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The American Society for Nondestructive Testingwww.asnt.org

Geosynthetic membranes orgeomembranes are 0.5 to 2.5 mm(0.02 to 0.1 in.) thick sheets ofplastic that are used to preventliquid leakage to the environmentprimarily in landfills, ponds, miningleach pads, and tanks (Fig. 1). Theyare also used in wastewatertreatment plants, potable waterreservoirs, dams, canals and asfloating covers. Leakage ingeomembranes can cause pollutionof groundwater, product loss, andeven failure of containmentsystems. Leaks can easily occurbecause of inadequate installation,damage from constructionactivities, and damage while placinga protective layer of earth on thegeomembrane. Nondestructivetesting techniques are used forpreservice testing of the seams andsheets for leaks. Often when aleakage problem is experienced,nondestructive leak locationtechniques are used to locate theleaks for repair.

Some flexible types ofgeomembranes are custom-made tofit an installation, but the more rigidgeomembranes are deployed in rollsand seamed together in the field.The seams can be extrusion welds,fusion welds, and sometimessolvent welds (Fig. 2). Often thegeomembranes are covered with alayer of earth materials.

Although not discussed in thisarticle, destructive testing is alsoperformed on the material andparticularly on the geomembraneseams to test the physicalcharacteristics. Destructive testingof seams requires cutting out alarge section of the seam, requiringlarge patches at the sample location.

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Nondestructive Testing of GeomembranesGlenn T. Darilek* and Daren L. Laine*

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* Leak Location Services, Inc.;16124 University Oak; San Antonio,Texas 78249; (210) 408-1241;results@llsi.com.

Figure 1. Nondestructive testing techniques are used to locate leaks ingeomembranes such as the one shown in this large pond.

From NDT Technician, Vol. 11, No. 2, pp: 1–5.Copyright © 2012 The American Society for Nondestructive Testing, Inc.

Nondestructive testing of geomembranes can be dividedinto two general categories. The first is testing as part ofconstruction quality control, specifically the testing of theseams and patches. The other category is leak locationmethods for locating leaks in the geomembrane panels aswell as in the seams.1

Nondestructive Testing of Seams

Nondestructive methods used to test the seams during theinstallation of the geomembranes include vacuum boxtesting, air channel pressure testing, air lance testing, andspark testing.

Vacuum Box Testing. Vacuum box testing is primarily usedto test extrusion welds on rigid geomembranes. A rigidrectangular box with an open bottom with rubber seals anda clear plastic window on top is placed on a section ofextrusion weld that has been wetted with a soap solution. Avacuum pump is used to draw a partial vacuum on the box.Leaks are visually indicated by bubbling in the soap film.Because of the labor involved, this method is primarily usedonly on patches. Some skill is needed to be able to get agood seal and to visually recognize leaks through thewindow, which is usually obscured with soap solution. It isonly applicable on relatively flat surfaces. ASTM D5641 is astandard practice for vacuum box testing.2

Pressure Testing of Seams. Double track fusion weldersproduce two closely-spaced seams at the same time. Thisallows the ends of the seams to be sealed for pressuretesting. An air pump is connected to a hose with a valve,pressure gauge and hollow needle at the end of the hose.The top layer of the seam is punctured with the hollow

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Figure 3. Electrical leak location method for leak testing ofgeosynthetic membranes.

Powersupply

Current source electrode

Geomembrane

Current flow

Leak

Current returnelectrode

Potentialmeasurement

Figure 2. Geomembrane seam welds: (a) fusion and(b) extrusion.

(a)

(b)

needle and the air pump pressurizesthe seam to the specified level. Thevalve is closed and the pressure ismonitored for a prescribed time. Aspecified pressure drop indicates aleak in the seam. Air pressure testingof double seams is conducted inaccordance with ASTM D5820.3

Pressure testing of dual-track seams inpolyvinyl chloride (PVC)geomembranes is conducted inaccordance with ASTM D7177.4

Air Lance Testing. Flexiblegeomembrane seams can be testedusing an air lance. An air lance is ahollow tube with an ell and orifice atthe end. A suitable air compressor isconnected to the air lance with an airhose. The orifice of the air lance isdirected towards the edge of theupper flap of the seam. Unbondedareas are indicated when the air flowcauses a vibration of thegeomembrane. ASTM D4437describes the air lance test as well asother nondestructive tests for flexiblegeomembranes.5

Spark Test. Applied in accordancewith ASTM D6365, the spark test isused particularly for seams that cannotbe tested using the othernondestructive tests, specifically forcurved surfaces, corners, andpenetrations through thegeomembrane.6 A wire or otherconductive strip is embedded at theedge of the top sheet of an extrusionweld. The end of the wire isgrounded. A holiday tester with acurrent-limited voltage of severalthousand volts is used with aconductive probe or brush. The probeor brush is passed along the weld beadand a spark discharge will occur ifthere is an air channel or void betweenthe probe and the grounded wire.

