Field & SeamField & Seam
The advantages and uses of fabricated geomembranes.
November 1, 2010
Timothy D. Stark, Luis F. Pazmino, Stanford Slifer and Duff Simbeck
In general, geomembranes come in two distinct types: The first is flexible enough for folding, transport to a job-site in large panels, unfolding and seaming to other panels for field testing. The second cannot be safely folded and instead ships as rolled goods. As a result, the majority of seaming and testing for the second type of geomembrane must occur in the field. This results in longer construction time (and costs) than for geomembranes that can be folded in the factory. This article discusses a few other advantages of fabricated geomembranes for containment applications, such as in landfills.
Factory Fabricated Geomembranes
Some geomembranes are flexible enough that they can be factory seamed, folded, and unfolded without creasing or damage to the liner and then seamed to other panels and tested as necessary in the field. These geomembranes are most commonly thin gauge (less than 45 mils thick). They are produced through calendering, lamination, and extrusion manufacturing. If a geomembrane is not flexible, creases in the geomembrane may weaken it, possibly resulting in cracking or accelerated aging.
Geomembranes that can be folded are usually seamed in a factory to reduce field seaming, cut installation time, and minimize field tests and patching. In addition, revised field testing procedures (e.g. air-channel testing) further reduce the number of destructive samples required on a job site. Installation time and testing is particularly important in harsh environments, under the threat of inclement weather, and in situations where an owner may want to extend the field installation season. These fabricated liners can facilitate installations in difficult climates and are less expensive to install than liners assembled completely in the field.
Folding and shipping large prefabricated panels allows for increased field production. Quick deployment of panels allows large areas of carefully prepared subgrade to be covered quickly and prevents degradation from weather and other operations. It also allows large areas of geosynthetic clay liners (GCLs) to be covered quickly, thus limiting premature hydration of the bentonite. Commonly, after completing only one field seam between two factory-fabricated panels, the area covered already exceeds one acre.
Flexible geomembranes are fabricated into panels in a controlled factory environment more suitable to high quality seaming than variable field environments. Generally, the size of the panels is only limited by allowable shipping and handling weight. It is common to ship panels that weigh 1,816 kg (4,000 lbs.) but panels as heavy as 4,086 kg (9,000 lbs.) have been shipped. On-site, fewer field seams are required to complete the installation since the panels are so large. Field seaming usually utilizes thermal fusion. Patches also use thermal fusion, though materials like PVC also allow for field solvent welds.
Factory Seaming Versus Field Seaming
Field-seamed geomembranes face a few more challenges than their factory-seamed counterparts. Wind, dirt, precipitation, temperature fluctuations, crew changes and machine performance can all affect field seaming. Bonding two sheets of geomembrane in the factory on a smooth, clean concrete floor enables (1) quicker and more consistent welding, (2) higher seam shear and peel strengths, and (3) less need for destructive testing and thus fewer patches in the completed liner. Excessive destructive testing presents more potential weak points in areas subjected to tensile stresses.
Fabricated geomembranes can be especially efficient for small projects where only one or two panels are enough line the entire project area. For example, 0.81 hectares (2 acres) can be completely covered, tested, patched and certified in one day with two or three panels. Savings on a small project include a smaller crew size, less mobilization cost, less equipment and less detail work. Generally, a field-assembled geomembrane will require a second day to complete the testing and detail work. In a given liner area, the amount of field seams may be 80 percent less than that required for a similar non-fabricated liner.
Estimated Installation Cost
In general, the cost of field-assembled geomembrane material is comparable to fabricated geomembrane material. However, the installed cost is usually higher for field-assembled geomembranes because of the greater scope of installation and field testing costs.
When a project requires CQA/CQC, a fabricated geomembrane installation reduces these third-party costs significantly. Simply put, large factory fabricated panels require fewer field seams, therefore incurring fewer third-party costs.
Factory fabrication has been performed since the 1950s and will be an important facet of the geomembrane industry for years to come. The use of fabricated geomembranes can facilitate installation of a containment system, closure cell, sports turf barrier, floating cover, pond liner of all types and field testing protocols. In addition, prefabricated panels and fewer field seams result in liner phases being completed more quickly and with fewer patch locations. Fabricated geomembranes generate lower installation and CQA costs because large panels reduce time and expense by field personnel, inspectors and general contractors. Finally, fabricated geomembranes show more consistent seam performance than field assembled liners because the majority of the welds are created in a controlled factory environment.
Timothy D. Stark is professor of civil and environmental engineering at the University of Illinois at Urbana-Champaign ([email protected]). Luis F. Pazmino is a Masters student in the Department of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign. Stanford Slifer is president of Watersaver in Commerce City, Colo. ([email protected]). Duff Simbeck is vice president of Simbeck and Associates in Mancos, Colo. ([email protected]).