Physical Properties of GeomembraneWVT10

7/28/2019 Physical Properties of GeomembraneWVT10

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679GEOSYNTHETICS INTERNATIONAL S 1996, VOL. 3, NO. 6

Technical Paper by K.R. Reddy, S.R. Bandi, J .J. Rohr,

M. FinyandJ . Siebken

FIELD EVALUATION OF PROTECTIVECOVERS FOR LANDFILL GEOMEMBRANELINERS UNDER CONSTRUCTION LOADING

ABSTRACT: Theperformanceof landfill geomembraneliner protective cover sys-temswithandwithoutageotextileisevaluatedusingfieldtests.Thephysical propertiesof the protectivecover soils andthe geomembraneliner before andafter field testingweredeterminedusinglaboratorytests. Thehydraulic propertiesof thegeomembrane

fieldsamplesweremeasuredusingwater vapor transmission(WVT) tests,andtheme-chanical propertiesweremeasuredusingmulti-axial tensiontestsandwidestriptensiletests. A low massper unit areageotextile was demonstrated to completely protect thegeomembranein this study.

KEYWORDS: Protectivecover,Drainagelayer,Linersystems,Geomembrane, Fieldtesting, Laboratory testing, Landfill.

AUTHORS: K.R. Reddy, Assistant Professor, Department of Civil and MaterialsEngineering,Universityof IllinoisatChicago,2095EngineeringResearchFacility,842WestTaylorStreet,Chicago,I llinois60607,USA,Telephone:1/312-996-4755,Telefax:1/312-996-2426; S.R. Bandi, Principal, Great Lakes Soil and EnvironmentalConsultants Inc., 3317Washington Street, Lansing, Illinois 60438, USA, Telephone:1/708-474-8860,Telefax:1/708-474-7790;J.J .Rohr,VicePresident,RustEnvironment

& Infrastructure, 3121ButterfieldRoad, Oak Brook, Illinois60521, USA, Telephone:1/630-574-5171, Telefax: 1/630-574-2007; M. Finy, Environmental Engineer, WasteManagement Inc., Brownsville, Ohio 43721, USA, Telephone: 1/614-787-2327,Telefax: 1/614-787-2335; andJ . Siebken, National Seal Company, P.O.Box1448,1255Monmouth Blvd., Galesburg, Illinois 61401, USA, Telephone: 1/309-343-3418,Telefax: 1/309-343-1536.

PUBLICATION: Geosyntheti cs I nternati onal is published bythe Industrial FabricsAssociation International, 345 Cedar St., Suite800, St. Paul, Minnesota55101-1088,USA, Telephone: 1/612-222-2508, Telefax: 1/612-222-8215. GeosyntheticsInternational is registered under ISSN 1072-6349.

DATES: Original manuscript received 28 May 1996, revised version received 24October 1996andaccepted26October 1996. Discussionopenuntil 1September 1997.

REFERENCE: Reddy, K.R., Bandi, S.R., Rohr, J.J., Finy, M. and Siebken, J., 1996,Field Evaluation of Protective Covers for Landfill Geomembrane Liners UnderConstruction Loading, Geosyntheti cs International, Vol. 3, No. 6, pp. 679-700.


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REDDY,BANDI,ROHR,FINY&SIEBKEN D LandfillGeomembraneLinerProtectiveCovers

680 GEOSYNTHETICS INTERNATIONAL S 1996, VOL. 3, NO. 6

1 INTRODUCTION

Liner systems for wastecontainmentfacilitiesserveas barriersthat preventsubsur-facecontamination byminimizingthemigration of wasteconstituentsintothesubsur-faceenvironment. Inthecaseof landfills, thelinersystemscanvaryfromasinglecom-posite system consisting of a compacted clay layer overlain by a geomembraneto acomposite system consisting of multiple layers of compacted clay, geomembranes,geotextiles, geonets, andgeocompositesas depicted in Figure 1. Theintegrity of theliner systemduringitsinstallation, as well as throughout itsentire lifetime, is of para-mount importance. Themost damagepronecomponentsof liner systems are thegeo-synthetic layers. In the caseof composite liner systems, protecting the geomembranefromtearingorpuncturingduringconstructionandafterwasteplacementiscritical fortheentireliner systemto functionproperly. Thegeomembranesaresubjectedtoheavy

Figure1. Protectivecover systemsfor landfill linersused in theUnitedStates: (a) singlecompositeliner; (b)doublecompositeliner.

Waste

Protective/Drainage soil

Compacted clay

Subsoil

Geotextile

Geomembrane

WasteWaste

Waste

Protective/Drainage soil

Compacted clay

Subsoil

Compacted clay

Geotextile

Geomembrane

Geotextile

Geomembrane

Geonet

Drainage pipe

Drainage pipe

(a)

(b)


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loadings from construction equipment particularly during installation. As a result, aprotective cover is requiredfor placementof materialsover thegeomembraneliner to

prevent damage from construction loading, and to prevent overstressing of the geo-membraneliner due to subsequent wasteplacement.

Various types of protectivecoversarecurrently being used for landfill liners. IntheUnitedStates, thegeneral practiceistouseeither ageotextileoraprotectivecover soilor bothover thegeomembraneliner.Theuseof ageotextileover ageomembranelinerincreasesthepunctureresistanceof thegeomembraneliner andcushions loadscausedby initial wastelift placement. Theprotectivecover soil preventsdirectcontactof theconstructionequipmentwiththegeomembraneliner,anddistributesthevehicularload-ingto alargerarea,thus,reducingthestressesonthegeomembraneliner.Theprotectivecover soil generally consistsof afree-draininggranular soil that also servesasadrain-agelayer for theleachatecollectionandremoval system(Figure1).Thedegreeof geo-membrane liner protection that the protective cover offers depends on the type andproperties of the geotextile, andthe compositionandthickness of the soil layer used.

An ideal protectivecover systemfor landfill geomembraneliners should fulfill thefollowing requirements both during construction and after subsequent waste place-ment:

S Prevent damageto thegeomembraneliner from stresses induced by drainagelayerconstruction and wasteplacement.

S Prevent damageto the geomembraneliner such as tearing, bursting, puncturing orimpact.

S Possess properties that allow placementwith minimumruttingandformation of ir-regularities, since easy access across large areas during construction is critical toachievecost effective, timely construction.

S Consistof afreedrainingmaterial thatcanalsoserveasaneffectivedrainagemediumfor the landfill leachate.

S Be capable of withstanding landfill construction, operation and closure conditionswithout undergoing major changes in physical properties.

