IntroductionTheMcArthurRiveruraniummineislocatedin theAthabascasandstonere-gioninthenorthernpartoftheprovinceof Saskatchewan,Canada.It is theworld'slargest,high-gradeuraniumde-positwithprovenandprobablereservesofmorethan473millionpoundsU30S.It ismajorityownedandoperatedbytheCamecoCorporation.

Thispaperpresentsacasestudyoftheengineeringthatwascarriedoutinorderto artificiallyfreezetheunder-groundorebodyattheminepriortoitbeingmined.Fulldetailsof theproject

AthabascaSandstone

.~Highpressurewater

~

SandandGravel

--

Case Study: Thermal Analysis of ArtificialGround Freezing at the McArthur RiverUranium Mine

G.P..Newman

havebeenpreviouslydescribedbyNewmanandMaishman(2000).Theprimaryfocusof thepaperistheappli-cationofanadvancedthermalanalysistoolthatgreatlysimplifiesthefiniteele-mentmodelingrequiredduringthede-signand.monitoringstagesof projectssuchas thatcarriedoutatMcArthurRiver.

BackgroundAccordingtoNewmanandMaishman(2000),theorebodyitselfislocated550metersto 620 metersundergroundwheretheground-waterpressureisap-

proximately5500kPa.Duetothepres-enceof ahangingwallfaultstructure,theorebodyis surroundedon threesidesby fairlydry,competentground.Theotherthreesidesarecomprisedofhighlyfracturedsandstonewithsignifi-cantamountsof rubble,flowingsandandclayregions.In ordertominetheore,itwasnecessarytocreateafrozenwallbarrieraroundthethreepoorsidesoftheorebody.Thefrozenwallbarrierwasdesignedtopermitdrainageofwa-terin theoreandconsequentlyreducewaterpressurespriorto mining.Thewallwasalsorequiredtoprovidestruc-

tural supportofweak,clay/ oreground near tomining cavities.FigureI showsacross-sectionoftheorebodyandneighboringgeol-ogy.

A mechanicalfreezingsystemiscomprisedof abrinecoolinganddistributionnet-workplusaseriesof brine freezepipesinstalledinthegroundto befrozen. Typicalgroundfreezingapplicationshaveinvolveddrillingfreezeholesfromsurfaceor near

.........SouthFreezeRow

DryBasementRock

640mLevefDeveloDment

60 GeotechnicalNews, June2003

Figure1.Cross-sectionoftheorebodyandneighboringgeology(afterNewmanandMaishman,2000)

~-.....

/1

Low Pressure Side

Brine

72 FreezePipes

~reeze Plant-Conceptualfreezing system

High Pressure Side

Brine

530m L.evel

Figure2. Illustrationof thehighandlowpressurebrinedistributionnetworks(afterNewmanandMaishman,2000)

surfaceandtheseactivitieshavebeenwelldocumented.Inaddition,thebrinecoolinganddistributionnetworkhastypicallyincludedanammoniacom-pressorwith ammoniato brineheatexchangers.Theprocessof installinga"typical"freezingsystematMcArthurRiverwasmademoredifficultduetothelocationof thefreezepipechamberunderground.

Thefreezingchamberislocated530metersbelowground,whichmeansthatthebrinepressureswithinthefreezepipesandassociatedbrinedistributionnetworkwouldequal5000kPaif con-necteddirectlytothesurfacerefrigera-tionplant.Thisis notpracticalfromadesignoroperationsperspective.In or-derto minimizethebrinefluidpres-sures, the underground brinedistributionsystemwasisolatedfromthesurfacebrinesystemusingshellandtubebrine-brineheatexchangers.

TheillustrationinFigure2showstherelativepositionofthe800Tonrefriger-ationcapacityfreezeplantonsurface,the12"ID brinesupplyandreturnlinesinstalledin theshaft,andoneof fourshellandtubeheatexchangersonthe530m level.The low-pressurebrinenetworkon the530m leveloperateswithina 150kPato600kPapressurerangeatflowratesrangingbetween130

m3/hrand550m3/hr.Thedesignbrinetemperaturewas-40degreesCelsius.

In orderto determinetheactualgrowthof thefreezewallitwasneces-saryto installthermocouplestringsatseverallocationsaroundthefreezingre-gion.Thethermocoupleswereloweredintoacasedholecontainingafinegroutmix priorto thegroutsetting.Eachstringwascomprisedoftwelvesensorslocatedatfive-meterintervalsdownanygiventemperaturemonitoringhole.Thisenabledthetemperaturedecaytobe monitoredoffsetfromthefreezepipesin varioustypesof ground(seeFigure I for comparisonof groundtypes).Groundtemperatureswerere-cordedeverysecondday.In thiscasestudy,theactualgroundtemperaturescanbeusedtoverifythenewthermalanalysis numerical tool. This isdescribedbelow.

Thermal Modelingand AnalysisTheTEMP/w two-dimensionalfiniteelement computer program(GEO-SLOPE,2002)hasbeenusedex-tensivelyby manypracticinggroundfreezingconsultantsfordesignofartifi-cialgroundfreezingprojects.Inthepastit wascommonto applythethermalboundaryconditionforafreezepipebyassuminga fixedtemperaturedecayfunctionforearlystagesoffreezingfol-

GEOSPEC

lowedby anassumptionthatthepipesurfaceis ascold,ornearlyascoldasthebrinefortheremainderofthefreez-ingperiod.

