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Leading the industry in geomembrane construction quality assurance with electric leak location methods
Technical note
Leading the industry in geomembrane construction quality assurance with electric leak location methods
14 Min read
The dipole method uses GPS-based acquisition to detect leaks through cover soil
Introduction
Geomembranes are used to provide a barrier of protection against potentially dangerous materials and groundwater and/or soil in large-scale containment facilities such as landfills, wastewater ponds, mining applications, secondary fuel storage and potable water reservoirs. While these geomembrane liners are critical to protecting the environment, they are vulnerable to damage. When a geomembrane contains even a small number of leaks, along with wrinkles that are typical in geomembrane installations, it performs only slightly better than a low-permeability soil liner (Giroud and Wallace, 2016).
Evidence shows that typical Construction Quality Assurance(CQA) protocols that exclude use of leak location surveys are insufficient at ensuring a geomembrane is intact post construction. Leaks caused by poor seaming methods, knife slices, punctures and equipment damage are routinely found by Electrical Leak Location (ELL) surveys.
The most significant damage to the geomembrane is often caused by equipment during placement of cover material. CQA efforts typically focus on the seams, whereas ELL surveys can test 100% of the lined area for leaks, both before and after cover material placement. Even with careful construction methods and a high level of CQA, there’s no guarantee a geomembrane has been installed without leaks unless ELL methods are applied as part of project construction.
Electrical Leak Location (ELL) methods
Electric leak location survey is a field proven technology use to locate leaks in installed geomembranes both before and after water or soil placement. Various ELL methods are available, but all operate on the principle that geomembranes are electrically isolative. Thus, when electricity is applied to the surface of the geomembrane and grounded to the layer beneath it, the path of electricity can be directly traced through any leaks present in the geomembrane. The ELL methods can be grouped into two different categories: 1) exposed geomembrane methods and 2) covered geomembrane methods. Descriptions of these methods, including advantages and limitations, can be found in ASTM D6747 Standard Guide for Selection of Techniques for Electrical Leak Location of Leaks in Geomembranes.
Covered geomembrane testing
Leak location surveys can be performed on geomembranes that have already been covered. During covered geomembrane testing, the current injector is placed in the material covering the geomembrane, the current return is placed in the underlying layer, and the cover material distributes the applied voltage throughout the survey area. Measurements of voltage are then acquired in a grid pattern throughout the survey area. The voltage potential will be highest at the current injector location and lowest at the location of a breach in the geomembrane. A dipole (an apparatus with two closely spaced measurement points) is used to measure the voltage gradient to magnify the slope of the voltage potential field, with a sharp downward slope indicating the direction of a leak (when voltage is measured from front to back).
The data collected by a dipole instrument is then organized into a voltage map of the survey area, as shown in Figure 1.
Figure 1: The dipole method uses GPS-based acquisition to detect leaks through cover soil
For dipole-method testing, a survey direction is chosen, and the dipole must remain in the same orientation as it collects voltage data in a grid pattern throughout the survey area. For the Figure 1 voltage map, the dipole travel direction is from bottom to top, with voltage measurements obtained between the front foot and back foot as the dipole is facing the direction of travel. The tick marks on the map show measurement acquisition locations. Negative voltage values are coded by changing from red to blue to black with increasing magnitude. Positive voltage values are coded by changing from green to yellow to white with increasing magnitude. If current is traveling from the current injector, the dipole will measure a negative/positive polarity throughout the survey area, with positive values below the current injector and negative values above the current injector. Locations where current is exiting the survey area (e.g. leaks) will appear as the opposite: a negative area directly below a positive area. These negative/positive areas will form circles above and below the leak location, separated by tightly spaced contour lines that look like a butterfly on its side.
Since a leak very close to the current injector can be masked by the high voltages around it, the current injector must be moved at least once to maintain distance from it. The greatest advantage to the covered geomembrane methods is that they’re performed after cover-material installation, detecting the largest leaks that would otherwise go undetected or while hydraulic head is applied to the geomembrane. The drawback to the covered methods is that the detection sensitivity is extremely dependent on site conditions, liner cross section and site materials—in addition to operator skill and methodology. ASTM D7007 was originally created for the dipole method inits simplest form. More recently, ASTM D8265 was published to control site conditions, dipole instrumentation and testing procedures. ASTM D8265 requires that a voltage map such as the one presented in Figure 1 be generated as part of final reporting, making the testing results reviewable by a third party. The report required by ASTM D8265 is the strongest quality-control document that can be generated to show the integrity of a lining system at the conclusion of construction.
Product solutions
As mentioned earlier, the material directly below the geomembrane must be sufficiently electrically conductive. In some projects, this isn’t possible. In this case, GSE LeakLocation Conductive geomembrane can be utilized to facilitate and enhance all the ELL methods. The installation of this product needs to follow specific steps as described in the GSE Leak Location Conductive Installation Guide to ensure an effective ELL survey. If these installation steps are not followed, then only the exposed ELL method of spark testing is feasible.