The current practice in the United States for design and construction of protectivecover systemsfor landfill geomembranelinershasseveral deficienciesas outlinedbe-low:

S Thespecifictypeof soilsthatcanbeusedasprotectivecovershasnotbeenwell docu-mented.Asaresult, thesuitabilityof locallyavailablesoilsfor useasprotectivecov-ers cannot be rationally evaluated. The protective cover soil must also serve as adrainagelayer: fromaregulatoryperspective,itshouldhaveahydraulicconductivitygreaterthan110-4m/s.Several soil typesmeetthiscriterion,however,anaddition-al criterion, in termsof physical orstrengthcharacteristics, is neededfor theevalua-tion of a soil as an effectivegeomembraneliner protectivematerial.

S A specific rationale for theselectionof thethickness of theprotectivecover soil has

not been formulated. Froma regulatory perspective, the minimumthickness of thedrainagelayer mustbe0.3m. Currently, thethicknessof protectivecover soilsrangefrom0.3mto0.9m. Theuseof smaller thicknessesmaycreatethepotential for geo-


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membraneliner damage, while theuseof larger cover soil thicknessesmay provetobeuneconomical.

S Fewquantitativestudiesinvestigatingthepossibilityof damagetogeomembranelin-ers due to constructionloadinghavebeen performed to date.

S A recommended construction procedure to avoid excessive construction-inducedstresses in the geomembraneliner currently does notexist.

A comprehensive research program on protective cover systems for landfill linerswasinitiatedin 1994attheUniversityof Illinois at Chicago(UIC). Thepurposeof thisresearchis:(i) toassessthedamagetolandfill liners,inparticulargeomembraneliners,under different protective cover conditionsboth during construction andafter subse-quentwasteplacement; and(ii) todeveloparational designmethodologyfor protectivecoversoilsthatinvolvesthedevelopmentof indicatorparametersand/ordesigncorrela-tionsthatdefinetheprotectivecover soil compositionandthicknessin order toprovideadequate geomembraneliner protection.

Thefocus of this paper is the field tests that were performed as part of this researchstudy. Thepurposeof thefieldtestswastoevaluatetheperformanceof differentprotec-tive cover systems under construction loading. This paper first summarizespreviousstudies that were conducted on protective cover systems, andthen providesdetails ofthefieldtesting procedures that werefollowed in this study to assess theperformanceof protective cover systems under constructionloading.Thelaboratorytests that wereperformed to evaluatethe constructiondamageincurred bythe protectivecover soilsandgeomembranelinersarethendescribed. Theresultsof thelaboratorytestsarethenused to determinethe effectivenessof each protective cover system.

2 BACKGROUND

IntheUnitedStates,either ageotextile, asoil layer,or botharecurrently usedasthe

protectivecover systemfor alandfill geomembraneliner system. Thethickness of thesoil layer must besuch that, in additionto preventing mechanical damageto thegeo-membraneliner,italsoallowsefficientdrainageof leachateintotheleachatecollectionpipes. Inother countries, suchasGermany, aprotectivecover isconstructedas asepa-ratelayerdirectly abovethegeomembraneliner.Thedrainagelayeristhenconstructedontop of theprotectivecover. According to German regulations, aprotectivecover isconsideredsuitableif thelocal deformationsproducedbythedrainagelayer donot in-duce a strain greater than 0.25% in the underlying geomembrane liner both duringconstruction andlandfill operations. This regulatory requirementspurred theinterestof several German investigators to develop andevaluatedifferenttypes of protectivecovers,andthus, awidevarietyof protectivecovermaterialshavebeensuggested.Fig-ure2showsa fewof thesuggestedprotectivecover materials:gravel, aconcrete-filledgeotextile, asand-filledgeotextile, agravel-filledgeotextile, orcompositelayerscom-

posed of differentfilaments andfabrics (Brummermannet al. 1994; Muller-RochholzandAsser 1994; Kirschner andKrelt 1994; Saathoff andSehrbrock 1994).A typical sequenceof landfill liner andprotectivecover constructionfollowedinthe

UnitedStatesis showninFigure3. Thelinersystemwhichconsistsof acompactedclaylayer anda geomembraneliner is first constructed. A geotextile is then installed over


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Figure3. Crosssectionofalandfill liner andprotectivecover systematdifferentstagesofconstructionanduse:(a)constructionoftheprotectivecover system;(b) placementofwasteover theprotectivecover system.

Figure2. Differentprotectivecoversfor landfill liner systemsusedin Germany.

Waste

Drainage layer

Drainage pipe

Protective coverGeomembrane

Mineral sealinglayers(minimum 3 lifts)

Subsoil

Different protective covers used:

Geotextile

Composite layers of different geotextiles

Gravel

Sand-filled geotextileGravel-filled geotextileGeosynthetic clay liner

Concrete-filled geotextile

Protective cover soil

Geotextile

Geomembrane

Drainage pipe

Compacted clay

Protective cover soil

Drainage pipe

Compacted clay

Geotextile

Geomembrane

Waste

(a)

(b)


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thegeomembraneliner. Theprotectivecover soil is initially positioned by enddump-ing,andbulldozers(dozers) areusedtospreadthesoil over theentirelandfill liner sur-

faceto thedesiredthickness.Duringconstruction,extremecaremustbetakentoprotectboth the geomembrane liner and the geotextile from mechanical damage due toconstruction equipment loading and placement of the protective cover soil. Theconstructionequipmentshouldnotbedrivendirectly onthegeotextileandtheunderly-ing geomembraneliner, anda minimumsoil cover should bemaintained at all times.

A fewstudiesonthelevel of protectionofferedbythegeotextilehavebeenreported(Koerneretal. 1986;Motanetal. 1993). Thesestudieshaveshownthattheperformanceof geomembraneliners with regard to tear strengthandpuncturestrength is increasedwhenageotextileis used;however,thestudyconductedbyMotanet al. (1993) didnotprovideconsistentresultsonthis beneficial behavior. Theeffectof introducingdiffer-enttypesof geosynthetics includinggeocompositesandgeosynthetic clay linersintheprotective cover systemwas evaluated byRichardson(1996) usingfieldexperiments.Ingeneral, improvedperformanceof thelandfill liner systemwasobservedwhenthese

geosynthetics were included in theprotective cover system.In most cases, thelevel of protectionoffered bythe differenttypes of geosyntheticsaloneisconsideredinadequatefor preventingmechanical damageto thegeomembraneliner bothduringandafter construction. Therefore, theprotectivecover soil, which isinstalledover thegeotextileor other geosynthetic component, becomesacritical com-ponentinthedesignof asafe andefficientlandfill liner system. To date, nosystematicstudy hasbeenreportedtodeterminethelevel of protection achievedbyusingvarioussoil types and soil thicknesses as protective cover soils (Ruetten et al. 1995). A widevariety of soilsarecurrently beingusedasprotectivecover soilsandthusit is essentialto evaluatetheir performance (Reddy et al. 1995).

3 FIELD TESTING PROCEDURE

Thestructural integrity of ageomembraneliner duringconstructioniscrucial for itsintended purposeof effectiveleachatecontainment. Inthis study, afield investigationwas performed to evaluatedifferent protective cover soils andto determine the mostefficient cover soil system that provides the maximum level of geomembrane linerprotection duringconstruction.

