© Alan Newman © Alan Newman PERSONALITY HEALTH and WELLBEING.
Newman 2003
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Introduction The McArthur River uranium mine is located in the Athabasca sandstone re- gion in the northern part of the province of Saskatchewan, Canada. It is the world's largest, high-grade uranium de- posit with proven and probable reserves of more than 473 million pounds U30S. It is majority owned and operated by the Cameco Corporation. This paper presents a case study of the engineering that was carried out in order to artificially freeze the under- ground ore body at the mine prior to it being mined. Full details of the project Athabasca Sandstone .~ High pressure water ~ Sand and Gravel -- Case Study: Thermal Analysis of Artificial Ground Freezing at the McArthur River Uranium Mine G.P..Newman have been previously described by Newman and Maishman (2000). The primary focus of the paper is the appli- cation of an advanced thermal analysis tool that greatly simplifies the finite ele- ment modeling required during the de- sign and .monitoring stages of projects such as that carried out at McArthur River. Background According to Newman and Maishman (2000), the ore body itself is located 550 meters to 620 meters underground where the ground-water pressure is ap- proximately 5500 kPa. Due to the pres- ence of a hanging wall fault structure, the ore body is surrounded on three sides by fairly dry, competent ground. The other three sides are comprised of highly fractured sandstone with signifi- cant amounts of rubble, flowing sand and clay regions. In order to mine the ore, it was necessary to create a frozen wall barrier around the three poor sides of the ore body. The frozen wall barrier was designed to permit drainage of wa- ter in the ore and consequently reduce water pressures prior to mining. The wall was also required to provide struc- tural support of weak, clay / ore ground near to mining cavities. Figure I shows a cross-section of the ore body and neighboring geol- ogy. A mechanical freezing system is comprised of a brine cooling and distribution net- work plus a series of brine freeze pipes installed in the ground to be frozen. Typical ground freezing applications have involved drilling freeze holes from surface or near ......... South Freeze Row Dry Basement Rock 640m LevefDeveloDment 60 GeotechnicalNews, June 2003 Figure 1. Cross-section of the ore body and neighboring geology (after Newman and Maishman, 2000)
description
Thermal Analysis of Artificial Ground Freezong at Mc Arthur River Uranium Mine
Transcript of Newman 2003
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IntroductionTheMcArthurRiveruraniummineislocatedin theAthabascasandstonere-gioninthenorthernpartoftheprovinceof Saskatchewan,Canada.It is theworld'slargest,high-gradeuraniumde-positwithprovenandprobablereservesofmorethan473millionpoundsU30S.It ismajorityownedandoperatedbytheCamecoCorporation.
Thispaperpresentsacasestudyoftheengineeringthatwascarriedoutinorderto artificiallyfreezetheunder-groundorebodyattheminepriortoitbeingmined.Fulldetailsof theproject
AthabascaSandstone
.~Highpressurewater
~
SandandGravel
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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
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Figure1.Cross-sectionoftheorebodyandneighboringgeology(afterNewmanandMaishman,2000)
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/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
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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.
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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
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