GCLDesignSeries Part 3

Designers Forum

GCL design seriesPart 3: GCL installationand durability

By Greg Richardson, Richard Thiel,and Richard Erickson

This is the final part of a three-part GFRseries summarizing key aspects of the in-dustrys first comprehensive GCL designguidance document (the GundSeal GCLDesign Manual, Thiel et al. 2001). Part I,published in the June/July issue of GFR,presented an overview of GCL hydraulicperformance and leakage issues. Part II,published in the August issue of GFR, fo-cused on slope stability of reinforced andunreinforced GCLs. Finally, Part III willexpand on GCL installation, durability,and effective construction quality control(CQC) and construction quality assurance(CQA) to observe and verify the installa-tion meets the design intent.

Material installationand construction moni-toring guidelinesProper material installation and coveringprocedures are essential to meet the designintent for effective environmental con-tainment and long-term performance.Good standard industry guidance regard-ing material handling, subgrade prepara-tion, panel deployment, alignment (Photos1), overlapping and seaming can be foundin ASTM D 5888, Standard Practice for Stor-

age and Handling of Geosynthetic Clay Liners,and ASTM D 6102, Standard Guide for In-stallation of Geosynthetic Clay Liners. Addi-tionally, guidelines are available from GCLmanufacturers regarding GCL handling andinstallation. This article attempts to high-light some of the everyday construction is-sues that may not be explicitly covered inthese standard industry references.

Engineering assumptions regardingGCL performance hinge on the geosyn-thetics integrity being maintained throughthe construction process. Geosyntheticsare manufactured with adequate durabil-ity to survive construction and service load-ings, provided that contractors andinstallers follow recommended procedures.Designers, installers, contractors andoperators must be aware that there are lim-its to the level of abuse that geosyntheticscan tolerate.

A few of the very practical issuesfaced in the field are highlighted in thisarticle, including: material handling (or, Can we useforklifts?); subgrade preparation (or, How big of arock can we leave under the liner?); permissible vehicle size for traffic overthe GCL (or, Can we drive on it?);

weather constraints (or, Is it OK after ithas been rained on?); seaming bentonite (or, Do we have touse it?); and soil covering (or, Can we use scrapersand only 150 mm of cover?).

Caveat: the subject of this article is by itsvery nature non-definitive and subject tomany site-specific conditions, and to theartful techniques of variously talented in-stallers. The opinions in the article arethose of the authors and are subject tohealthy doses of criticism.

Material handling (Canwe use forklifts?)The basic guiding principle for loading,unloading, and moving material aroundthe jobsite is to follow the manufacturersrecommendations. Generally, manufac-turers will not condone moving and stack-ing rolls using the forks on a forklift. If aninstaller says We do it all the time, thenthe designers response should be some-thing like Its not in the specifications,and if you want to do it that way, you needto submit a change order and written ap-proval from the manufacturer. Thatshould put an end to the discussion.

Material unloading at the jobsite shouldbe considered an important element ofCQA. Oftentimes the geosynthetics willarrive well in advance of the trained in-staller, and either the owner or the generalcontractor will unload the materials. Nei-ther of these two parties is necessarilytrained, equipped or sensitive to the nu-ances of unloading geosynthetics, particu-larly GCLs. Usually they will resort to aforklift. The results are often that the outerfive wraps of the GCL become speared withholes, and the inner cores of the rolls arebroken in two places because their weighthas been cantilevered over the ends of twoforks, making subsequent use of stinger-barsand axle-bars more difficult. This does nothappen just with neophyte contractors. Thesecond author recently performed CQAwith the most experienced landfill generalcontractor in his region, who has been con-structing landfills in conjunction withG

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Photo 1: Installations of a GCL over a protective geotextilecushion.

rocks are embedded in a soil matrix inthe subgrade, these rocks will tend to rollover and displace during final paneladjustments. Even though some of thesemay not technically be a problem for theGCL (because of its self-sealing ability),they can be a problem for the overlyinggeomembrane which will end up withundesirable, conspicuous bumps in thefinished product. A good technique fordifficult subgrade conditions is to waterthe subgrade approximately one hourin advance of GCL deployment, and thensmooth-drum roll the area just beforeGCL deployment. The exact timing ofwatering, roll ing and deploymentdepends on the site-specific soils andweather conditions.