3.1 Test Variables

The field testing procedure followed in this study included preparation of the sub-gradeandplacementof thegeomembraneliner ontheprepared subgrade. Next, apro-tectivecover soil layer wasconstructedonthegeomembraneliner inthesamemannerasitwouldbedoneduringaregulardrainagelayerplacementoperation.Thisprocedureinvolved pushingthesoil withadozer andperforming dozer maneuverssuch as brak-

ing, turning and repeated travel on the protective cover soil. Several tests were per-formed usingdifferent cover soils, differentdozers, and by either using or not usingageotextilebetweenthesoil andthegeomembraneliner. Thefollowingconditionsweretested:

1. mediumgravel, no geotextile, CAT D4 (light dozer);


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2. mediumgravel, geotextile, CAT D7 (heavy dozer);

3. mediumgravel, no geotextile, CAT D7;

4. finegravel, no geotextile, CAT D4;

5. finegravel, no geotextile, CAT D7;

6. finegravel, geotextile, CAT D4; and

7. finegravel, geotextile, CAT D7.

A uniform0.3mthick soil layer andsmooth1.5mmthick(60mil) highdensitypoly-ethylene(HDPE) geomembranewereusedinall of theabovetests.Thegeotextileusedwasanonwovenneedle-punchedpolypropylenegeotextilewithamassper unit areaof270g/m2. Thetwodifferenttypesof dozersusedforthis studywerea light dozer (CATD4 model) and a heavy dozer (CAT D7 model). Thespecifications of both dozersaresummarizedin Table 1.

3.2 Field TestingProcedure

Thefield testing was performed at Suburban South RecyclingandDisposal Facilityin Brownsville, Ohio, USA. A level test areawasidentified,andthesubgradewaspre-paredfor thetestpads. Thelevel areawaslargeenoughtoperformtestsonfourdifferentpads concurrently. Thesubgrade, which consisted of siltyclay withastiff toverystiffconsistency, was inspected for any protrusionsthat might damage thegeomembraneduringthetests. A 9.1mwidegeomembraneroll was placed along oneendof thetestpad and unrolled. A 6.1mlong piece was cut to forma 6.1m9.1 mgeomembranetestpad. Thegeomembranewas then visually examined for damages incurredduringtransportation and storage.

Sixstakesweredrivenintothegroundtoprovidecontrol depthsforthegranularmate-rial (protectivecover soil): four stakes wereplacedat thefour corners, andtwostakeswereplaced at the mid-point of the 9.1 m edges. A pieceof flagtape was tied to the

stakesat aheightof 0.3mfromthegroundsurfacetoassistthedozer operator withtheplacementof the 0.3mgranular soil layer. Figure4 showsthegeomembranetest padimmediately before placement of thegranular soil. Granular drainagesoil was piledalongthe6.1medgeof thetest pad. Whensufficient material wasplacedfor thedozertooperate, theoperatorpushedthesoil ontothegeomembraneto formarelatively uni-form0.3mthicklayeroveraportionof thegeomembrane. Additional granularsoil wasthen placed along theedge, andtheoperation was repeated until the entire geomem-branewascoveredwiththegranularsoil.Theoperatorthenusedthedozerbladetoleveland adjust the soil thickness to 0.3 m. Thephotographs in Figure5 show thegranulardrainagesoil being pushed onto thegeomembrane.

Table1. Specificationsofthedozersused in thisstudy.

Dozer properties Dozer D4 Dozer D7

Total mass (kg) 7,726 27,125Width of tracks (m) 0.6 0.9

Length of tracks (m) 2.0 3.2

Surfaceareaof tracksin contactwithsoil (m2) 2.6 5.8

Averagecontactpressurebetweentracksandsoil (kPa) 29 46


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Figure5. Photographsoftheplacementoftheprotectivecover soil onthegeomembrane:(a)soil placementonthegeomembrane; (b)dozer traversingover soil onthegeomembrane.

Figure4. Photographoffield testpadwiththegeomembrane.

(a)

(b)


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Thedozerthentraversedthegranular soil layerinboththeforwardandreversedirec-tions, proceedingfivetimesineachdirection(Figure6). This actionwasperformedto

simulatenormal dozer traffic onthegranular soil layer duringactual construction. Thedozerwas thenturned 180_ onthegranular soil layer bymaneuveringback andforth.This action also simulates normal dozer operations during placement of materials. Fi-nally, the dozer traversed back and forth onthe test pad fivemore times to completethe testing. At the end of eachmotion, break forces were applied to simulate brakingoperationsduring placementof protective cover materials. A few samplinglocationsonthetest padwherethedozer traversed themost wereselected. Thefirst fewinchesof thegranular material wereremovedat theselocationsusingabackhoewith arubberlinedbucket edge. A sideswipemotionof thebackhoewasusedtoremovethegravel.Thefinal few centimeters of thegranular material werecarefully removedwith asho-vel. Anapproximateareaof 0.9m0.9mwasclearedusingthismethod.Oncethegeo-membranesurfacewasexposed,itwascleanedusingaclothandvisuallyexaminedfordamage. A 0.6m0.6msamplewascutandlabelled.Theundersideof thesamplewas

examinedfordamagethatoccurreddueto protrusionsatthesubgradesurface. Samplesof thegranular material werecollectedbefore andafter testing. All geomembraneandsoil samples werecarefully transported to the laboratory for testing.

3.3 DamageAssessmentTests

3.3.1 Soil Testing

Samples of mediumandfinegravel were obtained before andafter field testing toevaluatetheirparticlesizedistributionsandhydraulicconductivityvalues.Theparticlesizedistributionsof theseprotectivecoversoilsareshowninFigure7,andthehydraulicconductivityvaluesweredeterminedtobe1.310-2m/sand6.610-3m/sfor theme-diumandfinegravels,respectively.Thedifferencesintheparticlesizedistributionsandthehydraulic conductivity valuesfor thesoilsbeforeandafter field testingwereinsig-

Figure6. Photographofadozer traversingthetestpad.


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Figure7. Particlesizedistributionoftheprotectivecover soilsusedin thisstudy.

nificant. Theseresultsshow that theprotectivecover soils did not undergo significantchanges due to construction operations.

3.3.2 Geomembrane Testing

In order to quantify thechange ingeomembraneproperties due to the placementof

theprotective cover soils andconstruction loading, several laboratorytests werecon-ductedonthegeomembranesamplestakenafterfieldtesting. Thesetestsincludedwa-ter vapor transmission (WVT) tests, multi-axial tension tests, and wide strip tensiletests. Theproceduresfollowedfor thesetestsarebrieflydescribedin thefollowingsec-tions.

