FlowableFill-ePipe010.pdf

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ePipewww.concrete-pipe.org

An educational document from the American Concrete Pipe Association for users and specifiers

2010 American Concrete Pipe Association, all rights reserved. Resource # e-010 10/10

Flowable Fill

Althoughowablellsarenotgenerallyrequiredforconcretepipeinstallations,thispaperhas

beenpreparedtoaddressthoseconditionswhereowablellmaybenecessary.Flowablell,also

knownassoil-cementslurry,controlledlowstrengthmaterial(CLSM),soil-cementgrout,unshrinkable

ll,owablemortar,controlleddensityll(CDF),plasticsoil-cement,andK-Krete,isacompoundthat

canbeusedasembedmentorbackllmaterialinlieuofcompactedsoil.Itismadeupofsoil,cemen-

titiousmaterialandwater,andisgenerallyusedinpipelineconstruction. ASTM Standard D 4832-02,

Standard Test Method for Preparation and Testing of Controlled Low Strength Material (CLSM) Test

Cylinders,denesowablellas:

ControlledLowStrengthMaterial(CLSM)Amixtureofsoil,cementitiousmaterials,water,

andsometimesadmixtures,thathardensintoamaterialwithahigherstrengththanthesoilbut

lessthan8400kPa(1200psi).Usedasareplacementforcompactedbackll,CLSMcanbe

placedasaslurry,amortar,oracompactedmaterialandtypicallyhasstrengthsof350to700kPa(50to100psi)formostapplications.[1]

Figure1.FlowableFillMix

The Use of Flowable Fill

Thedecisiontouseowablellnotonlydependsonthesurroundingsoilcharacteristicsbut

alsothepipecharacteristics.Rigidpipecanoftenprovidethemajorityofthestrengthofthesoil-pipe

structure,andthusitisoftenmoreeconomicaltousetheexistinginsitusoilratherthanowablell.Withexiblepipe,embedmentmaterialisamajorcontributortothesoil/pipestructure.Thus,theuse

ofowablellwithexiblepipewilllikelyresultinahigherqualitynishedstructureinamoretimeef-

cientmannerinsteadofcompactedsoilasembedmentandbackllmaterial.Althoughowablellis

oftenthoughtofasthesolutiontopoorinsitusoilconditions,itisimportanttorememberthatthesoil

envelopeisacombinationoftheowablellandthesoilinthetrenchwall,thuswidertrenchesmay

berequiredforpoorsoilsthatcannotbracetheowablell.Whendeterminingtheappropriatenessof

owablellforapipeinstallation,thestrengthcharacteristicsofthenativesoilshouldrstbedeter-

mined.AsstatedinASTM D 4832,The CLSM transfers the load from the pipe to the insitu material,


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2

so the native soil must be able to provide the necessary support for the pipe.

Flowablellcanbeusedinoneoftwoways:asanembedment

materialorasabackllmaterial.Whenemployedasanembed-

mentmaterial,owablellworkseitherasagaplleroratrench

ller.Whenitisagapller,itisusedasathickloadtransfer

materialbetweenthesurroundingsoilandthepipe.Inthisway

theowablellisabletoeliminatethevoids,whichforminthe

hauncharea,whilethesoilremainsastheprimarysidesupport

forthepipe.Conversely,whenowablellisusedasatrench

lleritcompletelyreplacesthevolumeoftheembedmentand

beddingsoil,effectivelydoingawaywiththeneedtocompactthe

soilinthishardtoreachhauncharea.Additionally,theowablellactsastheprimarysidesupportfortheinstalledpipe.

Ifthenativesoilisweak,itwouldnotbeappropriatetousethe

gap-llermethod(whichsimplyactsasaloadtransfermate-

rialbetweenthepipeandthesoil).The

owablellisthemainformofsupport

forexiblepipe(ifcompactedsoilisnot

used)so,whenthesoilisweak,moremustbeexcavatedandreplacedwith

owablell.Whenusedasanembedmentmaterialtheowablellshould

risetothesameheightthatthecompactedsoilwouldrise.

Usingowablellinsteadoftraditionalsoilcompaction,allowsforfaster

installationandensuresadequatesupportforthepipe.Duetothespeedwith

whichowablellcanstabilizeaninstalledpipe,thismethodisgenerallyused

whenpipesrununderstreetsandhighways.It

isalsousedintightspaceswherecompacting

soilisdifculttoachieve.