Recently,TEMP/w hasbeenmodi-fiedsothattheuseris abletoapplyaconvectiveheattransferanalysisforthepipe-groundinterface(oranyothersur-faceconvectiveheattransferprocess).Theamountof convectiveheattrans-ferredbetweenthe groundandthechilledbrineisdependentonthegroundtemperaturerelativeto thebrinetem-peratureanditsubsequentlydetermineswhatthenewgroundtemperaturewillbe.Basedonthisapproach,thegroundwillcoolatavariablerateandtoamini-mumvaluethatis determinedby thebrineflowparametersandthediffer-ence betweenbrine and groundtemperature.

Convectiveheattransferis com-prisedof two mechanisms:energytransferduetorandommolecularmo-tion(diffusion)andenergytransferduetothemotionofafluid(advection/con-vection).Inthiscasestudyanalysis,weareconcernedwith convectionheattransferbetweenfluid in motionin apipeandthepipewallwhentheyareatdifferenttemperatures.Technically,theconvectiveheattransferisoccurringbe-tweentheinternalpipewall andthefluidbutitisacceptabletocombinetheconductiveheattransferacrossthesteelpipewallwiththeconvectivecompo-nenttoarriveatacombinedconvectiveheattransfercoefficient.

Regardlessof thenatureof thecon-vectiveheattransferprocess,theappro-priaterateequationis

q=h(Ts- Tf)whereqistheunitheatflux (W 1m2);h isthecombinedconvectiveheattransfercoefficient(W/m20C);Tsis thepipe'sexternalsurfacetemperature(0C);andTf is the fluid temperature.In theTEMP/w program,theusermustinputtheoverallheattransfercoefficientaswellasafixedtemperatureortimede-pendenttemperaturefunctionfor thefluid.Thephysicalsizeof thepipecanalsobespecifiedasanoptionif theac-tualpipegeometryisomittedfromthefiniteelementmesh.

ThedatainTableI summarizesthethermalpropertiesusedin theoriginal

GeotechnicalNews. June2003 61

GEOSPEC

Temperature Comparison for Different Ground Types

Measured at 2m offset from freeze row In high grade ore/clay- Predicted Measured at 3.7m offset from freeze row in low grade sandy/clay- PredictedJj. Measured at 3.5m offset from freeze row in barren sandstone

4 6 8 10 12

Time (Months)

Figure 3. Comparison of computedand predicted ground temperaturesfor various rock types(measureddata after Newmanand Maishman, 2000)

thermalmodelling (NewmanandMaishman,2000)andin thecurrentanalysis.

It shouldbenotedherethatnoad-

Theseresultsclearlyshowthatthenewconvectiveheattransferboundarycon-ditionoptioncanbeappliedtoartificialgroundfreezingif oneknowsthebrine

justmentsto thefreezepipeboundaryconditiontemperaturesweremadeinthemodelwhencalibratingthethermalconductivityof theclay/ oreground.Theactualfreezepipetemperaturewascomputedasaresultof theconvectiveheatbeingremovedbythebrine.

Figure3 is a comparisonof com-putedandmeasuredgroundtempera-turesfor threeof themainrocktypesandinitialin-situgroundtemperatures.

62 GeotechnicalNews, June 2003

fluidtemperatureandfreezepipediam-eter.Theoverallconvectiveheattrans-fercoefficientcanreadilybecomputedbasedonthebrineflowrateandotherhydraulicflowproperties.

Conclusions

Artificialgroundfreezingof thehighgradeMcArthurRiveruraniumorede-positwascarriedoutunderextremeconditions.Duetoadvancesinthermal

finiteelementanalysis,it isnowpossi-bletoaccuratelydetermineheatlossesfromthesoilintothemechanicalfreez-ingsystemwithouthavingtomakeanassumptionabouttheappropriatetem-peratureboundaryconditiontouseintheanalysis.

ReferencesGeo-Slope,2002.TEMP/w Version5

UsersManual,Geo-SlopeInterna-tionalLtd.,Calgary,Canada.

Newman,GregandDerekMaishman.ArtificialGroundFreezingof theMcArthurRiverUraniumOreDe-posit.Proceedings:InternationalConferenceonGroundFreezingandFrostActioninSoils.Belgium.Sep-tember,2000.

G.P.Newman,P. Eng.,GEO-SLOPEInternationalLtd.,1400,633- 6th.AvenueS\V,Calgary,AlbertaT2P 2Y5,Tel:403-269-2002,Fax: 403-266-4851,http://www.geo-slope.com

25.0

20.0

15.0-0-; 10.0...::s1U 5.0...Q)CoE 0.0Q)I-

-5.0

-10.0

-15.0

0 2

Table1.ThermalPropertiesUsed in Analyses

Conductivity(W/ m0c) BulkVolumetricWaterContent(%)

SilicifiedSandstone 5.2 10

Clav/ Ore 1.2 50

BasementOuartzite 5.6 2

IL

BiTechispleasedtopublisht~:nint: annuallistingof ]PhD Thesesin engineering.- --- - - - --AnkaraUniversityDepartmentof GeologicalEngineering,06100,Tandogan/ Ankara-Turkey

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