A geomembrane is very low permeability synthetic membrane liner or barrier used with any

geotechnical engineering Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics for the solution of its respective engineering problems. It als ...

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 Geotextiles are permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain. Typically made from polypropylene or polyester, geotextile fabrics come in two basic forms: woven (r ...

with

asphalt Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term a ...

elastomer An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus and high failure strain compared with other materials. The term, a portmanteau of ''elastic p ...

or polymer sprays, or as multilayered bitumen geocomposites. Continuous polymer sheet geomembranes are, by far, the most common.

Manufacturing

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 Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex c ...

, calendering, and/or spread coating. Three methods used to manufacture geomembranes

Geomembranes dominate the sales of geosynthetic products, at US$1.8 billion per year worldwide, which is 35% of the market. The US market is currently divided between HDPE, LLDPE, fPP, PVC, CSPE-R, EPDM-R and others (such as EIA-R), and can be summarized as follows: (Note that M m² refers to millions of square meters.) *

high-density polyethylene High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a thermoplastic polymer produced from the monomer ethylene. It is sometimes called "alkathene" or "polythene" when used for HDPE pipes. With a high strength-to-density ratio, ...

(HDPE) ~ 35% or 105 M m² *

linear low-density polyethylene Linear low-density polyethylene (LLDPE) is a substantially linear polymer (polyethylene), with significant numbers of short branches, commonly made by copolymerization of ethylene with longer-chain olefins. Linear low-density polyethylene differs ...

(LLDPE) ~ 25% or 75 M m² * polyvinyl chloride (PVC) ~ 25% or 75 M m² * flexible

polypropylene Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene. Polypropylene belongs to the group of polyolefins and ...

(fPP) ~ 10% or 30 M m² *

chlorosulfonated polyethylene Hypalon is a chlorosulfonated polyethylene (CSPE) synthetic rubber (CSM) noted for its resistance to chemicals, temperature extremes, and ultraviolet light. It was a product of DuPont Performance Elastomers, a subsidiary of DuPont. Hypalon as it i ...

(CSPE) ~ 2% or 6 M m² * ethylene propylene diene terpolymer (EPDM) ~ 3% or 9 M m² 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 liner A landfill liner, or composite liner, is intended to be a low permeable barrier, which is laid down under engineered landfill sites. Until it deteriorates, the liner retards migration of leachate, and its toxic constituents, into underlying aqu ...

s 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%). Perhaps the greatest increases will be seen in the containment of

coal ash Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when de ...

and heap leach mining for precious metal capture.

Properties

The majority of generic geomembrane test methods that are referenced worldwide are by the ASTM International, American Society of Testing and Materials (

ASTM ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...

) due to their long history in this activity. More recent are test method developed by the International Organization for Standardization (

ISO ISO is the most common abbreviation for the International Organization for Standardization. ISO or Iso may also refer to: Business and finance * Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007 * 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

The main physical properties of geomembranes in the as-manufactured state are: * Thickness (smooth sheet, textured, asperity height) *

Density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

Melt flow index The Melt Flow Index (MFI) is a measure of the ease of flow of the melt of a thermoplastic polymer. It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied ...

* Mass per unit area (weight) * Vapor transmission (water and solvent).

Mechanical properties

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. * tensile strength and elongation (index, wide width, axisymmetric, and seams) *

tear resistance Tear resistance (or tear strength) is a measure of how well a material can withstand the effects of tearing. It is a useful engineering measurement for a wide variety of materials by many different test methods. Discussion For example, with rubbe ...

impact resistance In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing.puncture resistance * interface shear strength * anchorage strength * stress cracking (constant load and single point).

Endurance

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 A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Bre ...

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 An elastic modulus (also known as modulus of elasticity) is the unit of measurement of an object's or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is ...

, 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. * ultraviolet light exposure (laboratory of field) * radioactive degradation * biological degradation (animals, fungi or bacteria) * chemical degradation * thermal behavior (hot or cold) * oxidative degradation.

Lifetime

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: * ''Stress limit testing:'' A method by the HDPE pipe industry in the United States for determining the value of hydrostatic design basis stress. * ''Rate process method:'' Used in Europe for pipes and geomembranes, the method yields similar results as stress limit testing. * ''Hoechst multiparameter approach:'' A method that utilizes biaxial stresses and stress relaxation for lifetime prediction and can include seams as well. * ''Arrhenius modeling:'' A method for testing geomembranes (and other

geosynthetics Geosynthetics are synthetic products used to stabilize terrain. They are generally polymeric products used to solve civil engineering problems. This includes eight main product categories: geotextiles, geogrids, geonets, geomembranes, geosyntheti ...

) described in Koerner for both buried and exposed conditions.

Seaming

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 Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...

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

Geomembranes have been used in the following environmental, geotechnical, hydraulic, transportation, and private development applications: * As liners for potable water * As liners for reserve water (e.g., safe shutdown of nuclear facilities) * As liners for waste liquids (e.g., sewage sludge) * Liners for radioactive or hazardous waste liquid * As liners for secondary containment of underground storage tanks * As liners for solar ponds * As liners for brine solutions * As liners for the agriculture industry * As liners for the aquiculture industry, such as fish/shrimp pond * As liners for golf course water holes and sand bunkers * As liners for all types of decorative and architectural ponds * As liners for water conveyance canals * As liners for various waste conveyance canals * As liners for primary, secondary, and/or tertiary solid-waste landfills and waste piles * As liners for heap leach pads * As covers (caps) for solid-waste landfills * As covers for aerobic and anaerobic manure digesters in the agriculture industry * As covers for power plant coal ash * As liners for vertical walls: single or double with leak detection * As cutoffs within zoned earth dams for seepage control * As linings for emergency spillways * As waterproofing liners within tunnels and pipelines * As waterproof facing of earth and rockfill dams * As waterproof facing for roller compacted concrete dams * As waterproof facing for masonry and concrete dams * Within cofferdams for seepage control * As floating reservoirs for seepage control * As floating reservoir covers for preventing pollution * To contain and transport liquids in trucks * To contain and transport potable water and other liquids in the ocean * As a barrier to odors from landfills * As a barrier to vapors (radon, hydrocarbons, etc.) beneath buildings * To control expansive soils * To control frost-susceptible soils * To shield sinkhole-susceptible areas from flowing water * To prevent infiltration of water in sensitive areas * To form barrier tubes as dams * To face structural supports as temporary cofferdams * To conduct water flow into preferred paths * Beneath highways to prevent pollution from deicing salts * Beneath and adjacent to highways to capture hazardous liquid spills * As containment structures for temporary surcharges * To aid in establishing uniformity of subsurface compressibility and subsidence * Beneath asphalt overlays as a waterproofing layer * To contain seepage losses in existing above-ground tanks * As flexible forms where loss of material cannot be allowed.