Proper CQA procedures, in accor-dance with ASTM D 6102, should in-clude a final visual inspection of the sub-grade surface to identify unacceptablesurface protrusions (typically larger than12 mm), excessive rutting (typicallygreater than 25 mm), abrupt vertical dis-placement differences, or other areas thatmay damage the GCL during or afterinstallation. These areas should be elim-inated by removing protruding objects,smooth-drum compaction, or the place-ment of a protective soil/geotextilecushion layer prior to installation of theGCL (Photo 1).

All of that being said, the bentonite layerof GCLs generally has the swelling capac-ity to seal small punctures (generally

less than 25 mm) in-duced from above orbelow the liner andperform in accor-dance with theGiroud empiricalleakage equation(Giroud et al. 1997;and Giroud andBadu-Tweneboah1992) as describedin Part I of this GCLdesign series. Evenso, walking aroundand feeling lumpyrocks covered byGCLs and geomem-branes never inspiresconfidence, and theASTM recommen-

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geosynthetics installations for over a dozenyears. The owner of the landfill (a majornational firm) asked this general contrac-tor to unload the material in advance ofthe installer arriving on site, with the re-sults exactly described above.

The key to success in this case is to beprepared with the proper equipment be-fore the delivery trucks arrive on site.

Subgrade preparation(How big of a rockcan we leave underthe liner?)The relative thinness of GCLs requiresthat more attention be given to subgradepreparation than would be given duringthe construction of a compacted clayliner. In general, the subgrade prepara-tion specifications that are used forgeomembrane installations are adequatefor GCLs. Owners switching from tradi-tional compacted-clay-based liners toGCLs should be aware that more atten-tion must be given to subgrade prepara-tion than previous construction projects.Typical construction techniques for thefinal preparation of the subgrade surfaceinclude watering plus vibratory orsmooth-drum rolling of the subgrade soilsurface; final grade control is typicallyobtained with a smooth blade, rubber-tired grader, or its equivalent.

Loose rocks can also be a problem.Even though it may appear that loose

dations should be pursued as a general goal.

Permissible vehicle sizefor traffic over theGCL (Can we drive onit?)Installers and contractors invariably ask ifthey can drive on various geosynthetics.In response to this issue, there are no ab-solute answers, but general guidelinesshould be followed with the aim of pre-serving the integrity of the geosynthetics.For example, geosynthetics are not man-ufactured and designed to be driven over,and yet under carefully controlled cir-cumstances they could be driven over byfully loaded scrapers without causing anydamage. Would we say that is ever allowedin the specifications? Certainly not! Andyet some leeway could be given.

The authors have found that the bestcompromise is to specify that the onlyequipment allowed on geosynthetics arethose pieces of equipment specifically ap-proved in writing by the manufacturers,such as all-terrain vehicles (ATVs) (Photo2) unless field demonstrations convincethe engineer that other types of equipmentwill work. This requires that the installersubmit a written proposal and then exe-cute a field demonstration. If any condi-tions change from the initial demonstra-tion, additional demonstrations may beneeded. The situation to avoid is an in-staller coming to the field with an out-of-spec method saying, This is the way wealways do it

The largest variables that affect the useof equipment directly on GCLs are themoisture content of the bentonite and thetype of GCL. The drier the GCL, the lessit will be influenced by equipment wheelloads. Reinforced GCLs will fare much bet-ter with equipment traffic than unrein-forced GCLs. It is common that ATVs andsix-wheel gators are allowed directly ondry fabric-encased GCLs. Although thisequipment is sometimes allowed ongeomembrane-supported GCLs, it is bet-ter to have a 0.5- to 0.75-mm slip sheetbetween the vehicle and the bentonitewith these materials if the bentonite sideis facing up. Larger equipment, such as aloader, is also allowed on fabric-encasedGCLs, but only if the GCL is dry, there is

Photo 2: Deployment of a geomembrane over aGCL with an all-terrain vehicle.

a good subgrade, and turns are very gen-tle. Some types of rubber-cleated trackequipment have also been used.

Even with reinforced GCLs, if the ben-tonite becomes moist to the extent that itbehaves like a plastic putty, virtually noequipment should be driven on it. Thebentonite will become displaced, and thereinforcement may be damaged in thewheel paths.