The GSE Leak Location Conductive installation guidelines include two basic functions of isolation and ground connectivity into a grid. Isolation refers to the upper exposed flap after welding panels together. The bottom of the exposed flap can carry current and act like a hole in the geomembrane.
1. Isolation: Three areas of isolation on the exposed flap need to be completed, including fusion welds (using aniso-wedge), patches on fusion welds and the flap at the anchor trench.
2. Ground Connectivity: The two types of connections needed are panel connectors and patch connectors underneath the geomembrane. Typically, a 3 ft x 3 ft(1 m x 1 m) piece of the same conductive geomembrane is flipped upside down and placed in the middle of two panels. The patch-connector piece should be the width of the hole and long enough to have the patch connected to both panels with at least a foot of excess material.
Other types of conductive geosynthetic products are available for testing of a primary geomembrane of a double-lined facility. One downside of these products is that they lack the intimate contact that the conductive-backed geomembrane features. They also lack the industry track record that the conductive-backed geomembrane product has earned through several decades.
Site preparation and testing support
For all ELL methods, there must be a continuously conductive layer directly under the geomembrane being tested. Earthen material conductivity (in-situ soils, prepared subgrade) is rarely a challenge. However, problems can arise with double-lined installations. A conductive-backed geomembrane should be specified as the primary geomembrane in most cases. An encapsulated geosynthetic clay liner (GCL) (i.e., sealed between a secondary and primary geomembrane) can be problematic. Factory moisture content should be sufficient, but if the GCL isn’t covered on a jobsite and sits exposed in an arid environment, it can desiccate to the point of no longer being electrically conductive. In this case, control of GCL moisture should be part of the project construction, as detailed by Beck et. al.(2008).
Exposed geomembrane testing
Exposed geomembrane testing typically is performed immediately after liner installation, but can also be performed during installation and leading up to completion. The survey area should be uncluttered. For the high-voltage-based methods, the geomembrane must be clean and dry. No outside testing support is required. For the water-based methods, the burden of the water supply is usually put on the general contractor. A source of water (e.g., water truck) must be provided along with a series of hoses that reach the entire survey area. Additionally, laborers are required to assist with movement of the hoses. Some water can exist in the survey area prior to water-puddle testing but should be limited to scattered puddles.
Covered geomembrane testing
The site requirements for covered geomembrane electrical surveys can be summarized by four requirements:
1. The material covering the geomembrane must be sufficiently conductive.
2. The material underneath the geomembrane must be sufficiently conductive.
3. There must be sufficiently conductive material inside the leak(s).
4. The material above the geomembrane must not touch the material below the geomembrane except through the leaks.
The more the current can be encouraged to travel through the leaks, and only the leaks, the more sensitive the survey will be.
For all covered geomembrane ELL methods, the cover material must be isolated from the underlying conductive layer. This is typically achieved by leaving a width of uncovered geosynthetics along the entire perimeter of the survey area. An example of such an “isolation gap” is shown in Figure 2. The current injector is placed in the survey area, with the return electrode connected to the underlying conductive layer. In the case of a single-lined facility, this would be the surrounding ground.
The current always follows the path of least resistance from the current-injector electrode to the return electrode. This is why the survey-area material must be isolated; the only path between the electrodes should be through any leaks present in the geomembrane. Electricity travels the path of least resistance; if any conductive feature (e.g., soil, water, metal pipe, etc.) allows current to flow from the inside to the outside, a portion of the current may or may not flow through the holes present, which is a prerequisite for detection.
Other than providing survey-area isolation, testing support includes moisture-conditioning the cover material (in the case of earthen material cover) and the excavation of leak locations.
Figure 2: Perimeter isolation gap around soil-covered survey area
How to specify and design for ELL methods
Project specifications for ELL methods on covered geomembranes should include, at a minimum:
1. Specify the use of ASTM D8265.
2. Require adequate moisture in material above the geomembrane and through any leaks.
3. Require a sufficiently conductive layer below the geomembrane or a conductive geomembrane and electrical contact with this layer (if double-lined).
4. Require electrical isolation between the material covering the geomembrane and the layer underneath it.
Most project specifications put the burden of moisture content and electrical isolation on the general contractor. A typical specification is to leave a 1-foot-wide gap in the cover material along all edges. Electrical contact to the material under the geomembrane for double-lined facilities entails installing a system of copper wires as part of liner installation. The project-specific copper-wire layout should be generated by the ELL contractor.
One of the most-common questions for preparation of a dipole survey is how much water should be applied to the cover material. The answer is specific to the site materials, but the minimum moisture content is usually in the range of 0.5-6.0 percent to have sufficient electrical conductivity through typical earthen materials.
