UTILIZING PVC GEOMEMBRANES FOR LANDFILL AND POND LINERS
by Fred P. Rohe
Environmental Protection Inc.
Abstract
Flexible PVC liners have been successfully used for containment applications since the 1960’s.
This paper will cover the fabrication and installation of a typical PVC geomembrane for containment applications. The multimedia presentation will consist of a PowerPoint presentation using video and photographs.
The benefits of using PVC versus some of the alternative geomembranes will be discussed.
Introduction
The large scale development of PVC geomembranes began in the 1960’s with the use of PVC film to prevent seepage of water from canals and reservoirs used for irrigation in the western United States. The US Department of Interior, Bureau of Reclamation [1] first began experimenting with PVC geomembranes in 1957.
In the Bureau of Reclamation’s 35+ years of geomembrane experience, PVC geomembranes have proven especially effective where limited access, short downtime, long haul distances, and potential for freezing and thawing are factors.
The primary use of PVC geomembrane by the Bureau of Reclamation has been for lining irrigation canals in the southwestern United States. Hundreds of miles of canals are in use today lined with 10 mil and 20 mil PVC. These canals are unique structures because:
1. They are long and narrow
2. They have limited access for work
3. They have steep side slopes
Historically, PVC has been the most widely used geomembrane for canal applications for the following reasons:
1. Availability in large sheets - PVC can be factory fabricated into panels up to 30 M wide and 100 M long. Panels can be accordion folded in both directions to facilitate shipping and handling in narrow confines associated with canal construction. These large panels minimize field seaming.
2. PVC is highly flexible and retains this property over a wide range of temperatures, which permits it to conform to the subgrade better than other geomembrane materials, which were available at the time of selection, such as HDPE and EPDM.
3. PVC is easily field-spliced and repaired with a solvent-type cement.
4. PVC also has good puncture, abrasive, and tear-resistant properties, which are important to minimize damage during installation.
5. PVC geomembrane installation does not require sophisticated equipment or skilled labor.
These same material properties are advantageous to many other applications requiring containment liners. PVC geomembrane is used in all types of water and waste containment applications, including landfills, wastewater treatment lagoons, oil exploration, aquaculture, and irrigation ponds.
Theory
PVC geomembrane film can be factory pre-fabricated into large panels prior to shipment to the project site. Panels as large as 2,000 M2 (20,000 Ft 2 ) are common. This reduces the amount of field welding required by up to 80% in many installations. Factory fabrication is not affected by the adverse weather conditions often encountered in the field therefore, quality of welding can be controlled and assured. In many situations, a one piece liner can be installed without any requirements for field welding.
Factory Fabrication
Calendered PVC film is manufactured in thickness’ of .25 mm (10 mil) up to 1.5 mm (60 mil) and widths from 1.75 to 2.5 M (5 Ft. to 8 Ft.). The most common thickness’ used in containment applications are .5 mm and .75 mm (20 and 30 mil) . Manufacturers are required to provide certifications that the PVC film meets or exceeds all raw material testing requirements of the PVC Geomembrane Institute’s PGI-1197 specification for PVC geomembrane liners. Fabricators are also required to provide certifications that all factory welds meet or exceed material testing requirements.
The PVC film is converted into panels by PVC geomembrane fabricators using chemical fusion welding, dielectric welding, hot air or hot wedge welding. The process used depends on the particular fabrication firm, but all materials and welding are required to meet or exceed the PVC Geomembrane Institute PGI-1197 specification for material properties and seam weld strength.
Panels of PVC geomembrane are fabricated to fit the particular project dimensions. Panels can be built to fit irregular curves, accommodate unusual geometric shapes, or made into very large panels to minimize welding in the field.
Factory weld testing [7] includes destructive testing of samples taken from a minimum of every 1,000 M (3,000 Ft.) of factory seam, or once per panel. Samples are tested for bonded seam strength and seam peel adhesion according to PGI-1197. The PGI Specification 1197 [6] replaces NSF-54, which is no longer supported by the National Sanitation Foundation.
Factory Welding Minimum Peel Strength:
20 mil - 12.5 LB/in
30 mil - 15
40 mil - 15
50 mil - 15
60 mil - 15
Factory Welding Minimum Shear Strength:
20 mil - 38.4 LB/in
30 mil - 58.4
40 mil - 77.6
50 mil - 96
60 mil - 116
Nondestructive testing is also performed over the full length of every weld. Records from each liner panel can be traced from the serial number of each panel back to the original PVC batch produced and to the resin used to produce the material.
After completion of the welding, the PVC panel is accordion folded in both directions and placed on a sturdy wooden pallet for transport to the project site. The panel is packaged to protect the material from weather and damage during transportation and storage. Each panel is labeled with a serial number and panel designation corresponding to the panel location drawing for its particular project.
Field Installation
PVC panels arrive at the site and are deployed according to a panel layout diagram for that project. The deployment simply involves unfolding the panel in reverse of the fabrication process. The panel is unfolded off the pallet along its length, and then using manpower, it is unfolded across its width, much like placing the tarp on a baseball diamond during a rain delay. This unfolding is quick, simple and allows large areas of liner to be placed each day.
Panels are overlapped approximately .3M (12 In.) for field welding. The perimeter of the panel is usually placed in an anchor trench which is backfilled with soil to hold the panel in place during welding and placing of the cover soil on top of the geomembrane.