Weather constraints(Is it OK after it has

been rained on?)Upon approval of the GCL materialthrough the manufacturers certifications,conformance testing, and on-site inspec-tion, its installation is very quick andstraightforward. One of the most criticalCQC/QA items during installation is toensure that the bentonite does not becomeoverly hydrated (mainly from precipita-tion). There are two reasons for this: (1)increased potential for damage to the GCLproduct during installation if the bentoniteis moist and soft (described above), and(2) slope stability issues for encapsulated

bentonite designs. One partial fallacy is that the GCL

is ruined if it becomes hydrated from rainbefore being covered. Another partial fal-lacy is that a GCL is just fine if it becomeshydrated from rain. The real answer is:it depends

Sodium bentonite is typically consid-ered hydrated when the moisture con-tent exceeds 50%. At a moisture con-tent greater than 50%, localizedbentonite thinning may occur due totransient construction pressures wherethere is inadequate soil cover above theGCL. Therefore, if the hydrated ben-tonite can be allowed to dry to a pointthat allows careful covering of the GCLwith soil materials without creating un-even bearing stresses that cause it toshear, then it should be fine. If con-struction must proceed in a way that hy-drated bentonite will experience smallbearing capacity failures evidenced bybentonite thinning (e.g., such as a foot-print in the mud), then it probablyshould be removed and replaced.

A typical installation specification mayinclude a maximum installed bentonitemoisture content of 30% to ensure min-imal bentonite prehydration. A localizedhydrated area of the GCL bentonite layermay be rectified by allowing that area tosun/air dry prior to covering, increasingthe overlap distance for a hydrated areaadjacent to roll edges, or supplementingthe hydrated area with additional granu-lar bentonite.

In general it is in everyones interest toget the GCL covered with no hydrationfrom rain. This is accomplished by: covering the rolls of GCL with protec-tive tarps as soon as they arrive on site; requiring the GCL that is deployed becovered the same day with geomembraneor soil; and constructing a project from the high endto the low end.

Seaming bentonite(Do we have to useit?)This issue is still not resolved even today(although who knows, maybe it will be bythe time this article is printed). Here arethe authors opinions: Geomembrane-supported GCLs: no

Photo 4: Soil cover placement over a GCL.GFR

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Photo 3: Final manual alignment of a deployed GCL.

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seaming bentonite is required because thebentonite is virtually exposed. Even thevery light spunbonded geotextile (spiderweb) sometimes adhered to the surface ofthe bentonite does not inhibit the sealingability at seams. Geotextile supported GCLs:(1) Woven-to-woven geotextile interface:no seaming bentonite is required.(2) Woven-to-nonwoven geotextile in-terface: more research required. At thispoint, when in doubt, leave it in! The costand extra work is truly minimal, so itshould be required in this case.(3) Nonwoven-to-nonwoven geotextileinterface: seaming bentonite required.

Soil covering (Can weuse scrapers and only150 mm of cover?)All GCL installations must include a soilcover to eliminate bentonite free swellingand maintain the hydraulic performanceof the bentonite. If the cover soil is gran-ular drainage material, it should be free ofsharp or angular objects greater than 12mm that may cause damage to the liner.The cover should include a minimum 300-mm-thick soil layer placed in a timelymanner after deployment of the GCL.Photo 4 illustrates typical soil cover place-ment over a GCL.

In addition to physical protection of theinstalled GCL liner, the objective of soilcover placement is to prevent the bentonitefrom hydrating with no confining normalload. The three crucial durability issues re-lated to covering a GCL with soil are: covering in a timely manner; covering in a careful manner; and covering with an adequate soil thick-ness. These factors, discussed below, shouldbe considered, and installation proceduresshould be closely followed so that the fun-damental design performance of the GCLis not affected.

Soil covering in a timely mannerThe objective of covering a GCL in a

timely manner is to prevent the ben-tonite from hydrating with no confiningpressure. With an encapsulated bentonitebetween geomembranes, however, the issueis significantly reduced, if not eliminatedaltogether, as long as the bentonite is cov-ered with the overlying geomembrane by

the end of each working day.How one defines timely manner de-

pends upon the moisture conditions of thesubgrade and the type of GCL. Construc-tion quality assurance (CQA) specifica-tions should set maximum allowable ex-posure times before soil covering, and everyinstance of exceeding these exposure timesshould be verified by field inspection.

