US$1.8 billion per year worldwide, which is 35% of the market.[2] The US market is currently divided between HDPE, LLDPE, fPP, PVC, CSPE-R, EPDM-R and others (such as EIA-R and BGMs), and can be summarized as follows:[citation needed] (Note that M m2 refers to millions of square meters.)
The above represents approximately $1.8 billion in worldwide sales. Projections for future geomembrane usage are strongly dependent on the application and geographical location. Landfill liners and covers in North America and Europe will probably see modest growth (~ 5%), while in other parts of the world growth could be dramatic (10–15%).[citation needed] Perhaps the greatest increases will be seen in the containment of coal ash and heap leach mining for precious metal capture.
Properties
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The majority of generic geomembrane test methods that are referenced worldwide are by the ASTM International|American Society of Testing and Materials (ASTM) due to their long history in this activity. More recent are test method developed by the International Organization for Standardization (ISO). Lastly, the Geosynthetic Research Institute (GRI) has developed test methods that are only for test methods not addressed by ASTM or ISO. Of course, individual countries and manufacturers often have specific (and sometimes) proprietary test methods.
Physical properties
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The main physical properties of geomembranes in the as-manufactured state are:
Mechanical properties
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There are a number of mechanical tests that have been developed to determine the strength of polymeric sheet materials. Many have been adopted for use in evaluating geomembranes. They represent both quality control and design, i.e., index versus performance tests.
Endurance
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Any phenomenon that causes polymeric chain scission, bond breaking, additive depletion, or extraction within the geomembrane must be considered as compromising to its long-term performance. There are a number of potential concerns in this regard. While each is material-specific, the general behavior trend is to cause the geomembrane to become brittle in its stress-strain behavior over time. There are several mechanical properties to track in monitoring such long term degradation: the decrease in elongation at failure, the increase in modulus of elasticity, the increase (then decrease) in stress at failure (i.e., strength), and the general loss of ductility. Obviously, many of the physical and mechanical properties could be used to monitor the polymeric degradation process.
Lifetime
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Geomembranes degrade slowly enough that their lifetime behavior is as yet uncharted. Thus, accelerated testing, either by high stress, elevated temperatures and/or aggressive liquids, is the only way to determine how the material will behave long-term. Lifetime prediction methods use the following means of interpreting the data:
self-published source
]Seaming
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The fundamental mechanism of seaming polymeric geomembrane sheets together is to temporarily reorganize the polymer structure (by melting or softening) of the two opposing surfaces to be joined in a controlled manner that, after the application of pressure, results in the two sheets being bonded together. This reorganization results from an input of energy that originates from either thermal or chemical processes. These processes may involve the addition of additional polymer in the area to be bonded.
Ideally, seaming two geomembrane sheets should result in no net loss of tensile strength across the two sheets, and the joined sheets should perform as one single geomembrane sheet. However, due to stress concentrations resulting from the seam geometry, current seaming techniques may result in minor tensile strength and/or elongation loss relative to the parent sheet. The characteristics of the seamed area are a function of the type of geomembrane and the seaming technique used.
Applications
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Geomembrane installation as part of the construction of a base liner system of a landfill[2]Geomembranes have been used in the following environmental, geotechnical, hydraulic, transportation, and private development applications:
See also
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References
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Further reading
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A new class of geosynthetic has recently emerged known as GCCMs (Geosynthetic Cementitious Composite Mats) defined by the ASTM D-35 committee in 2017 as `a factoryassembled geosynthetic composite consisting of a cementitious layer contained within a layer or layers of geosynthetic materials that becomes hardened'. GCCMs consist of a three-dimensional fibre structure filled with a dry cement/concrete mix, overlain by a hydrophilic filter layer and underlain by a watertight membrane, which is typically a polymeric film. The material is delivered in its dry format and unrolled into place using similar installation techniques to traditional geosynthetics. Once in place, it is hydrated by spraying with water and the cement/concrete mix hardens. The result is a watertight polymeric film which is overlain by a protective fibre-reinforced concrete layer. GCCMs have been in use since 2009 and are predominantly used for the lining of water channels for small scale drainage. More recently a variant of GCCMs has emerged which integrates a geomembrane liner onto the rear surface which allows the joints to be thermally welded. These are known as Geosynthetic Cementitious Composite Barriers (GCCBs). It is estimated that Egypt has more than 110,000 kilometers of canals comprised of approximately 30,000 km of public canals (first and second level) and 80,000 km of private third-level canals (mesqas) and irrigation ditches. A common problem associated with canals, is seepage. Seepage can result directly in water loss through the network or result in waterlogging of adjacent land. In the case of land used for cultivation, waterlogging can reduce crop yields or cause salinization of the soils. This does not only occur in earthen canals, but also in concrete lined canals, particularly those that have experienced cracking, scour, panel separation or damage. It is also a common misconception that concrete lining of canals is an effective method of mitigating seepage losses. The 25-year study performed by the USBR indicates that concrete over geomembrane has a 95% effectiveness at reducing seepage through canals1. This abstract introduces a revolutionary new class of materials called Geosynthetic Cementitious Composite Mats (GCCM's), specifically Type II GCCM's to ASTM D8364 for lining of bulk water transportation canals. The Type II GCCM in question consists of concrete encapsulated by between two geotextile layers with a minimum 1mm thick LLDPE geomembrane backing which can be thermally welded to produce a testable and low permeability joint, per ASTM D5820, with air channel testing to ensure a leak free installation. Because it is a composite of concrete and geomembrane in a single application, installation can occur as a one-stepprocess imparting both cost and time savings to the project. The abrasion resistance of the concrete layer is 3.5 times that of typical 20Mpa concrete typically used for canal applications. With a design life of more than 50 years, this new product classification will provide a feasible, long-term solution to help preserve and protect fresh, clean water, one of Egypt's most precious - and ever more scarce - natural resources.
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