Water Vapor Transmission Tests. TheWVT test was selected toprovideinformationon the changes in the permeability of the geomembranedue to damage. A WVT testcandetectminutecracksinthegeomembranethat maynot bevisible tothenakedeye.Theoretically, adamagedgeomembraneshouldhaveahighervapor transmissioncapa-bility than a geomembranethat is not damaged.

TheWVT testswereperformedinaccordancewithASTM E 96(ProcedureB). Three108mmdiametercircularspecimenswerecutfromeachof theexhumedgeomembrane

samplesandfromavirgin geomembranesample. Thethicknessof eachgeomembranespecimenwasrecordedastheaverageof fivethicknessmeasurements. Usingwax, thegeomembranespecimenswereattachedto aluminumcupsthat containeddistilledwa-ter. Thewax sealed the junction between the geomembranespecimen andthe cup. Aflangewasfittedoverthegeomembranespecimenandtightenedagainstthecuptohold


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thegeomembranesecurelyinplace. This apparatusleftageomembraneareaof 0.0071m2 throughwhich thewater contained in thecup could migrate. In addition, acontrol

specimen that consisted of an aluminumdisc sealedto an aluminumcup in the samemanner asthegeomembranespecimenswasused. Thecupswerestoredunder aPlexi-glasshoodinstandardlaboratoryconditions. Airwascontinuallycirculatedthroughthehoodwithasufficientvelocitytomaintain uniformconditionsat all test locations. Thetemperatureandrelativehumidity wererecordedandchangedlessthan0.6_C and2%,respectively, for all of the tests. Periodically, the dishes were removed fromthe hoodandweighedonascalereadableto0.0001g.Thepositionsof thespecimensinthehoodwere rotated at each weighing. Theambient temperature andrelative humidity wererecorded when thespecimens were weighed.

Water vapor transmission is defined as the slopeof the regression line ona weightversustime plot divided bythe exposed area of the test specimens. When the plotsofweight versus timewere constructed, adjustmentsweremadeusingthe control speci-mensto compensatefor variabilityinthetestconditionsandwater vaporthatmayhave

escaped through theseal. Compensationswere madeby subtractingtheinitial weightof thecontrol specimenfromtheweightof thecontrol specimenateachweighinginter-val andthensubtractingthisvaluefromtheweightof eachgeomembranespecimenforagiveninterval. Plotsof themodifiedweightversustimedataweremadefor eachgeo-membranespecimen, andalinewasfit tothedatausingtheleast-squaresmethod.Theslopeof eachlinewascalculatedanddividedbytheexposedareaof thegeomembranetestspecimensto give thewater vapor transmission rate.

Geomembranepermeanceis defined as thewater vapor transmissiondivided bythevapor pressuredifferential acrossthegeomembraneliner, whichisthedrivingforceofthe water vapor transmission, andis calculated usingthe following equation:

(1)Permeance= WVTS(R1 R2)

where: WVT=watervaportransmission(g/day-m2);S=saturationvapor pressure(Pa);R1=relativehumidityinsidethecup;andR2=relativehumidity outsidethecup.Thegeomembranepermeancewas calculated for this testusingtheaveragerecordedtem-peratures andrelative humiditieswhich were 23.1_C and35.9%, respectively. Thus,the following values wereused: S=64,329 Pa, R1=1.0 (100%) andR2=0.359.

Permeabilityis definedin ASTM E 96asthepermeancemultipliedbythegeomem-braneliner thickness. Thisdefinitionof permeability is apropertyof thegeomembraneandwould beunchanged by the geomembranethickness; however, the terms perme-abilityandpermeanceareoftenusedinterchangeably.By convention, thepermeabilityof the geomembrane is reported in units of cm/s-Pa bydividing the permeanceof thegeomembranebythedensityof water(1g/cm3). Thispermeability isactuallytheper-meance of the geomembraneandwill decreasewith increasing thicknessof thegeo-membrane.

Multi-Axial Tension Tests. Themulti-axial tensiontestwas selected becausethetestresults do not significantly depend onthe orientationof nicks, cutsor indentations inthegeomembrane. ThistestwasperformedinaccordancewithASTM D 5617. Thetestuses a 0.3mor 0.6mdiameter specimen attached to a pressurevessel. Thespecimen


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undergoesout-of-planedeformationwhenthechamberpressureisincreased. Thepres-sureis increaseduntil thespecimenis ruptured, which is associated withasuddenloss

in pressure. The deformation versus pressure data is used to calculate the stress andstrainexperiencedbythegeomembranespecimen.Thestress-strainvaluesobtainedareusedtoevaluatetheperformanceof geomembranelinersduringplacementof thegran-ulardrainagelayer.A detaileddescriptionof thetestprocedureanddatareductionpro-cedureis given in ASTM D 5617.

Wi de Str ip Tensi le Tests. Thewidestrip tensiletest wasselectedbecauseit is simpleandwidely used. This test was performedin accordancetoASTM D 4885andwasse-lectedtoprovideacomparisonof thetensilepropertiesof geomembranesamplestakenfromthefieldaftertestingwiththoseof virgingeomembranesamplesinorder toassessthedamageto thegeomembranesamplesunder differentprotective cover conditions.By comparingthewidestriptensiletestresultswiththemulti-axial testresults,thesuit-ability of eachtestingmethodfor geomembranedamageassessmentcanbeevaluated.

A detaileddescriptionof thewidestriptensiletestprocedureanddatareductionproce-dureis given in ASTM D 4885. It should be noted that the test standardrecommendsexpressingthetensilestrengthin forceper unit width; however, for thepurposeof thispaper it is expressed as stress, i.e. forceper unit cross sectional area.

4 RESULTSAND DISCUSSION

AsmentionedinSection3.3.1,thesoilsdidnotundergo significantchangesinphysi-cal andhydraulic properties. Theresultsof themulti-axial tension testsandwidestriptensile tests performed on the geomembrane specimens are summarized in Table 2.Thesetest results areused to assess thedamageto thegeomembraneliner. Theresultsof the WVT testsarealso discussed below.

4.1 Water Vapor TransmissionTests

A total of 24WVT testswereperformed, includingthree tests onvirgin specimens.Figure8showstheaveragepermeancevaluesof geomembranespecimensfordifferentprotectivecover conditions.Theseresultsshowthatthedifferencesbetweenthespeci-mensaretoo small tobedetected bythis test. Theapparentvariation inthetest resultsis due to the fact that the WVT ratewas so low (on average eachspecimen only lostapproximately 4 mg of water over the duration of thetest) that even small errors inweight measurements aresignificant. Theoverall averagepermeanceof theexhumedgeomembranespecimensis 6.3410-15 cm/s-Pawhichagreed well with the averagepermeanceof 7.7010-15 cm/s-Pafor thevirgin geomembranespecimens. Thesetestresultsshowthatthepermeabilityof thegeomembranedid notchangeduetothediffer-entprotectivecover conditionsused in this study.


