Geomembranes: Wikis

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Geomembranes are a kind of geosynthetic material. They are impermeable membranes used widely as cut-offs and liners. Until recent years, geomembranes were used mostly as canal and pond liners.

Contents

Current applications

One of the largest current applications is at landfill sites for the containment of hazardous or municipal wastes and their leachates.[1] In many of these applications geomembranes are employed with geotextile or mesh underliners which reinforce or protect the more flexible geomembrane whilst also acting as an escape route for gases and leachates generated in certain wastes.

Composition

Geomembranes are made of various materials. Some common geomembrane materials are Low-Density Polyethylene (LDPE), High-Density Polyethylene (HDPE), Polyvinyl Chloride (PVC), Polyurea and Polypropylene (PP). Another type of geomembrane is bituminous geomembrane, which is actually a layered product of glass and bitumem-impregnated non-woven geotextile.

Characteristics

Each type of geomembrane material has different characteristics which affect installation procedures, lifespan and performance. For example, PVC geomembranes are very flexible and as a result can conform to uneven surfaces without becoming punctured. LDPE, on the other hand, is very susceptible to UV radiation, and therefore should not be used in applications where it will be exposed or else it will become brittle and fragile.

Physical Properties/Testing

The physical material properties of geomembranes include thickness, density, water vapor transmission, solvent vapor transmission, and melt flow index. The thickness can be measured by using a standard thickness test in which a thickness gauge under twenty kPa is applied for five seconds. All densities for PVC and polyethylene (PE) are less than one, so it is reasonable find the mass per unit volume instead of the density for most geomembranes. The water vapor transmission is the amount of water that can permeate the geomembrane. The solvent vapor index is the measurement of the flow of vapors besides water vapor through the geomembrane liner. The melt flow index is the measurement of the fluidity of the molten geomembrane. It is measured by heating the polymer until it is liquid. Once it has been heated, it is then pushed through a small orifice under a constant load for ten minutes. The higher the melt flow index is, the lower the density. (Redding et al, 690)

References

Reddy, Krishna R., Sharma, Hardi D. Geoenvironmental Engineering: Site Remediation, Waste Containment, and Emerging Waste Management Technologies. New Jersey: John Wiley and Sons, Inc., 2004.

Mechanical Properties/Testing

The mechanical properties of the geomembrane include the behavior of the membrane under a tensile load; tear, impact, and puncture resistance; interface shear strength between geomembranes and other materials; environmental stress cracking behavior; and shear and peal mode seam failures. The mechanical properties are some of the most important properties to be tested due to their impact on how the geomembrane will hold up under the direct load of municipal solid waste, or sometimes hazardous waste materials. (Redding et al, 691) The tensile testing of geomembranes is important because it informs the experimenter the allowable tensile stress a geomembrane can be put under before a tensile rupture occurs. A study done by Ryan Berg and Rudolph Bonaparte detail a common way of testing tensile strength. The testing was done on polyethylene (PE) geomembranes. The procedure requires that first a creep test is performed on the material at a variety of temperatures. Once this is complete, rupture tests are completed and stress-rupture-time relationships can be found. From the creep and rupture tests, a derived geomembrane stress-strain relationship can be found. Using this number, the reduction factor for chemicals, the weld factor for seam strength, the reduction factor for installation, and the overall factor of safety, a formula is used to find the allowable tensile stress. (301 ) Another method for performing tensile loading tests consists of using uniaxial tests on geomembranes and literally stretching them to their tensile limit. A test similar to this was performed by Rust, Visser, and Wesseloo. The test was performed on high density polyethylene (HDPE). Due to viscoelastic nature of the HDPE, as the strain is increased, the strength and stiffness increased as well. The strain was then measured by detecting the distance the specimen was stretched before failure. From here, a stress-strain relationship can be developed using a mathematical model. (275) The interface shear strength between a geomembrane and other materials provides important information that is needed to determine the stability analysis in the design of landfills slopes. To perform a test on the interface shear between a geomembrane and another surface, shearing blocks are typically used. These can apply pressure from the top and bottom to the interface surfaces. In an experiment done by Patrick Fox and Robert Kim, the other surface used, besides the geomembrane, was a geosynthetic clay liner (GCL). The geomembrane used was called a GMX, which is composed of very flexible polyethylene (VFPE). Testing was done on shear blocks and at four normal stress levels. The results from this experiment provide important characteristics of how a geomembrane constructed in the same way as a GMX would interact with a geosynthetic clay liner. In this particular situation, the test showed a smooth transition from the start load, to the peak shear strength, and then on to the residual shear strength on the stress-displacement relationship curve. This means the test was of good quality and can be trusted. (Fox et al, 462) Shear strength between a geomembrane and a soil is also of utmost importance. It is responsible for keeping the slopes of the landfill from failing and collapsing in on themselves. “One of the easiest methods of improving the shear strength at the geomembrane-soil interface is to use a textured geomembrane instead of a non-textured geomembrane.” (Fleming et al, 274) This allows the geomembrane, in essence, to “grip” the soil and have a strong hold on it. The strength of this interface can be tested by placing the soil and the geomembrane next to each other and putting normal forces on both. Once this is complete a shearing force can be applied and measured. (Fleming et al, 276) The puncture resistance of a geomembrane is very important. Most of the time, the geomembrane is laid directly onto the ground surface. This means if there are any rocks, pieces of glass, metal scraps, etc. on the ground they will be directly in contact with the geomembrane. The waste put in the landfill is also laid directly on top of the geomembrane in some cases. Waste can many times be sharp and tough, making the geomembrane susceptible to puncture. For these reasons, the geomembrane must be tested for puncture resistance. One such test used is called the truncated cone test. In this test, a specimen of geomembrane is placed in a chamber and held in place by sixteen clamps. It is placed at the elevation of three truncated cones. The chamber is then filled with water. Air pressure or water pressure is used to press the cones through the geomembrane. A pressure valve is able to measure the pressure difference between the point right before and right after the puncture. This is the puncture resistance measurement of the geomembrane. (Boerman et al, 483 )

