This issue of the newsletter critiques a report of the experiences of The Bureau of Reclamation with geomembranes for seepage control in canals, reservoirs, and dam rehabilitation. This report also presents design criteria, construction procedures, and O&M (Operation and Maintenance) considerations for buried geomembrane canal linings. While this synopsis of the report describes their success with PVC geomembranes, you may want to refer to a full copy of the report for more detail. We have included information on how to obtain the report.
Reclamation's earliest PVC geomembrane installation was a small experimental section installed in 1957, on the Shoshone Project in Wyoming. The first PVC installation under a construction specification was in 1968 on the Helena Valley Canal in Montana. The geomembrane was an alternative to the hot, spray applied asphalt membrane. Because the energy crisis in the 1970s caused a significant increase in the cost of petroleum products and limited the source of supply, the asphalt membrane was deleted from specifications.
PVC geomembranes have been important to the Bureau ever since then. During this time, PVC has been the material of choice in 47 of 51 projects cited in the report. The report highlights and details seven canal projects, three reservoir applications, two dam rehabilitations, six special applications, and five laboratory studies.
In addition to reporting on PVC successes on specific projects, the report also contains highly informative sections on:
- Design Criteria for Buried Membrane Linings
- Construction Procedures
These two sections are as concise and useful coverage of these topics as we've seen, and alone would be sufficient reason to obtain a copy of the report.
Bureau Enjoys Satisfactory Results
Results indicate that buried PVC geomembranes are providing satisfactory service for seepage control, and that they are viable alternatives in areas not suitable for concrete or compacted earth linings. The slope stability of the protective soil cover on steeper slopes has been an issue, and as with other geomembrane applications, additional studies are needed concerning ways to assure good stability. Laboratory studies and field observations confirm that loss of some plasticizer results in slight stiffening of the PVC geomembrane with the associated reduction in elongation, increased tensile strength, and a decrease in low temperature impact. However, since this occurs over a long period of time, it does not significantly affect its performance, if properly installed.
Although, for many years, the Bureau used .25-mm (10 mil), the currently recommended geomembrane for standard buried canal applications is 0.50-mm (20-mil) thick PVC. The PVC should conform to the physical property requirements listed in PVC Geomembrane Institute PVC Specification 1197.
Comparison with other Materials
When compared with other materials, the PVC geomembranes compared favorably over other materials. See the table of comparison results, below.
Some Specific Findings
Buried PVC geomembranes are providing satisfactory seepage control in Reclamation canals, dams, and reservoirs. These linings are viable alternatives to concrete and compacted earth linings. The rate of aging (plasticizer loss) of PVC geomembranes depends primarily on the following factors:
- PVC geomembrane source
- 56-day laboratory volatility tests indicate differences in plasticizer loss rates among PVC linings from different manufacturers.
- Thickness of PVC membrane
- Results of 56-day laboratory volatility tests also indicate that the plasticizer loss rates generally decrease with an increase in geomembrane thickness.
- Location in canal
- Samples obtained from within the water prism generally exhibited less aging than those obtained above the waterline.
- Condition of Subgrade
Samples obtained from areas where the geomembrane has been placed over a fairly smooth subgrade exhibited less aging than those installed over a coarser base.
Geomembranes frequently require some type of soil cover to protect them from the elements, animal traffic, vandalism, and mechanical damage during cleaning operations.
In keeping with the trend of reducing the use of solvent-base construction materials (oil base paints, cutback asphalts, etc.) the field seaming of geomembranes, including PVC, by chemical methods is expected to decrease in the next few years and eventually will be eliminated. To prepare for this eventuality, industry is now beginning to develop heat-seaming methods for use in the field.
Comparison with other Materials
When compared with other materials, the PVC geomembranes compared favorably over other materials.
Biaxial Flexibility Very Good
Uniaxial Elongation Good
Conformance to Substrate Very Good (Best rating to the materials shown)
UV Resistance Generally covered -- long term exposure requires specific formulation
Thermal Expansion Low to Moderate
Shear Friction Low -- compared to textured products
Ease of Handling Prefabricated Panels -- easy
Seaming Methods Chemical and Thermal
Point Puncture Resistance Good
Successes in Canal Applications
Canals are unique structures because they are long and narrow, have limited access for work, and have steep side slopes. Historically, PVC has been the most widely used geomembrane for canal applications for the following reasons:
- PVC is available in large sheets which minimize field seaming.
- 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.
- PVC is easily field spliced and repaired with a solvent-type cement.
- PVC has good puncture, abrasive and tear-resistant properties, which are important to minimize damage during installation.
- PVC geomembrane installation does not require sophisticated equipment or skilled labor.
Bottom-only geomembrane linings have been shown to be an effective, low-cost method of reducing canal seepage in loessial soils in some areas of Kansas and Nebraska. A field study in which 0.25 -mm (10-mil) PVC was placed only on the canal invert was performed by Reclamation's Nebraska-Kansas Projects Office and showed a 50 to 55 percent seepage reduction. This seepage control method can be easily accomplished by local field personnel.
Success at the Coachella Canal
The liner can be installed underwater. For a number of years, Reclamation has been interested in developing seepage control methods for leaky, unlined irrigation canals that cannot be easily dewatered for lining because of water delivery commitments. The search for a practical and economical solution to this problem has led to several approaches, such as: an underwater installation of a prefabricated bituminous canal lining, and chemical soil sealants. PVC proved to be the most effective approach in these situations.
The report describes the project at the Coachella Canal, near Niland, California. It provides technical data as well as diagrams, photos and methodology.
The successful completion of a 1.5-mile long demonstration section on this California project has shown the feasibility of the underwater lining of operating canals as a seepage control method. Refinement of techniques will allow the underwater lining of canals to find wide use, both in the southwestern United States and worldwide. The underwater lining technology is also applicable to placement in the dry. For example, the concrete/geomembrane could be used to avoid over excavation in expansive clays, shales, gypsiferous and loessial soils, and high sulfate soils.
Environmental Protection, Inc provides this PGI Technical Bulletin for your information.
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