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LandfillGeomembraneLinerProtectiveCoversREDDY,BANDI,ROHR,FINY&SIEBKEN D

691GEOSYNTHETICS INTERNATIONAL S1996, VOL. 3,NO. 6

Table2.

Sum

maryofmulti-axialandwidestriptensiletestresultsforthegeomembranefields

pecimens.

Multi-axialtensio

ntesting

Widestriptensiletesting

Testco

nditionfor

1.5mmthicksmooth

HDPEg

eomembrane

Elongationat

burst

(%)

Tensilestressat

burst

(MPa)

Yieldstrain

(%)

Yieldstress

(M

Pa)

Strainatbreak

(%)

Stressatbreak

(MPa)

Average

Change

(%)

Av

erage

Change

(%)

Average

Change

(%)

Average

Change

(%)

Average

Change

(%)

Average

Change

(%)

Virgingeomembrane

25.33

N/A

15.14

N/A

19.5

N/A

14.33

N/A

1210.8

N/A

21.32

N/A

Afterfieldtestwithmediumgravel,

nogeotextileandlightdozer

19.1

--24.6

1

6.2

7.0

19.6

0.5

14.66

2.3

452.4

--62.6

11.69

--

45.2


nogeotextile,heavydozer

15.16

--46.7

1

6.9

11.6

19.6

0.5

14.41

0.5

464.4

--61.6

12.19

--

42.8


geotextile,lightdozer

26.9

6.2

14.62

--3.5

21.0

7.7

14.31

--0.2

1048.2

--13.4

19.06

--

10.6

Afterfieldtest

withfinegravel,no


20.13

--20.5

15.75

4.0

18.8

--3.6

13.96

--2.6

578.9

--52.2

13.47

--

36.8

Afterfieldtest

withfinegravel,no

geotextile,heavydozer

18.25

--28.0

15.69

3.6

21.6

10.8

14.35

0.1

505.2

--58.3

12.48

--

41.4

Afterfieldtestwithfinegravel,


30.12

18.9

14.55

--3.9

21.1

8.2

14.07

--1.8

1141.3

--5.7

20.17

--5.4

Afterfieldtestwithfinegravel,

geotextile,heavydozer

29.2

15.3

14.68

--3.1

19.1

--2.1

14.84

3.6

1026.7

--15.2

15.76

--

26.1

Note:

N/A=no

tapplicable.


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Figure8. Averagegeomembranepermeancebasedonwater vapor transmissiontests.

4.2 Multi-Axial TensionTests

A total of 24 multi-axial tension tests were conducted on geomembranespecimensobtained fromthe field including three tests on virgin geomembranespecimens. Out

of these, 15testswereperformedusing0.3mdiameter specimensand9testswerecon-ductedusing0.6mdiameter specimens. Thetest resultsfor the0.3and0.6mdiameterspecimensfromthesamegeomembranefield sample, andavirgin geomembranesam-plearecomparedinFigures9aand9b. Figure9aisaplotof geomembranetensilestressat burst while Figure9b is a plot of geomembraneelongationat burst. This dataindi-cates that the results obtained using 0.3 and 0.6 m diameter specimens are in closeagreement.

Figure10ais aplot of theaveragegeomembranespecimentensilestressat burst forall of theprotectivecoverconditionsusedinthisstudy. Theaveragegeomembraneten-sile stress at burst for the virgin geomembrane specimen is also plotted in Figure10afor comparison purposes. Figure 10a shows that the average geomembrane tensilestressesforthefieldspecimensareapproximatelythesameasthosevaluesforthevirgingeomembranespecimens. Thegeomembranespecimensfromfield samplesthat were

testedwithoutageotextileasprotectionfailedatslightly highertensilestressesthanthevirgingeomembranespecimens,whereasthegeomembranespecimensfromfieldsam-ples that were tested with a geotextile as protection failed at slightly lower tensilestresses than thevirgin geomembranespecimen. Theseslight differences in geomem-


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Figure9. Multi-axial tensiontestresultsshowingtheeffectofgeomembranespecimensizeon: (a)tensilestressatburst; (b)elongationatburst.

Note: Finegravel was used in thesefield tests.

(a)

(b)

branetensilestress at burst may beattributed to themorebrittle or stiffer conditionofthe geomembranein theabsenceof a geotextile.

Figure 10b shows a comparison of the averagegeomembranespecimen elongationat burst for all of the protective cover conditionsused in this study. Theaveragegeo-membraneelongation at burstfor thevirgin geomembranespecimensisalsogivenfor


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that of thevirgin geomembranespecimens. Theseresults clearly reflect theprotectionprovidedbyusingageotextileintheprotectivecover system. Thegeomembranespeci-

men elongation at burst is the lowest at 15.2% for the case of medium gravel and aheavier dozer, andthehighest at 32.1% for thecaseof finegravel, alighter dozer anda 270 g/m2 geotextile. Althoughno visible physical damageoccurred, the protectivecover systemconsistingof amediumgravel layer constructeddirectlyonthegeomem-braneusingaheavy dozercausedsignificantchangesinthemulti-axial tensileproper-ties of thegeomembraneliner.

4.3 WideStripTensileTests

A total of 21widestriptensiletestswereconductedongeomembrane specimensob-tained fromthe field. In addition, three testswereconducted onvirgin geomembranespecimens. Based on the stress-strain data, the following values were calculated: (i)yield stress; (ii) yield strain; (iii) 10%secant modulus; (iv) stressat break; (v) strain at

break; and, (vi) load/widthat break. Theaveragevalues of thesedifferentparametersareplottedin Figures11through16, respectively, for thevariousprotectivecovercon-ditions.Thetestresultsfor thevirgingeomembranespecimensarealsoshownontheseplots for comparison purposes.

Figure11isaplot of theyield stresses for thegeomembranespecimenstestedunderdifferentprotectivecoverconditions. Figure11showsthattheyieldstressforgeomem-branespecimensunder differentprotectivecover conditionsisapproximately equal tothe yield stressfor the virgin geomembranespecimens. Thesame observation is alsomadefortheyieldstrainandthe10%secantmodulusplotsinFigures12and13,respec-

Figure11. Averagegeomembranespecimenyieldstressbasedonwidestrip tensiletests.


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Figure12. Average geomembranespecimen percent strain at yield based on widestriptensiletests.

Figure 13. Average geomembranespecimen 10% secant modulus based on wide striptensiletests.


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tively. This result indicates that thegeomembranespecimenyield stress andstrain donot change for differentprotectivecover systems tested in this study.

