By Abigail Beck, TRI Liner Integrity Services — New equipment has been developed that behaves like a spark tester, but doesn’t have to be performed on conductive-backed geomembrane. It has the same requirements as the water puddle and water lance methods in terms of a conductive layer underlying the geomembrane, including soil. It is not a spark tester, which is used exclusively on conductive-backed geomembrane; yet, it creates an electrical spark.
The terminology invented to differentiate this new technology from its predecessors is the arc tester. The arc tester can locate leaks even when there is a separation between the geomembrane and the underlying conductive layer of up to one inch, which ultimately makes it much more sensitive than the water-based methods. This method has been used privately for over a decade in Europe, completely replacing the water puddle and water lance methods, and is now being offered commercially for the first time.
On a side-by-side comparison to the water-based methods, the arc testing method wins out in almost every category. The water-based leak location methods completely rely on a leaking geomembrane to detect leaks. This means that in the few seconds that water is sprayed or pushed along the geomembrane, the water must go through the leak and make good contact with the subgrade. There are plenty of leak types that won’t leak this quickly, like knife slits or pinholes. The water-based methods can locate pinholes when conditions are good, but there are a handful of times when the methods have missed leaks due to the fact that the water could not flow through the geomembrane quickly or completely enough for detection. A reliable size for detection using the water-based methods is a 1 mm diameter leak, as specified in the ASTM standards. But pinhole leaks are not only a reality; they are a big problem for many geomembrane installations.
One example where pinholes posed a huge problem is a landfill where the action leakage rate (ALR) is 5 gpad. Typical conservative action leakage rates are in the range of 20 gpad, so 5 gpad is actually quite difficult to attain. At this particular landfill, an electrical leak location survey was not performed during the installation of the geomembrane. After the site exceeded its ALR, several dipole surveys were performed per ASTM D7007. A handful of pinhole leaks were found, which were collectively creating the excessive leakage. Typical dipole sensitivity is in the range of a 6.4 mm diameter leak, so after several surveys had been performed unsuccessfully, extreme (and very costly) measures were taken to increase the survey sensitivity in order to finally locate the leaks. The dipole method is simply not appropriate to locate pinhole leaks, which could have been located and repaired as part of geomembrane installation. Water-based methods can commonly find pinhole leaks, but not reliably.
PUTTING IT TO THE TEST
A direct comparison test was organized between the water-based methods and the arc tester. The comparison test took place at a very challenging exposed geomembrane survey site consisting of a corrugated metal tank lined with a PVC geomembrane, as shown in Photos 2 and 3.
This site was yet another example where pinhole leaks were creating the difference between a successful project with a happy client and a problematic site. This particular tank was only leaking about a litre of water every few hours, meaning that the leak was very small. The small size of the leak and the lack of good contact between the loose PVC tank lining and the corrugated steel wall, separated by a cushion geotextile, made this one of the most challenging situations possible for the exposed geomembrane leak location methods.
In the past, this tank installer hired a third party to locate a leak in one problematic installation, but the cost of hiring a third party with specialized equipment prompted the company to investigate equipment purchase options. Since the water puddle method is not effective on extreme slopes and the client wanted to be able to test the side walls of his tank installations, the two equipment choices were the water lance and the arc tester. The water dipole method could be used very effectively, but that method would require filling the tank with water, which can be costly and time-consuming when it is feasible at all. Since I had not seen the arc tester in action first-hand, I could not give an honest evaluation of whether the water lance or arc testing method would better suit his application, so I proposed this comparison test.
The tank installer welcomed the opportunity to compare two different sets of electrical leak location equipment in order to evaluate first-hand which method would be appropriate for his purposes. The arc tester is not effective in the presence of water, so the plan was to perform the survey with the arc tester and then confirm whether or not the water lance could also detect the leak. If both methods failed, then the tank would have to be filled with water in order to force water through the leaks and the water dipole method would be used as a last resort.
The tank installer pointed out which wall he thought the leakage was coming from, based on which side of the exterior of the tank the leakage was reporting to. Within about ten minutes, the arc tester had located a pinhole leak on the wall of the tank. The water lance equipment was then set up to maximum sensitivity and the wall area was sprayed with water at a typical survey rate application. No leak was registered by the water lance. Since the known leak location had been circled on the wall of the tank as a result of the arc test, the water lance equipment was sprayed directly at the leak location. There was still no signal. The water lance conductive nozzle was then pressed directly onto the leak to force the geomembrane to come into close contact with the wall of the tank and finally a signal was registered. It would be impractical to survey the entire installation with the nozzle pressed directly on every square centimeter of the survey area and the requirement for a continuous water source also made this method cumbersome. It became obvious that the arc tester was the superior testing method for this application.
After the direct comparison sold the arc testing equipment to the tank installer, the leak survey was continued along the walls and floor of the tank, where he located four more pinholes. Photo 3 shows the tank installer using the arc testing equipment on the walls of the tank.
CONSISTENTLY SUPERIOR RESULTS
This is not news. Side-by-side comparisons between the water-based methods and the arc tester have been performed in front of clients and the outcome is always the same. The arc tester has even been used to perform an electrical leak survey at a site where the water lance method had been previously employed. The result was the location of a few hundred pinhole leaks that the water lance method had failed to pick up.
One revolutionary aspect of this new technology is that it is very easy to operate. In fact, after about five minutes of “training,” the tank installer was locating pinhole leaks like a pro. The arc tester is a close cousin of the spark tester. It has undergone about a decade more of evolution and field testing than the spark tester and is much easier to use. The only equipment adjustment required is a knob that is set by the thickness of the liner to be tested.
As J.P. Giroud stated during the Liner Integrity Panel discussion at Geosynthetics 2013, “All installed geomembrane liners should be subjected to an electric leak location survey.” With the availability of this new arc testing equipment, this statement has the means of becoming a reality. Damage incurred to the geomembrane during cover material placement is a completely different ballgame, but this is at least a step in the right direction.
Abigail Beck is the Director of TRI Liner Integrity Services, a division of TRI Environmental. For information on the company’s survey training and education, or to inquire about purchasing or renting liner survey equipment, visit www.linersurvey.net or contact firstname.lastname@example.org.