Leak Location Testing

Imagine the task of locating a smallleak in a geomembrane-lined pond aslarge as the one shown in Fig. 1. Thesecond category of nondestructivetesting of geomembranes routinelyaccomplishes that. Leak locationtesting using electrical methods is

sometimes called electrical leak locationtesting or geoelectric leak location. Itgoes without saying that the mostimportant test of a geomembrane,whose only function is to preventliquid leakage, is to test for leaks.Geoelectric methods test for leaks inthe seams and patches as well as in thegeomembrane panels.Figure 3 shows the basic

geomembrane leak location method isto connect a direct current powersupply to one electrode in contactwith a conductive material under thegeomembrane and another electrodein contact with conductive materialabove the geomembrane. Althoughthe two media are actually resistive,they are electrically conductivecompared to the electrical resistanceof a leak. Because the geomembraneis an electrical insulator, with no leaksno electrical current will flow, and theresulting electrical potentials on thegeomembrane are uniform. But if thegeomembrane has leaks, electricalcurrent will flow through the leaks,causing a characteristic anomaly in thepotential at the leaks. Various probesare used to detect or map theseanomalies to locate the leaks.Various implementations have been

developed to test bare geomembranes,geomembranes covered with shallowand deep water, and geomembranescovered with earth materials. There isno standardization of the equipmentused for the leak location testing.Instead, performance-based standardsare used. The equipment and surveyparameters are tested to determine theleak detection distance using asimulated or actual leak of a specifiedsize. Simulated leaks are electricalequivalents of a leak in ageomembrane. Leak locationmeasurements are made to determinethe maximum leak detection distancefor the specified artificial or actualleak size. The leak location surveys areconducted so that measurements aremade within that distance of everypoint on the geomembrane. Thevarious methods and the advantagesand limitations of each are presentedin ASTM D6747.7

Water Puddle Method for BareGeomembranes

For bare geomembranes, leaks aredetected by pushing a puddle of waterover the geomembrane using asqueegee or other device. A lowvoltage electrical power supply isconnected to a conductive mediaunder the geomembrane such as earthground. The other output of thepower supply is connected through anelectronic current monitoring detectorto the squeegee in contact with apuddle of water. The puddle of wateris pushed ahead of the squeegee andwhen the water puddle passes over aleak in the geomembrane, the waterflows through the leak and contactsearth ground. This completes a circuitand the resulting current is monitoredby the electronic detector. Thedetector typically converts the increasein the current to an audible toneindication and a meter reading.Figure 4 shows the operation of awater puddle system. Leaks with adiameter of 1 mm (0.04 in.) areroutinely detected.8

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Figure 4. Water puddle system usedto detect leaks in bare geomembrane.

A similar method uses a stream ofwater or water lance that is scannedover the geomembrane instead ofpushing a puddle.9

Spark Testing of ConductiveGeomembrane

A proprietary geomembrane isavailable with a conductive surfacelayer that is installed with theconductive layer downward. Thegeomembrane is tested in accordancewith ASTM D7240 using a holidaytester similar to that described for thespark testing of seams above.10 In thiscase a wide metallic brush is used tosweep the geomembrane. When a leakis encountered, a spark dischargeindicates a leak.

Leak Location with Water on theGeomembrane11

ASTM D7007 contains standardpractices for performing leak locationsurveys with water on thegeomembrane. Leaks are detected

using one of several dipole probes.One type is a 2.5 m (8 ft) probe that isused while wading in the water(Fig. 4). Another wadingimplementation uses two electrodeson two arms that are hinged together.Other probes include a towed probefor deep water and a plumb bobprobe for vertical walls. Typically the power supply provides

100 to 400 volts across thegeomembrane. The probes arescanned on the geomembrane. Anelectronic detector measures thevoltage and sometimes polarity of thevoltage on the probes. Typicaldetectors convert the signal to a meterreading and an audible tone thatincreases with leak signal amplitude.When a leak signal is detected, theprobes are maneuvered to obtain themaximum signal, which correspondsto the exact location of the leak. Withwading surveys the leaks are typicallymarked with small sand bagsconnected to a float with a string.Typically, leaks with a diameter of1.3 mm (0.05 in.) and even smaller canbe easily detected.