Figures 14 and 15present thevalues of the averagegeomembranespecimen stressandstrain at break, respectively, obtained from wide strip tensile tests performed ongeomembranefieldspecimensunderdifferentprotectivecoverconditions. Theaveragestressandstrainatbreak for thevirgin geomembranespecimensarealsoshowninFig-ures14and15. FromFigure14, it is observedthat all of thegeomembranefield speci-mens experienced lessstressat break as compared to the virgin geomembranespeci-mens. Thedifferenceinthestressatbreakbetweenthevirgin geomembranespecimensandthegeomembranefield specimenswaslessfor geomembranefieldspecimensthathad aprotectivecover with ageotextileas compared to thegeomembranefield speci-mensthathadaprotectivecover withoutageotextile. Themaximumdifferenceingeo-membrane stress occurs when the protective cover consists of medium gravel ascompared to fine gravel. The use of either a light or heavy dozer during constructionappears to haveno significant influence onthe geomembranestressat break. Similar

trends alsooccur for both geomembranestrain at break andgeomembraneload/widthat break (Figures 15and 16, respectively). The widestrip tensile test results demon-stratethat damagecaused to the geomembraneliners is not reflected in the geomem-braneyield stresses andstrains; however, thedamagepotential is reflectedinthegeo-membraneultimateor break stressesand strains.

5 SUMMARY AND CONCLUSIONS

Thisresearchinvestigatedtheperformanceof different protectivecoversfor landfillgeomembrane liners under construction loading conditions. Both medium and finegravels wereused as the protective cover soils and were tested with and without thepresenceof ageotextileover thegeomembraneliner. Field testingwasperformed at alandfill siteusing bothlightandheavy dozers. After fieldtesting, geomembranesam-

pleswereexhumedandvisually inspected.Noneof thegeomembranesampleshadap-parentdamageintheformof tearsor holes; however, all of thegeomembranesamplesthat were tested without a geotextile had surficial scratches anddents.

Laboratorytests wereperformed to determinethephysical propertiesof theprotec-tive cover soils and the geomembrane liner bothbeforeandafter field testing. Thesetestresultsareused to quantify thedamagethat occurredduringconstruction loading.Theresultsof theparticlesizedistributionandhydraulic conductivity testsonthepro-tectivecover soil samples revealedthat nosignificantchanges in theproperties of thesoils took placeunder constructionloadingconditions.

Thephysical propertiesof thegeomembranesamplesweredeterminedby water va-por transmissiontests, multi-axial tension testsandwidestrip tensiletests. Theresultsof the water vapor transmission tests show that the geomembrane liner permeabilityvaluesdo notchangeby beingin contactwithany of theprotectivecoversused in this

investigation. Thegeomembranespecimen stressat burstvalues determined fromthemulti-axial tests, andthegeomembraneyieldstressandstrainvalues determinedfromthewidestriptensile testsalsodonotchangewiththedifferentprotectivecover condi-tionsused. However,thegeomembraneelongation atburstdeterminedfromthemulti-axial tensiontestsandthegeomembraneultimateor break stressandstraindetermined


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Figure 14. Average geomembranespecimen ultimate stress based on wide strip tensiletests.

Figure15. Average geomembranespecimen ultimate strain based on wide strip tensiletests.


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Figure16. Averagegeomembranespecimenload/widthatburstbasedonwidestriptensiletests.

from the wide strip tensile tests indicate that the geomembrane samples underwentphysical changesduetotheconstructionloading. Eventhoughthesephysical propertyvalues (geomembrane elongation at burst and ultimate stress and strain) decreasedwithouttheuseof ageotextile, suchchangesmaynotadverselyaffecttheperformanceof thegeomembrane. However, thepresenceof ageotextileas lightas 270g/m2 inthe

protectivecover systemcompletelyprotectsthegeomembranefromconstructionload-ing, aswasdeterminedbyfieldtesting. Thisstudyrecommendsthatageotextilebecon-sidered for use inthegeomembraneliner protective cover systemwhen gravel and/orheavy construction equipment areused.

ACKNOWLEDGEMENTS

Thefinancial supportforthisprojectwasreceivedfromWMX TechnologiesInc. andis gratefully acknowledged.Theassistanceof personnel at SouthSuburban RDF, RustEnvironment& Infrastructure, andNational Seal Companyisalsogreatly appreciated.

REFERENCES

ASTM D 4885, Standard Test M ethod for Determini ng Perfor mance Strength of Geo-membranesby theWide Stri p Tensil eM ethod, AmericanSocietyforTestingandMa-terials, WestConshohocken, Pennsylvania, USA.


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ASTM D 5617, Test Method for Multi-Axial Tension Test for Geosynthetics, Ameri-can Society for Testing andMaterials, West Conshohocken, Pennsylvania, USA.

ASTM E 96, Standard Test M ethods for Water Vapor Transmi ssion of Mater i als - Pro-cedureB - Water M ethod at 23_C,AmericanSocietyforTestingandMaterials,WestConshohocken, Pennsylvania, USA.

Brummermann, K., Blumel, W. andSloewahse, C., 1994, ProtectionLayersfor Geo-membranes:EffectivenessandTestingProcedure, Proceedings of the Fi fth Interna-ti onal Conference on Geotextil es, Geomembranesand Relat ed Products, Vol. 1, Sin-gapore, September 1994, pp. 1003-1006.

Kirschner,R. andK relt, V., 1994, Innovative, ProtectiveMattressesfor Landfill Geo-membranes, Proceedingsof the Fi fth International Conferenceon Geotextil es, Geo-membranes and Related Products, Vol. 1, Singapore, September 1994, pp.1015-1018.

Koerner, R.M., Monteleone, M.J ., Schmidt, R.K. and Roethe, A.T., 1986, Puncture

and Impact Resistance of Geosynthetics, Proceedings of the Third InternationalConference on Geotextil es, Vol. 3, Vienna, Austria, April 1986, pp. 677-682.

Motan, E.S., Reed, L.S. andLundell, C.M., 1993, GeomembraneProtection by Non-wovenGeotextiles,Proceedings of Geosyntheti cs 93,IFAI, Vol. 2, Vancouver,Brit-ish Columbia, Canada, March1993, pp. 887-900.

Muller-Rochholz, J. andAsser,J.D., 1994,SandfilledGeosyntheticsfortheProtectionof Landfill Liners, Proceedings of the Fifth International Conference on Geotex-ti les, Geomembranes and Related Pr oducts, Vol. 1, Singapore, September 1994, pp.1011-1014.

Reddy, K.R., Kolloju, P., Ruetten, M. andBandi, S., 1995, Characterizationof Protec-tiveCover Soils for Landfill Geomembrane Liners, Proceedings of the Third Geo-technical and Geoenvir onmental Conference, Cleveland, Ohio, USA, May1995, pp.117-143.

Richardson,G.N., 1996,FieldEvaluationof Geosynthetic ProtectionCushions,Geo-technical Fabri cs Report, IFAI, Vol. 14, No. 2, pp. 20-25.

Ruetten,M., Bandi, S. andReddy, K.R., 1995, Rational DesignApproachforLandfillProtectiveSoil Cover, Proceedings of the Eighteenth Internati onal M adison WasteConference, Madison, Wisconsin, USA, September 1995, pp. 302-308.

