沉管隧道介绍(英文)课件

ImmersedTunnels

Typically,animmersedtunnelismadebysinking

precastconcreteboxesintoadredgedchannelandjoiningthemupunderwater.Tunnelsectionsinconvenientlengths,usually90to150meters,areplacedintoapre-dredgedtrench,joined,connectedandprotectedbybackfillingtheexcavation.Thesectionsmaybefabricatedinshipyards,indrydocks,orintemporaryconstructionbasinservingasdrydocks.

Immersedtunnelsare,moreadvantegousasasubaquoussolutioninsoftsoilsincreasinglyusedalternativetotraditionallyusedshieldtunnelling,withouthavingtherisksassociatedwithpressurechambersandinrushofwater.alsosuitableinwaterdeeperthanitispossiblewiththeshieldmethod,whichessentiallyisrestrictedtolessthan30mofwater(concerningthemaximumairpressureatwhichworkerscansafelywork).Advantegousasthereislesslossinheightthanwithtunnellingdeeplyundertheriverbed,andthetunnelmaythereforebeshorteroverall.Themostprofoundeffectofanimmersedtunnelontheenvironmentconcernstheelementitismeanttobypass—water.Theinfluenceofthetunnelonthegroundwaterandthesurfacewaterintheareaplaysapredominantroleinthetunneldesignandconstructionmethods.Anaspectofmorerecentconcernaffectingconstructionisthepossiblepresenceofcontaminatedsoilsthatmustberemovedforthetunneltrench.Waysofremovingthesesoilsandtransportingthemtodepositoriesthatareespeciallyequippedtoreceivethemareenvironmentalproblemsrequiringnoveltechniquesandqualitycontrolprocedures.Themoretraditionalenvironmentalaspectsarethoseencounteredonanyconstructionjob:noise,dirt,andtraffichindrance.Thetopofthetunnelshouldbeprotectedbyadequateprotectivebackfill,extendingabout30moneachsideofthestructureandconfinedwithindykesorbunds.Thefillmustbeprotectedagainsterosionbycurrentswitharockblanket,protectiverockdykesorothermeans.Tidesandcurrenteffectsofthewaterwaymustbeevaluatedtodetermineconditionsduringdredgingandtubesinkingoperations.Importantly,dredgingandbackfillingoperationsshouldbeexecutedinsuchamannerastolimitdisturbanceinthenaturalecologicalbalanceattheconstructionsite.Governmentalagencieshavingjurisdictionoverenvironmentalprotection,naturalresourcesorlocalconditionsmustbeconsultedandapprovalofauthoritiesshouldbeobtainedinthepreliminarydesignstage.FoundationThefoundationmethodtobeusedmustbechosenwithdueconsideration,firstofall,forthesubsoilconditionsandthedegreetowhichthetunnelwillbesubjecttodynamicloadings,andearthquakeloadingsinparticular.Pilefoundationsareanoptionbutthissolutionhasbeenusedforafewtunnelsonly.Forbothsteelandconcretetypesoftunnels,themaintasksare:

Excavationofatunneltrenchtospecificationsandtokeepitfreeofsiltationthatmaybedetrimentaltothepermanentfoundationuntilthisfoundationhasbeenconstructedandthetunnelhasbeenbroughttorestonit.Constructionofwatertightanddurabletunnelelements.Installationofthetunnelelementsinthetunneltrench.Constructionofwatertightanddurablejointsbetweenthetunnelelements.Constructionofadurablefoundationforthetunnel.TunnelTrenchDredging