For example, for nonreinforced GCLs(worst-case situation) the following generalrecommendations address the three basicmoisture characteristics of agronomic soils,the intent being to cover the GCL beforebentonite hydration and migration due toconstruction loads would cause concern: If the subgrade is relatively dry (ap-proaching the wilting point moisturecontent that makes vegetation growth dif-ficult), the GCL should be covered withinfive days. If the subgrade is damp to moist (ap-proaching the field capacity moisturecontent that allows lush vegetation), theGCL should be covered within three days. If the subgrade is moist to wet (ap-proaching saturation), it is advisable tocover the GCL by the following morning.

For reinforced GCLs, all of these timeframes can be exceeded. Installers, engi-neers, and CQA personnel are advised toobserve the condition of the bentonite anddetermine at what point it becomes mal-leable (greater than about 4050% mois-ture content) and have it covered beforethat point.

Soil covering in a careful mannerThe high level of performance demon-

strated by composite liners with GCLs as-sumes that certain size defects in thegeomembrane would be rendered benignby the underlying bentonite from the GCL.Very large defects through a GCL, how-ever, might be beyond a GCLs sealing abil-ity. Spinning wheels or tracks on con-struction equipment, for example, couldrip a large gap in a geosynthetics-only lin-ing system. Therefore, industry-acceptedconstruction installation and monitoringpractices should be followed (such as thoseestablished by ASTM D 6102, or the man-ufacturers recommendations) to preventthese types of defects from occurring.

After a minimum 300 mm of soil is inplace over the liner system, the potentialfor further construction- or operations-in-

duced damage becomes remote. To ensurethe liner systems integrity, therefore, it iscrucial that the placement and spreadingof the cover soil layer over the GCL com-posite-liner geomembrane be properly ex-ecuted. To eliminate the possibility of largethrough-liner defects for the project, twoinstallation and monitoring practicesshould be followed: Develop appropriate construction spec-ifications that alert the installer, generalcontractor, and owner to the specific ac-tions and activities that should be takenand avoided. Provide for a high level of CQC andCQA during liner deployment and soilcover operations. Typically, this involveshaving a ground person directly monitorthe cover soil placement operations.

Thicker soil covering for high traffic areasand roads

As discussed, hydrated bentonite maymigrate and thin in response to differen-tial stresses, depending on the magnitudeof confinement and the degree of differ-ential stress. With the encapsulated-ben-tonite design (geomembrane-bentonite-geomembrane) and isolation of thebentonite from potential prehydration, thisis of minimal concern.

Areas above the liner system that expe-rience heavy construction loads should berequired to have adequate soil cover to pro-tect the liner system beneath the wheelpaths. At least 300 mm of cover soil is gen-erally adequate for track equipment. Forheavier traffic areas and haul routes, a min-imum soil cover of 600 mm to 900 mmshould be required, depending on the in-tensity of usage and the size of the equip-ment. The extra material on the haul routescan subsequently be spread out with a dozerat the end of the construction project.

CQC and CQAGeneral CQA Considerations for GCLs

As stated by Koerner and Daniel (1993),Far fewer things can go wrong with theinstallation of a GCL compared to place-ment and compaction of a compacted clayliner. GCLs are not only much easier tomonitor during construction, but also pro-vide a higher level of visible and quantifi-able quality assurance in verifying the in-stalled manufactured product meets the

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design intent.Insuring quality in the constructed project

is often divided into two categories: materialverification and construction monitoring.

Material verificationProjects commonly require that a man-

ufactured material be verified to meet theproject specifications before it is accepted.There are three levels of verification thatcan be applied, depending on the level ofconfidence that the owner requires to con-firm a product meets a given specification. Certification letter: A letter from thesupplier stating that the material meets specific performance standards is often ac-cepted by the owner or purchaser. Review of the manufacturers test results: An owner or purchaser may wish to re-quire that the supplier submit the resultsof the manufacturers quality control(MQC) testing conducted for the rolls sup-plied for the project. Independent conformance testing: Sam-ples of the material may be taken by theowner or purchaser and subjected to vari-ous quality assurance (QA) tests. Thesetests are referred to as conformance tests,used to verify that the material conforms toproject and material specifications.