Saathoff, F. andSehrbrock, U., 1994, Indicatorsfor Selectionof ProtectionLayersforGeomembranes, Proceedings of the Fi fth International Conference on Geotextil es,Geomembranes and Related Products, Vol. 1, Singapore, September 1994, pp.1019-1022.


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Discussions and Closure

FIELD EVALUATION OF PROTECTIVE COVERSFOR LANDFILL GEOMEMBRANE LINERS UNDERCONSTRUCTION LOADING

TECHNICAL NOTE UNDER DISCUSSION: Reddy, K.R., Bandi, S.R., Rohr, J.J.,Finy, M. and Siebken, J., 1996, Field Evaluation of Protective Covers for Landfill

Geomembrane Liners Under Construction Loading, Geosynthetics International, Vol.3, No. 6, pp. 679-700.

DISCUSSERS: R.S. Thiel, Thiel Engineering, PO Box 1010, Oregon House,California 95962, USA, Telephone: 1/916-692-9114, Telefax: 1/916-692-9115.

K. Badu-Tweneboah, Senior Project Engineer, J.P. Giroud, Senior Principal, and D.S.Carlson, Senior Laboratory Technician, GeoSyntec Consultants, 621 N.W. 53rd Street,

Suite 650, Boca Raton, Florida 33487, USA, Telephone: 1/561-995-0900, Telefax:1/561-995-0925, and G.R. Schmertmann, Senior Project Engineer, GeoSyntecConsultants, 1100 Lake Hearn Drive, N.E., Suite 200, Atlanta, Georgia 30342, USA,

Telephone: 1/404-705-9500, Telefax: 1/404-705-9400, E-mail:[email protected], [email protected], [email protected], and

[email protected], respectively.

PUBLICATION: Geosynthetics International is published by the Industrial Fabrics

Association International, 1801 County Road B West, Roseville, Minnesota55113-4061, USA, Telephone: 1/612-222-2508, Telefax: 1/612-631-9334.

Geosynthetics International is registered under ISSN 1072-6349.

REFERENCES OF DISCUSSIONS AND CLOSURE: R.S., Thiel, 1997,

Discussion of Field Evaluation of Protective Covers for Landfill GeomembraneLiners Under Construction Loading by Reddy, K.R., Bandi, S.R., Rohr, J.J., Finy, M.and Siebken, J., Geosynthetics International, Vol. 4, No. 5, p. 543.

Badu-Tweneboah, K., Giroud, J.P., Carlson, D.S. and Schmertmann, G.R., 1997,

Discussion of Field Evaluation of Protective Covers for Landfill GeomembraneLiners Under Construction Loading by Reddy, K.R., Bandi, S.R., Rohr, J.J., Finy, M.and Siebken, J., Geosynthetics International, Vol. 4, No. 5, pp. 543-544.

Reddy, K.R., Bandi, S.R., Rohr, J.J., Finy, M. and Siebken, J., 1997, Closure ofDiscussions of Field Evaluation of Protective Covers for Landfill Geomembrane

Liners Under Construction Loading, Geosynthetics International, Vol. 4, No. 5, pp.

545-546.


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DISCUSSIONS AND CLOSURE D Landfill Geomembrane Liner Protective Covers


Discussion by R.S. Thiel

The discusser enjoyed the authors paper because the discusser has also performed

such field loading programs. However, one important description was missing: the an-gularity of the rock used for the medium and fine gravels in the test program. Certainly,

there will be a difference in the damage caused to an underlying geomembrane depend-ing on whether or not the gravel is angular or round. The discusser believes that the pa-

per would be a much more valuable reference if the angularity of the gravels used in thetest program were classified as being angular, subangular, subround, round, or flat.

Discussion by K. Badu-Tweneboah, J.P. Giroud,

D.S.Carlson and G.R. Schmertmann

The authors should be commended for presenting an interesting paper that providesvaluable information on the effect of granular soil layers on geomembrane performan-

ce. Granular soil layers, when subjected to loading from construction equipment or oth-er sources, can apply high concentrated stresses to an underlying geomembrane at the

locations where soil particles are in contact with the geomembrane. These high concen-trated stresses can damage the geomembrane, thereby affecting its performance in twoways: (i) puncturing it, thereby causing an immediate decrease in its effectiveness as a

low-permeability barrier; and (ii) weakening it, thereby making it more vulnerable torupture under long-term service conditions. In their study, the authors used water vapor

transmission (WVT) tests, multi-axial tension (hydrostatic) tests, and wide-strip tensiletests to evaluate the effects of several granular soils under a range of construction load-

ings on a 1.5 mm thick high density polyethylene (HDPE) geomembrane.The authors conducted the WVT tests to detect changes in the permeability of the

geomembrane resulting from punctures caused by exposure to the different soils andloadings. The WVT tests indicated that there were no detectable changes in permeabili-ty. This is an interesting finding because it suggests that HDPE geomembranes can con-

tinue to perform as effective low-permeability barriers after sustaining light to moder-ate mechanical damage. The WVT tests were not, however, useful for differentiating

between the degree of damage caused by the different soils and loadings due to the lack

of detectable changes in permeability. Accordingly, the rest of this discussion is devoted

to mechanical tests, although it is recognized that the WVT test may be useful for soils

and loadings more likely to puncture the geomembrane. The discussers note that themulti-axial tension test is also useful in detecting punctures because the hydrostatic

pressure in the multi-axial tension test cannot be sustained with a punctured geomem-brane specimen.

The authors used the stress-strain data from two mechanical tests, the multi-axial ten-

sion test and the wide-strip tensile test, to evaluate the relative degree of damage caused

by the different soils and loadings. However, the authors did not use a criterion to deter-mine, based on the test results, whether the level of mechanical damage undergone by atested geomembrane specimen is acceptable. The discussers have experience in the useof the multi-axial tension test for evaluating mechanically damaged geomembranes and


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have developed a simple criterion to determine if a certain type or level of mechanical

damage is acceptable.

To determine if a certain type or level of mechanical damage is acceptable, oneshould evaluate the degree to which the geomembrane has been weakened and mademore vulnerable to rupture under long-term service conditions. The ability to resist rup-

ture depends on the deformability of the geomembrane, which is reflected in themagni-tude of strain at yield and at rupture. A decrease in the geomembrane deformability re-

sults in a potential decrease in its ability to resist rupture. Based on their experience ininterpreting multi-axial tension tests on mechanically damaged geomembranes, thediscussers have noted that a reliable indicator of significant decrease in the deformabil-

ity of an HDPE geomembrane is a change in the geomembrane rupture mode from thatobserved in undamaged (virgin) specimens. A virgin geomembrane with a uniform

thickness must rupture at the topof the dome formed by the hydrostatically inflated geo-membrane because this is the location where the tensile stresses are highest. If a geo-membrane specimen ruptures at a location other than the top of the dome, this indicates

that the specimen has been previously weakened at that location, for example as a resultof localized mechanical damage. Indeed, the discussers have observed that: (i) HDPE

geomembranes with severe mechanical damage always rupture at a location other thanthe top of the dome (except, of course, in cases where the location of severe geomem-

brane damage happens to be at the center of the specimen); and (ii) HDPE geomem-branes with light mechanical damage have a mode of rupture in the multi-axial tensiontest that is similar to that of a virgin geomembrane. The discussers have also observed

that the most significant decreases in the deformability of an HDPE geomembrane oc-cur when geomembrane specimens rupture at locations other than the top of the dome.