References

Berg, Ryan R. Bonaparte, Rudolph. “Long-Term Allowable Tensile Stresses for Polyethylene Geomembranes.” Geotextiles and Geomembranes. 12 (1993) : 287-306

Boerman, T.R. Conner, C.J. Stark, T.D. “Technical Note: Puncture Resistance of PVC Geomembranes Using the Truncated Cone Test.” Geosynthetics International. 15.6 (2008) : 480-486

Fleming, I.R. Jogi, M.B. Sharma, J.S. “ Shear Strength of Geomembrane-Soil Interface Under Unsaturated Conditions.” Geotextiles and Geomembranes. 24 (2006) : 274-284

Fox, Patrick J. Kim, Robert H. “Effect of Progressive Failure on Measured Shear Strength of Geomembrane/GCL Interface.” Journal of Geotechnical and Geoenvironmental Engineering. 134.4 (2008) : 459-469

Rust, E. Visser, A.T. Wesseloo, J. “A Mathematical Model for the Strain-Rate Dependent Stress- Strain Response of HDPE Geomembranes.” Geotextiles and Geomembranes. 22 (2004) : 273-295

Endurance Properties

The thermal, chemical, and biological properties can all be categorized under the umbrella of endurance properties. These material properties explain how well the geomembrane liner will withstand elements over time. These elements can be such things as ultraviolet rays from the sun, which break down the compounds in the geomembranes causing severe polymer damage. This is cause by the short wavelengths of the ultraviolet light. Chemical resistance is an extremely important aspect as well. The liner must be able to withstand the chemicals that are part of the leachate. Hot and cold temperatures can also affect the mechanical and physical properties of geomembrane liners. For example, in extremely cold temperatures geomembranes become less flexible and seaming becomes more difficult. (Redding et al, 695) The reason for the aging and degradation in the geomembrane is due to the development of the semi-crystalline polymers into more crystal and brittle bonds as the polymers move towards equilibrium. (Rowe et al, 78) This means that from the application perspective, the chemical resistance of the geomembrane is the most important quality. As the geomembrane chemically breaks down, the mechanical and physical properties all get much weaker, making the geomembrane much more susceptible to tearing and failure. To test this, it is typical to remove a piece of geomembrane from a liner that has been withstanding normal conditions for a number of years and run normal physical and material properties testing on it. When this was tested, it was found that while tensile loading tests showed tensile strength increase at both the yielding point and the failure point, both strain and tear resistance experienced a decrease. (Rowe et al, 86) Many times to test durability, tests are run on geomembranes which were previously part of a landfill liner. This was the case with the study done by Newman and Stark. The study took a ten year old specimen and ran tests including a tensile loading test and tear resistance test. As with the Rowe and Sangam study, it was found that the ten year time wear on the specimen did not negatively affect the tensile strength in any way. In the tear resistance test, there was little change. This showed that there is no impact of plasticizer migration. (Newman et al, 103)