Whenusedasabackllmaterial,owablell

canbeadvantageousbecause,providedhigh

early-strengthcementisused,excavationandpavingcanbecompletedinoneday.

Primary Advantages

Therearemanyadvantagestousingowablellratherthancompact-

edsoil.

1. Costsassociatedwiththemovingofexcavatedsoilarereduced.

2. Thetime,manpower,andequipmentneededtovibrate(ofteneven

Figure2.FlowableFillDry

Figure3.FlowableFill

Installation

Figure4.FlowableFill


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thisisnotnecessary)theowablellaremuchlessthanthatneededtoassuresufcientcom-

pactionofsoil.

3.Testingassociatedwithdeterminingthestrengthoftheowablellismoreefcientthanthe

testingassociatedwithdeterminingthestrengthofthecompactedsoil.

4.Becausetheplacementofowablellgenerallyresultsinaccurateinstallationonthersttry,

theproblemsandsubsequentcostsassociatedwithre-compactingandre-testingthesur-

roundingsoilareeliminatedwhendealingwithexiblepipeproducts.

5.Itcanbemadewithlocalsoil,whichcancontainupto20-25%non-plasticorslightlyplastic

nes.Thepresenceofthesenessuspendsthesandinthemixture,enablingeasierowand

prohibitingsegregationofthell.

6.Forinstallationssuchasexiblepipethatrequireconsiderablesoilsupport,embedmentsoils

oftenhavetobeimportedtothesite,increasingthecostandmakingowablellamoreeco-

nomicalmethod.

Design Issues

Aswithallmethodstherearealsodisadvantagestousingowablellasanembedmentmaterial.

1.Onedisadvantageisthattestingthestrengthoftheowablellisusuallydonesevendays

afterthemixisused.Sinceinstallationislikelytoalreadyhavebeencompleted,anyproblems

revealedduringtestingcouldbedifculttocorrectsincethepipehasnormallyalreadybeen

overlledbythistime.Thiscanimpactexiblepipeproductsastheydependonthestrengthof

thepipezonematerialtoprimarilycarrytheexternalloads.

2.Althoughowablelliseasiertoplace,itusuallytakesabouttwotofourhoursbeforeaninitial

setisachieved.Henceoverllingcannotbecompleteduntiltheinitialsetisaccomplished.Iftheoverllisnotplacedontheowablellforeighthoursormore,a6incoverofmoistearth

shouldcovertheslurryuntiltheoverllisapplied.

3.Flowablellisnotwaterproof.Inmostapplications,itwillleakandwickwater.Therefore,de-

signersandownersshouldnotcountonowablelltosealleakingjoints.

4.Duringconstruction,owablellshouldbeinstalledwithequalvolumesofmaterialonboth

sidesofthepipesimilartoacompactedsoilbackllproceduretopreventmovementorextra

stressestothepipe.

Figure5.LargeSteelTie-DownsforHDPEPipe


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Mix Proportions

Justlikeconcrete,aowablellmixdesigndependsonthecharacteristicsofthemixmaterials,the

requiredstrengthforinstallation,andthenecessaryowability.Thecommondesiredstrengthforow-

ablellisbetween350-700kN/m2(50-100psi)at7days.Oncethemixspecicationsareestablished,

anymethodofbatchingandmixingcanbeusedsolongastheconsistencyisacceptablebeforethe

mixisplaced.Thebestsoiltouseforowablellissiltysandwithamaximumof30%nes.Cement

contentwillbearound3-6%bydrymassofthesoil(usuallybetween1-1bagsofcementpercubic

yard).Whenareductionincementcontentisdesired,admixturescanbeaddedtotheowablell;

thisalsoimprovestheowcharacteristicsofthemixture.Flyashcanbeusedtoreducetheamount

ofcementusedinthemixaswell,howeverduetoitsstrengthvariability,testsmustbemorethorough

andfrequent.Ifthemixistobepumpedtoitslocation,Bentonitecanbeaddedtoimprovetheow

characteristicsoftheowablellthroughthedeliveryhose.Commonmixproportionsarelistedbelow.