From the foregoing analysis, the discussers have developed the following criterion:the mechanical damage undergone by a geomembrane is deemed unacceptable if thegeomembrane specimen tested in a large diameter (e.g. d 0.5 m) multi-axial tension

test exhibits rupture at a location other than the top of thedome and if this location is thatof visible mechanical damage. (The requirement for a large diameter is to ensure that

the specimen contains enough locations of mechanical damage to be representative.)The discussers have used the approach and the criterion described above on several oc-

casions and they found that the proposed criterion makes it possible to clearly and ob-jectively determine whether a certain type or level of mechanical damage is acceptable.Details on the development of the above criterion and an example from several studies

to evaluate the effect of a granular soil layer on the performance of an HDPE geomem-

brane are presented in a paper by Badu-Tweneboah et al. (1998).

REFERENCE

Badu-Tweneboah, K., Giroud, J.P., Carlson, D.S. and Schmertmann, G.R., 1998, Eval-

uation of the Effectiveness of HDPE Geomembrane Liner Protection, Proceedings

of the Sixth International Conference on Geosynthetics, IFAI, Atlanta, Georgia, USA,March 1998 (to be published).


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Closure by K.R. Reddy, S.R. Bandi, J.J. Rohr, M. Finy and

J. SiebkenThe authors of the paper are thankful to the first discusser for expressing interest and

facilitating discussion on the paper. The authors of the paper agree with the observationthat there will be a difference in the damage caused to an underlying geomembrane de-

pending on the shape of the soil particles. Both soils used in the field study, medium andfine gravel, are characterized as poorly graded soils with mean particle sizes of approxi-

mately 27 and 14 mm, respectively. The shape of the particles in both soils can be de-scribed as subrounded. Further laboratory investigation into the protective performance

of various cover soils consisting of particles of different sizes and shapes under simu-lated long-term load conditions has been recently completed at the University of Illinoisat Chicago and will be reported in a future publication. This recent study showed that

particle size can significantly affect the performance of the cover soil in protecting theunderlying HDPE geomembrane liner.

The authors of the paper are also thankful to the second set of discussers for express-

ing interest in the paper. The authors of the paper agree with the discussers assertion

that an extensively damaged geomembrane when tested in a large diameter multi-axial

tension test should fail away from the center and in the area of visual mechanical da-mage. For such situations, the elongation at burst is expected to be significantly lower-

ed. All of these observations are pertinent and should be included in any set of damageassessment criteria. The forthcoming paper by the discussers should be interesting.

The authors of the paper agree that water vapor transmission (WVT) tests did not re-sult in a meaningful differentiation of damage to the geomembranes caused by differentsoils and loadings. However, a more sensitive test may have detected the difference in

WVT induced by damage to the geomembrane. Koerner and Allen (1997) suggestedthat the standard test method ASTM F 1249 (Standard Test Method for Water Vapor

Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sen-sor) is a more appropriate method for determining WVT for geomembranes.

Research performed by the authors of the paper showed that the failure of the ex-humed geomembrane specimens in multi-axial tension tests usually initiated in the up-per portion of the dome; however, the modes of failure differed. Even virgin geomem-

brane specimens had different failure modes, and a summary of the observed failuremodes for the virgin and exhumed geomembranes is shown in Table 1. It should be

noted that a failure location at an edge indicates that either the specimen size is not ade-quate or there is a problem with the gripping mechanism. As seen in Table 1, none of thetests performed, even with 0.3 m diameter specimens, resulted in edge failures.

Since the modes of failure did not necessarily indicate if physical changes in the geo-membrane had taken place, the degree of protection offered by the cover systems was

assessed by analyzing differences in the stress and elongation at burst values of the ex-

humed specimens as compared to values of virgin geomembrane specimens. In addi-tion, the ultimate or break stress and strain values determined from wide-strip tensiletests were used to describe the degree of protection offered by the cover systems. Thedevelopment of criteria based on the stress and elongation at burst values from multi-

axial tension tests and/or the ultimate stress and strain values from wide-strip tensile


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tests would be helpful in determining if a protective cover system is acceptable or unac-

ceptable. However, further research is needed in order to determine these criteria based

on the functionality of the geomembrane (e.g. hydraulic and chemical containment)and the site specific conditions (e.g. subgrade, liner design, construction loads, wasteloads). The most common practice has been to usethe research results to select a protec-

tive cover system that offers protection such that the properties of the exhumed geo-membrane are relatively comparable to the virgin geomembrane.

REFERENCES

ASTMD 5617, Standard Test Method for Multi-Axial Tension Test for Geosynthetics,

American Society for Testing and Materials, West Conshohocken, Pennsylvania,

USA.

ASTM F 1249, Standard Test Method for Water Vapor Transmission Rate Through

Plastic Film and Sheeting Using a Modulated Infrared Sensor, American Societyfor Testing and Materials, West Conshohocken, Pennsylvania, USA.

Koerner, G. and Allen, S., 1997, The Water Vapor Transmission Testing Controversy,

Geotechnical Fabrics Report, IFAI, Vol. 15, No. 2, pp. 8-12.

Table 1. Failure modes observed during multi-axial tension testing of the exhumed and

virgin 1.5 mm thick HDPE geomembranes.

Failure modeCover

soilGeotextile Loading 0.3 m diameter

specimens

0.6 m diameter

specimens

Failure

location

None Heavy dozer H-CAT, XD-T N-EF

MediumNone Light dozer H-CAT, TD-T N-EFgrave

270 g/m2 Light dozer H-CAT, H-CAT N-EF

None Heavy dozer H-CAT, HOLE XD-T, H-CAT N-EF

Fine None Light dozer HOLE HOLE, HOLE N-EF

gravel 270 g/m2 Heavy dozer H-CAT, H-CAT MD-T, HOLE N-EF

270 g/m2 Light dozer H-CAT, H-CAT H-CAT, H-CAT N-EF

Virgin geomembrane HOLE, H-CAT H-CAT N-EF

Notes: Alltestsperformed according toASTM D 5617. N-EF = non-edgefailure; MD-T= machinedirection

tear; TD-T = transversedirection tear; XD-T = multi-directional tear; H-CAT = hole incat eye;HOLE= circular

or elliptical hole.

Documents

Physical Properties of GeomembraneWVT10