Typical index tests performed as part ofMQC for GCLs are found in ASTM D5889. When quality assurance confor-mance testing is performed, it often in-cludes some of these index tests, usuallyless frequent than the manufacturers qual-ity assurance testing for the GCL. In ad-dition to the typical QC/QA index testsand testing frequency performed by man-ufacturers, designers should consider con-ducting performance tests to verify thatproject-specific requirements are met forcritical projects. For example, direct sheartests are sometimes performed with mate-rials produced for a particular project toverify that the actual materials meet min-imum design expectations.

Monitoring during installationThe following monitoring activities are

typically performed to achieve a qualityGCL installation: proper material storage and handling bentonite moisture content proper subgrade preparation

specified overlapped seam distance adequate seaming bentonite, if required absence of bentonite prehydration proper repairs and patches absence of dislodged bentonite absence of debris in the overlapped seams minimal material wrinkles and fish-mouths proper attachment to structures proper material anchorage at slopes adequate bentonite protection andtimely covering only permitted equipment directlyon geosynthetics

CQA observation of soil cover operationsGiven that most l ining system

damage occurs during soil cover activi-ties, the CQA party is often required toprovide a ground person to observe soilcover operations. The primary functionof CQA monitoring is to verify thatthe soil spreading operations are per-formed in accordance with the specifi-cations and do not cause damage to thelining system.

Contractor observation of soil cover operationsIn the absence of CQA monitoring

during soil cover operations, the con-tractor performing soil covering shouldprovide a ground person in front of thespreading activities at all times. Theprimary responsibilities of the groundperson are: to establish and maintain adequate gradecontrol of the cover soil layer; to manually reduce or flatten wrinklesin the installed liner in advance ofsoil spreading; to identify and caution against any po-tential damage to the lining system.

Post-installation electrical defect detection surveyFor GCL-geomembrane composite

liner installations with welded geomem-brane seams, a simple but highly effec-tive electric defect-detection survey canbe performed. Electric defect detection isperformed after the soil cover is placedabove the lining system to locate po-tential installation and/or soil cover dam-age. This technology is not applicable tosingle-composite overlapped GM-GCL

applications because the overlaps inter-rupt the required continuous electricalinsulator provided by the geomembrane.For designers interested in pursuing thistechnology for sensitive projects thatinclude a geomembrane with weldedseams, additional information on elec-tric defect detection surveys is providedby Thiel et al. (2001).

Summary The authors hope that this final segmentof the three-part series on GCL design guid-ance alerts potential users of GCLs to thecritical need not to ignore manufacturersand ASTM guidelines for handling and in-stallation of GCLs. It is also hoped that wehave brought to light some other subtlepoints from our experience that may not beas explicitly covered by standard industryliterature for the benefits of GCL installa-tions. All engineers designing with and spec-ifying GCLs are encouraged to spend timein the field and visit GCL manufacturingfacilities to develop an appropriate appre-ciation of the benefits and limitations ofthese very useful materials.

References

ASTM D 5888. Storage and Handling

of Geosynthetic Clay Liners.

ASTM D 5889. Quality Control of

Geosynthetic Clay Liners.

ASTM D 6102. Guide for Installation

of Geosynthetic Clay Liners.

Giroud, J.P., and K. Badu-

Tweneboah. 1992. Rate of leakage

through a composite liner due to

geomembrane defects. Geotextiles

and Geomembranes, vol. 11, no.

1:128.

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

and K.L. Soderman. 1997. Com-

parison of leachate flow through

compacted clay liners and geosyn-

thetic clay liners in landfill liner

systems. Geosynthetics Interna-

tional, vol. 4, nos. 3 and 4:

391431.

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Koerner, R.M., and D.E. Daniel. 1993.

Technical equivalency assessment

of GCLs to CCLs. Proceedings of

the 7th GRI Seminar: Geosynthetic

Liner Systems: Innovations, Con-

cerns, and Designs. Geosynthetic

Research Institute, Drexel Uni-

versity, Folsom, Pa. 255275.

Thiel, R., D.E. Daniel, R.B. Erickson,

E. Kavazanjian, Jr., and J.P.

Giroud. 2001. The GSE GundSeal

GCL Design

Manual. GSE Lining Technology Inc.,

Houston, Texas.

Richard Thiel is president of ThielEngineering.

Richard B. Erickson is sales manager, GCLsfor GSE Lining Technology Inc.

Gregory Richardson is principal of GNRichardson and Assoc.

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GCLDesignSeries Part 3