Leak Location with Earth Materialson Geomembrane11

Geomembranes are often coveredwith earth materials for protection,particularly for landfills. Leak locationtesting is used to detect the majordamage that can be caused by heavymachinery placing the earth materialson the geomembrane. Instead ofscanning the measurement probealong the geomembrane,measurements are made on thesurface of the earth material (Fig. 6) using two electrodes spaced a fixeddistance apart. This implementation issometimes called the dipole method.Point-by-point pote ntialmeasurements are made with thedipole probe and a portable digitaldata acquisition system along equallyspaced survey lines. The data isdownloaded to a computer forstorage, plotting and analysis. When asuspect area is indicated in the data,manual measurements are made tofurther pinpoint the leak position.Typically, leaks with a diameter of6.4 mm (0.25 in.) are routinelydetected, and much smaller leaks aredetected depending on how close thesurvey line happens to be from theleak.

Conclusion

Nondestructive testing ofgeomembrane seams is an importantpart of construction quality control ofgeomembrane installations. Severalstates are requiring geomembrane leaklocation testing for landfills and pondson new installations and periodicallywhen in service. Leak location testingis also widely applied as part ofconstruction quality assurance and tosolve leakage problems. Engineers,owners, and regulators understand theimportance of testing the onlyfunction of a geomembrane, which isto prevent leakage.

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Figure 5. Dipole probe used to detectleaks in geomembrane-liner inwater-filled tank.

Figure 6. Dipole probe used to detectdamage to geomembrane coveredwith earth materials.

References

1. Nondestructive Testing Handbook,third edition: Vol. 1, Leak Testing.Columbus, OH: American Societyfor Nondestructive Testing (1998):p 592-597.

2. ASTM D5641, Standard Practicefor Geomembrane Seam Evaluationby Vacuum Chamber. WestConshohocken, PA: AmericanSociety for Testing and Materials(2011).

3. ASTM D5820, Standard Practicefor Pressurized Air ChannelEvaluation of Dual SeamedGeomembranes. WestConshohocken, PA: AmericanSociety for Testing and Materials(2011).

4. ASTM D7177, StandardSpecification for Air ChannelEvaluation of Polyvinyl Chloride(PVC) Dual Track SeamedGeomembranes. WestConshohocken, PA: AmericanSociety for Testing and Materials(2010).

5.ASTM D4437, Standard Practice forNondestructive Testing (NDT) forDetermining the Integrity of SeamsUsed in Joining Flexible PolymericSheet Geomembranes. WestConshohocken, PA: AmericanSociety for Testing and Materials(2008).

6. ASTM D6365, Standard Practicefor the Nondestructive Testing ofGeomembrane Seams using theSpark Test. West Conshohocken,PA: American Society for Testingand Materials (2011).

7. ASTM D6747, Standard Guide forSelection of Techniques for ElectricalDetection of Potential Leak Paths inGeomembrane. WestConshohocken, PA: AmericanSociety for Testing and Materials(2004).

8. ASTM D7002, Standard Practicefor Leak Location on ExposedGeomembranes Using the WaterPuddle System. WestConshohocken, PA: AmericanSociety for Testing and Materials(2010).

9.ASTM D7703, Standard Practice forElectrical Leak Location on ExposedGeomembranes Using the WaterLance System. WestConshohocken, PA: AmericanSociety for Testing and Materials(2011).

10.ASTM D7240, Standard Practicefor Leak Location usingGeomembranes with an InsulatingLayer in Intimate Contact with aConductive Layer via ElectricalCapacitance Technique (ConductiveGeomembrane Spark Test). WestConshohocken, PA: AmericanSociety for Testing and Materials(2011).

11.ASTM D7007, Standard Practicesfor Electrical Methods for LocatingLeaks in Geomembranes Coveredwith Water or Earth Materials.West Conshohocken, PA:American Society for Testing andMaterials (2009).

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