There are three primary methods of field seaming PVC geomembrane panels;
1. Adhesive seaming
2. Chemical fusion seaming
3. Thermal fusion welding
Adhesive seaming of PVC is the oldest and simplest method of sealing geomembrane panels. The process involves coating each surface of the panels to be joined with a bodied solvent adhesive. The coated pieces are then placed together and pressure is applied by a roller or other device to mate the pieces together and create a bond.
Chemical fusion welding utilizes a chemical fusion agent (solvent) to dissolve the surfaces of the pieces of material to be joined. The chemical fusion agent is introduced between the two sheets of PVC to be welded, and while the surface of the material is molten, pressure is applied to mate the two surfaces together. This provides a homogeneous bond between the two sheets after the fusion agent evaporates. This method is used extensively in the fabrication and installation of PVC geomembranes.
Thermal fusion welding of PVC has been used in Europe for many years. Its use for the field welding of PVC geomembranes in the United States began around 1990. Thermal fusion welders consist of a hot wedge or hot air device that heats the surface of the material to a molten state, while traveling along the length of the seam. Pressure rollers follow the heating device to press the two pieces of PVC geomembrane together while the surfaces of each sheet are melted. This provides a homogenous bond between the two pieces.
There are several advantages to the thermal welding method over the adhesive or chemical methods:
-
Significantly higher peel strength can be achieved.
-
Weld strength is more uniform
-
Two parallel welds can be made allowing for air pressure testing between the welds.
After welding is completed, samples are taken from the field seams at a rate of one per 150 M (500 Ft.) for destructive testing of peel and shear strength.
Field Welding Minimum Peel Strength:
all gauges - 10 LB/in
Field Welding Minimum Shear Strength:
all gauges - 80% of specified tensile strength
Each field seam is also non-destructively tested over its entire length to insure continuity and identify any voids in the seam. Non-destructive testing of adhesive and chemical seams is performed using an air lance. This apparatus directs a high pressure stream of air at the edge of the seam in order to detect any voids in the weld. This test requires a minimum of 50 psi air pressure through a 3/16" diameter nozzle.
Non-destructive testing of dual track thermal welds [8] is performed by sealing one end of the channel formed between the two parallel welds, and introducing air pressure from the opposite end of the weld. The air pressure varies depending on the thickness of the material. Minimum pressure is 15 psi and maximum is 30 psi. Any drop in pressure is recorded over the two minute duration of the test. A maximum amount of pressure drop is specified for each thickness of PVC geomembrane. Any void in the seam, even as small as a pin hole, will not allow the air pressure to be maintained.
After the non-destructive testing has been performed, any defective areas are repaired and then re-tested to insure there are no voids in the field seam.
When liner testing has been completed and accepted the geomembrane is then covered with .3 M (12 in.) of clean soil to protect the geomembrane from damage. The soil cover can be sand or clay that is free of stones, roots, rubbish or any other item that may penetrate the liner. Placement of the cover material can be accomplished using rubber tired or tracked machines as long as a minimum layer of cover soil is maintained between the equipment and the liner.
Performance
The US Bureau of Reclamation has been the best source of information on the long term durability of PVC geomembranes. Testing conducted by the Bureau [2] on samples of 10 mil PVC in service in irrigation canals for over 29 years indicate the buried material has retained its physical properties and is still performing satisfactorily.
Environmental Protection, Inc. and Occidental Chemical Corporation [10] also performed testing 25 year old 10 mil PVC geomembrane used in a golf course irrigation pond in northern Michigan. After 25 years or service the testing confirmed that the material still exceeded today’s specifications for minimum material properties and seam peel and shear strength.
The PVC Geomembrane Institute and its member companies along with the Minnesota Department of Natural Resources are conducting a 30 year assessment of the long term durability of PVC geomembrane. This long term test program, begun in 1995, will provide valuable information on the continuing performance of PVC in containment applications.
Conclusions
Today’s PVC geomembranes provide an economical solution to today’s rigorous environmental containment needs. Thicknesses of 30 and 40 mils provide more durability than the thinner materials used thirty years ago. PVC is easily fabricated, installed and tested to assure clients receive the highest quality and most secure liner installation available today.
Acknowledgements
PVC Geomembrane Institute
References
[1] REC-ERC-95-01, USE OF GEOMEMBRANES IN BUREAU OF RECLAMATION CANALS, RESERVOIRS, AND DAM REHABILITATION, US Department of Interior, Bureau of Reclamation, Denver, CO.
[2] REC-ERC-84-1, PERFORMANCE OF PLASTIC CANAL LININGS, US Department of Interior, Bureau of Reclamation, Denver, CO.
[3] Successful History of PVC Geomembranes Bureau of Reclamation: 1968 - 1995 - March 1996, PVC Geomembrane Institute Technical Bulletin
[4] CONSTRUCTION QUALITY CONTROL DOCUMENT, PVC Geomembrane Institute, - Dated April 1995
[5] Technical Guidance Document, "Inspection Techniques for the Fabrication of Geomembrane Field Seams", EPA/530/SW-91/051, U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, D.C., May 1991.
[6] New Standards for PVC Geomembranes Introduced - August 1997, PVC Geomembrane Institute Technical Bulletin
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