More accurate liner integrity surveys using electrically conductive geomembrane
Geosynthetics | October 2012
By David Gallagher and Abigail Beck
The electrical liner integrity (ELI) survey is currently the most effective and practical means of locating leaks in installed geomembranes after cover soil installation, as detailed in ASTM D7007. The technology functions along the principle that polyethylene geomembranes are electrically insulative. By applying an electrical potential across a geomembrane, an electrical current will flow only if a hole is present in the geomembrane, allowing the current a less-resistive path through the geomembrane. Various survey methods then track this current flow to pinpoint a leak.
When an ELI survey is performed in an application where a cover material has already been placed over the liner, it requires that a sufficiently conductive material—both above and below—is in intimate contact with the geomembrane. This traditionally meant the cover material above and subgrade below. However, due to various site conditions nonconductive liners can have difficulties meeting these requirements. For instance, with wrinkling or bridging of the liner, air gaps can exist beneath the liner that prevent the intimate contact required. Also, in cases where there is dry subgrade or encapsulated geosynthetic clay liners (GCL), the material beneath the liner may not be sufficiently conductive to conduct the leak survey (Peggs, 2007). It is also because of these two requirements that covered ELI surveys cannot be performed easily on double-lined ponds and their slopes cannot be surveyed at all when using traditional geomembranes.
Recent studies, outlined below, show that covered ELI surveys are more thorough and accurate over a variety of soil-moisture conditions and liner-contact situations with an electrically conducive geomembrane.
Improving ELI survey quality
The essence of covered ELI survey quality lies in the size and number of leaks that are located in the geomembrane during a survey for subsequent repair. To compare the performance of both traditional and electrically conductive geomembranes, the authors chose to employ the sensitivity test portion ASTM method D7007 to quantify survey quality by measuring the signal-to-noise ratio (signal) of identical holes in identical soil conditions when either type of geomembrane is used. A higher signal means that smaller leaks can be located compared with a lower signal, based on the authors’ field experience. To eliminate other variables, all of these comparisons were made in a controlled environment using a bench-scale rig in a climate-controlled lab.
Site conditions vary greatly and a sufficient level of conductivity is a requirement of ELI surveys, so it is important that the relationship between survey performance and subgrade conductivity is well understood. To evaluate this, surveys were performed using both types of liner on subgrades of various moisture contents. At each moisture content, a signal was measured using traditional liner as well as the electrically conductive liner and for ease of understanding these two signals were compared to each other to produce an Improvement Factor at each moisture content. For instance, an Improvement Factor of 2.0 means the signal when the conductive geomembrane was used was twice as strong as when the traditional geomembrane was used. Continuing this data collection at increasing subgrade moisture contents populated the chart in Figure 1, which shows the improvement in signal strength when the conductive geomembrane was used vs. a traditional geomembrane.
At one moisture content, the signal strength was measured at increasing lateral distances from the hole. The greater signal at the hole was maintained at increasing distances from the hole as shown in Figure 2. This translates to a greater likelihood that a leak will be found by surveyors even if they are not surveying directly over the hole.
Unlike the prior tests, where improvement was studied in a situation—moist, conductive subgrade in intimate contact with the liner—where traditional liner had historically been successful, the next test was to evaluate performance in finding leaks in situations where a traditional liner had struggled in the past, particularly when there is not intimate contact between a hole in the geomembrane and the subgrade. In this test, signals were measured multiple times for each contact scenario and the averages were recorded. It was found that when the hole was located over an imperfection in the subgrade that prevented intimate contact, as shown in Figure 3, leak signals were eight times stronger when conductive geomembrane was used. Similarly, when the hole is located under a seam flap, as shown in Figure 4, leak signals were 10 times greater when conductive geomembrane was used. Lastly, in a situation similar to Figure 5, where the hole is located on a wrinkle, a quantifiable comparison could not be made because no signal was detected when traditional liner was used.
The results of these tests have shown that an electrically conductive geomembrane can be used to significantly increase the quality of ELI surveys used to locate holes in a liner system as well as allowing the use of ELI surveys in applications that were previously impossible such as multiple-layer liner systems. The full detailed report of this study will be shared at the Geosynthetics 2013 conference in Long Beach, Calif., April 1–4, 2013.
The industry has begun to take notice of these improvements and recently a project was completed at a construction and demolition landfill in southern Minnesota. At this site, the electrically conducive geomembrane was installed on top of a GCL and below a geocomposite that was subsequently layered by approximately 4ft of cover soil.
An ELI survey was then performed, which resulted in the discovery of a small cut in the liner, as shown in Figures 6 and 7. Detailed information on this project was shared at the EuroGeo5 conference in Valencia, Spain, Sept. 16–19, 2012, “New Electrically Conductive Geomembrane for Post-Installation Liner Integrity Surveys” (Ramsey et al., 2012).
David Gallagher is a product development engineer with GSE Lining Technology in Houston, Texas.
Abigail Beck, M.S., P.E., is a senior engineer at Geo-Logic Associates in Grass Valley, Calif.
Beck A., Gallagher D., Kramer E. (2013), “Leak Location Liner Performance Evaluation,” Proc. of Geosynthetics 2013 Conference, April 1-4, 2013.
Peggs, Ian D. (2007). “Liner integrity/leak-location survey: The significance of boundary conditions,” Geosynthetics, February-March 2007, pp. 34-38.
Ramsey B. J., Peggs I., Gallagher D. et al. (2012), “New Electrically Conductive Geomembrane for Post-Installation Liner Integrity Surveys,” Proc. of the 5th European Geosynthetics Conference, September 16-19, 2012.