Cement Fly Ash Soil Water

Cleanconcretesand,noyash100 0 3000600(1sackmix)

CleanConcretesand,yash 50 300 2700600(1/2sackmix)

Siltysand,noyash 50 0 3000600(1/2sackmix)

Temperature Conditions

Incoldclimatesitisespeciallyimportant,whendesigningthesoil-pipesystem,toconsidertempera-

tureeffects.Theambientairtemperatureshouldremainabove40F(4C)whenplacingowablell,

howeverifthetemperatureis35F(2C)and rising,theowablellcanstillbeplacedsafely.Thetemperatureoftheowablellitselfmustbeaminimumof50F(10C)atthetimeofplacing.Flow-

ablellsmustbekeptfromfreezinginorderforthemtoacceptablyllinallthegapsandsupport

thepipe.Coveringtheowablellwithaninsulationblanketbeforetheinitialsetandwithalayerof

moistsoilaftertheinitialsetcanpreventsuchfreezing.Whentemperaturesarebelow50F(10C)

themoistsoillayershouldbe18inthick.Additionallytheowablellshouldnotbeplacedintrenches

wherethetrenchbottomorwallsarefrozenorcontainfrozenmaterial.

Pipe Flotation

Flotationofthepipeisanotherconcernrelatedwiththeuseofowablell.Whenplacingthell,theheighttowhichtheowreachesandtheweightofthepipeallcontributeindeterminingwhetheror

notthepipewilloat.Thepotentialforpipeotationcanbecalculatedusingthedensityoftheow-

ablell(approximately130lbs/ft 3).Ifthepotentialforoatingissignicant,variousapproachescanbe

takentopreventitsoccurrence.Theowablellcouldbeplacedinliftsthatwouldcontroltheamount

ofslurryplacedinthetrench.Whentheslurryreachestheheightatwhichotationwascalculated

tooccur,theslurrywouldbestoppedandallowedtoreachaninitialset.Atthistimetheremaining

owablellcouldbeaddedwithoutpossibilityofotationduetotheadhesionbetweenthepipeand

therstlayerofowablell.Anothermethodwouldbetoplacesandbags,oranothertypeofweight,


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ontopoftheinstalledpipe.Alsothepipecould

belledwithwater,whichwouldaddadequate

weighttothepipe,prohibitingotation.Finally,

exteriorinhibitorscouldbeused;suchasplac-

ingre-barsinanXshapeoverthepipeand

tyingthemtogetherwheretheycross,orplacing

horizontalbarsontopofthepipeandwedging

thebarsagainstthesideofthetrenchwall.Itis

recommendedthatatestsectionatthebegin-

ningofthejob,beusedtoestablishthemode

bywhichotationcouldbeprevented.

Placing

Themethodofplacingthematerialisimportant

becausethestrengthandlongevityofthepipedependontheuniformityoftheller.Non-uniformity

meansthatpressurepointscouldform,reducingthelifetimeofthepipe.Onemethodofplacingthe

owablellistoraisethepipeontwosandbagssothattherearegapsunderthepipe.Thenallow

thelltoowunderthepipeuntilthellisseenonbothsidesofthepipe.Afterllinginthegaps

betweentheinsitusoilandthepipe,thellcanbevibratedasnecessary(ifthellbleedsexcessively

orwhenthemixisstiffer,8-10inslump,roddingisnecessary).Manytimesitisimpracticaltolaythe

pipeontwosandbags;inthiscasethepipecanbelaiddirectlyonthetrenchbottom.Because,in

thiscase,thereisnotenoughspaceunderthepipefortheowablelltoow,visualinspectionmust

sufceindeterminingiftheowablellisllinginallthegaps.

Conclusion

Theuseofowablellcanreduceconstructionconcernsandcostsassociatedwithpoorinsitusoils

thatwouldotherwiserequireexpensiveimportedembedmentmaterials,limitedspace,orlower

strengthpipematerials.Thebenetsineaseofconstructiondonotcomewithoutduediligenceinthe

designoftheinstallation.Whenowablellisuseditisnotonlyimportantthatthematerialismanu-

facturedandplacedappropriately,butthatappropriatedesignissuesrelatedtothequalityofthepipe

andinsitusoilmaterialaretakenintoaccountwhenestablishingthesoil-pipestructure.

References:

1. 2003.StandardTestMethodforPreparationandTestingofControlledLowStrengthMaterial(CLSM)TestCylinders.

AnnualBookofASTMStandards2003.EditorsDreyfuss,RobertH.et.al.Vol04.08,pgs939-943.

2. Howard,Amster.2002.Soil-CementSlurry(FlowableFill).PipelineInstallation.EdHoward,JillS.,RelativityPublish-

ing.LakewoodCO.pgs13-1thru13-15.

Figure6.OtayRiver60inHDPE

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