If the material is conductive, the survey can be performed, but this doesn’t mean it will be successful in finding leaks with only the minimum moisture content for conductivity. For geomembranes covered by geocomposite or wrinkly geomembranes, it is likely the material will have to be saturated to cause any holes in the geomembrane to actually leak either shortly before or during the survey. If even a small amount of head over the geomembrane can be applied directly before the survey, the smallest leaks should be detectable, provided the site is well isolated. If this is not feasible an exposed geomembrane method should be used before cover material placement to ensure all leaks are detected. The main design decision regarding ELL methods is which method(s) to choose. This choice will depend not only on project configuration but also the performance goal. The Allowable Leakage Rate (ALR) should be used to determine which materials and methods are included in the design. Sites with less tolerance for leakage will require a more rigorous approach.
In tandem with specifying ELL, wrinkles must be addressed in the design since the combination of wrinkles and leaks plays the most-significant role in facility leakage. A statistical analysis performed for landfill expansion cells showed that for ALRs of 20 gallons per acre per day (GPAD), both exposed and covered ELL methods should be used.
For ALRs of 5 GPAD, a wrinkle-reduction strategy should be employed along with both exposed and covered ELL methods. If the goal is zero leakage, then wrinkles must be eliminated along with both exposed and covered ELL methods (Beck, 2015). One practical way to achieve this is by utilizing conductive-backed geomembrane. Conductive backed geomembrane doesn’t actually eliminate the wrinkles, but it eliminates the leakage problems they create by enabling the detection of leaks on wrinkles.
Double-lined facilities pose challenges to ELL methods because they’re typically built without a conductive layer between the geomembranes. If no conductive layer is specified to enable testing of the primary geomembrane, then the layer between the two liners must be filled with water to enable testing. This entails placing water above the primary geomembrane to avoid liner uplift. As a result, only the water-covered dipole method can be used, and only in the areas covered by water. Exposed liner testing can’t be performed.
Landfill cells are particularly difficult to thoroughly test since they’re typically built next to an existing landfill cell. The area where the old cell connects to the new cell is called a “tie-in”area. This area is prone to damage during excavation of existing materials, and welding is difficult in this area due to wet, dirty and uneven conditions. This area also commonly falls inside the isolation gap required for dipole testing, so it doesn’t get tested when only the dipole method is specified. Therefore, even if an exposed-liner testing method isn’t required for performance goals, the tie-in area should be tested using the arc-testing method.
Conclusion
A geomembrane will only function as intended if leaks can be minimized or eliminated before a facility is put into operation. The extent to which geomembranes and composite liners leak is dependent on design, construction and construction quality assurance (and whether and how well ELL is implemented). The only way to ensure containment goals are being achieved is to specify ELL as part of project construction and ensure those methods are properly applied.
References
1. ASTM D6747. “Standard Guide for Electrical Leak Location of Leaks in Geomembranes”
2. ASTM D7002. “Standard Practice for Electrical Leak Location on Exposed Geomembranes Using the Water Puddle Method”
3. ASTM D7007. “Standard Practices for Electrical Methods for Locating Leaks in Geomembranes Covered with Water or Earthen Materials”
4. ASTM D7240. “Standard Practice for Electrical Leak Location using Geomembranes with an Insulating Layer in Intimate Contact with a Conductive
Layer via Electrical Capacitance Technique (Conductive-Backed Geomembrane Spark Test)”
5. ASTM D7703. “Standard Practice for Electrical Leak Location on Exposed Geomembranes Using the Water Lance Method”
6. ASTM D7953. “Standard Practice for Electrical Leak Location on Exposed Geomembranes Using the Arc Testing Method”
7. ASTM D8265. “Standard Practices for Electrical Methods for Mapping Leaks in Installed Geomembranes”
8. Beck, A., Kramer, E. and Smith, M. (2008). “Specifications for Moisture Content of GCL to Perform Electrical Leak Location Surveys,”
Proceedings of EuroGeo4, Edinburgh, Scotland
9. Beck, A. (2015). “Available Technologies to Approach Zero Leaks,” Geosynthetics 2015 Conference Proceedings, Feb. 15-18, 2015, Portland, Ore
10. Forget, B., Rollin, A.L., and Jacquelin, T. (2005). “Lessons Learned from 10 Years of Leak Detection Surveys on Geomembranes,”
Proceedings of the Sardinia Conference
11. Gilson, A. (2019). “Electrical Leak Location Testing for Zero Leak Verification,” Proceedings of IFAI Geosynthetics Conference, Houston
12. Giroud, J.P. and Wallace, R.B. (2016). “Quantified impact of geomembrane wrinkles on leakage through composite liners,”
Proceedings of the 3rd Pan-American Conference on Geosynthetics, April 10-13, 2016, Miami Beach, Fla
13. Solmax Leak Location Conductive Installation Guide