References

Newman, E.J. Stark, T.D. “Ten Year PVC Geomembrane Durability.” Geosynthetics International. 16.2 (2009) : 97-108

Rowe, R Kerry. Sangam, Henri P. “Durability of HDPE Geomembranes.” Geotextiles and Geomembranes. 20 (2002) : 77-95

Seaming and Seam Testing

The seaming aspect of geomembranes is as important, if not more important, than the actual properties of the geomembrane. When a geomembrane is placed, the seams are the weakest part. For this reason, the properties of the seams are vital in determining the total strength of the liner. There are two main types of tests: one is a destructive test and the other is a non-destructive test. A typical destructive test is called a peel and shear test. In this, the seal is, as the name suggests, peeled back until it shears from the attached geomembrane. A non-destructive test, unlike the destructive test, does not permanently damage the geomembrane. It also, unfortunately, doesn’t test for seam strength. It is responsible for testing continuity and locating holes in the seam. (Redding et al, 698) Destructive seaming is a common way for testing seams. It is critical to do because, although non-destructive testing is useful in determining the continuity and if there are defects, destructive seam testing is the only way to get the most accurate measurement of the seam strength and quality of the seam. There are three modes for testing seam strength: shear, “T” peel, and 180 degree peel. The “T” peel is sometimes referred to as the 90 degree peel. Placing one end of the seam in one set of clamps and one end in another set, the seam is effectively ripped apart and the amount of force the sample is put under is quantified. From this number the experimenter can find both the maximum and residual stress for the seam. (Haxo et al, 376) The more preferred way for testing seams is through non-destructive testing. This occurs when the seam is tested but the geomembrane is not destroyed and the same geomembrane seam tested can be used for the liner. Some common types are a vacuum test, air lance test, and pressure test. The vacuum test is typically used on the more crystalline thermoplastic materials, such as high density polyethylene (HDPE) and linear low density polyethylene (LLDPE). In a vacuum test, the geomembrane and seam are coated with a soapy solution. Next whatever section is being tested is put into a vacuum box which is sealed and vacuumed. If bubbles appear on the surface of the geomembrane seam, it means there is a leak somewhere and air is escaping from seam. (Overmann, 418) An air lance test is also very popular and used on flexible, thermoplastic materials, such as PVC. To perform an air lance test, “a stream of air is forced through a nozzle at the end of a lance. The stream of air is directed along the exposed edge of overlapped geomembrane material. Unbonded areas are detected as the air passes through the defective area.” (Overmann, 419) A pressurized test is yet another option, which is growing in popularity and techniques that are available. This method can only be used on double-tracked seams. “The test is performed by first sealing both ends of the channel that is between the two tracks of the length of the seam to be tested. A rigid needle is then inserted into one end of the seam and an air-tight seal is formed around it.” (Overmann, 421) There are a valve and a pressure gauge which are attached to the needle. The pressure that is withstood inside the seam is then measured over time. (Overmann, 422 ) Although non-destructive tests cannot perform as well as destructive seam tests in determining the strength of the weld, in the case of the pressurized dual seam test, the test can be said to be almost as sufficient. When this test is completed, there is a relationship between the welded seam burst strength and the seam peel strength. (Barroso et al, 16) This allows the seam to not be destroyed but still obtain an accurate seam strength. Another type of common test is called the gas permeation pouch test. To perform this test, gas is pressurized in the gap between the two seams on a geomembrane. An analysis of the rate of the depressurization of the gas between the two seams will give a time constant of change. This can be used to determine the shear strength of the seam. Although this test is extremely accurate, it cannot replace the mechanical tests for seams, for example, the peel test. (Barroso et al, 20)

References

Barroso, M. Pierson, P. Lopes, M.G. “A Non-Destructive Method for Testing Non-Flexible Dual Geomembrane Seams Using Gas Permeation.” Geosynthetics International. 13.1 (2006) : 15-22

Haxo, Jr, Henry E. Kamp, Lawrence C. “Destructive Testing of Geomembrane Seams: Shear and Peel Testing of Seam Strength.” Geotextiles and Geomembranes. 9 (1990) : 369-395

Overmann, Leo K. “Geomembrane Seam and Nondestructive Tests: Construction Quality Control (CQC) Perspective.” Geotextiles and Geomembranes. 9 (1990) : 415-429

References

See also

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