Thedredgingworksrequiredfortheconstructionofanimmersedtunnelwillnormallycomprisesome,orall,ofthefollowingitems:Dredgingofacastingorlaunchingbasin.Dredgingoftestpitsinthewaterwayforevaluationofsiltationoftunneltrench.Wideningoftheexistingnavigationchannelinordertoprovidetemporarynavigationchannelsoutsidethemarineworkingarea.Compensationgroutingtomakeupforthereductionofthewaterwaycrosssectioncausedbythepermanenttunnelworks,andtherebyavoidingchangesinhydrographicalandbiologicalconditionsinthewaterway.Dredgingofthetunneltrenchfortheimmersedtunnelsection.Dredgingofanaccesschannelbetweenthecasting/launchingbasinandthetunneltrench.Maintenancedredging. Thedredgingvolumeisgenerallyintheorderoflmillionm3perkmforatypicalfour-lanemotorwaytunnel. Theexcavationmustprovidespacefortheprefabricatedtunnelbody;thesandorgravelfoundationunderthebodyaswellsastheprotectivebackfillonthesidesandonthetopofthetunnel. Becausethetopofthebackfillhastobekeptbelowtheexistingorfuturenavigationchannelprofile,atrenchbottomlevelatbetween25and30mbelowLowWaterlevelisquitecommon.Immersedtunnelsindeeporopenseamayrequirespeciallybuiltdredgers.

Exceptforcaseswhereverysoftsubsoil,deemedunsuitableforsupportofthetunnel,hastoberemovedandreplacedbysuitablematerials,thegeneralrequirementsforthedredgingofthetrenchbottomare:

Aclean,evensurface,ascloseaspossibletotheupperacceptablelimitinordertoavoidtheeconomicconsequencesofhavingtofilloverdredgedareas;Aminimumdisturbanceoftheremainingexposeduppersoillayersinthetrenchbottom,inordertolimitthechangesinthegeotechnicalcharacteristicsofthesubsoil.

Thepossiblephysicaldisturbanceandsofteningoftheexposedsoillayersinthetrenchbottom,particularlyincohesivesubsoils,canhaveaconsiderableinfluenceonthegeotechnicalbehaviourofthesesoillayerslater-and,hence,onthequalityofthetunnelsupportasawhole.Thisinturninfluencesthedesignofthestructuraltunnelbodyand,thus,eventuallytheoveralleconomy.Thesetechnicalrequirementsaremetby:

Usingthepropertypeofdredger(s).Carefulcontrollingthepositionofthecuttingtool,bearinginmindthatthedredgingnormallyhastobedoneintidalwatersandsometimesinwaterssubjecttoswellandwaves.Carefulplanningthedredgingoperationinordertoavoidundesirablefailuresoftheslopes.Timingofthedredgingoperation,inordertolimitthetimethatthetrenchbottomisexposedand,atthesametime,tolimitthesedimentationcausedbysubsequentdredgingnearby.ConstructionofTunnelElementsThetunnelelementsaremadefullyorpartiallybuoyantbymeansoftemporarybulkheadsinstalledattheelementends.Inadditiontoprovidingproperstructuralstrengthandcontrollingtheweightoftheelement,themaindesignandconstructiontaskofthereinforcedconcretetunnelistoprovideawatertightstructure.

Formanyyears,theanswerwastowrapthetunnelelementinawatertightmembranecomposedofsteelonthebottom,outerwallsandevenontheroof.Alternatively,bituminousmembranehasbeenusedontheouterwallsandroof.Inrecentyears,reinforcedconcretetunnelswithoutamembraneatallarebeingused.Aboveall,thiswillrequiresophisticatedcontrolofconcretetemperatureduringhardeningtoavoidcracking.Inordertoreducethedevelopmentofcracksduringhardening,primarilyinthewallswhentheyarecastafterthebottomslab,coolingofthelowerpartofthewallshasbeenthepracticeformanyyears.Insulationoftheformworkandcarefulsequencingofstrippingoftheformsarealsousedtocontroltheconcretetemperature.Improvedfieldconcretetechnologyaimedatminimisingthedevelopmentofcracksduringhardening,combinedwithmoderateprestressing,seemstobethecoursetofollow.Tunnelelementsareconstructedinthedry,forexampleinacastingbasin,afabricationyard,onaship-liftplatformorinafactoryunit.

CastingBasins

Thetunnelelementscanbeprefabricatedinacastingbasinorinadrydock.Forshorterroadwayandrailroadtunnels,theelementsarenormallycastinonebatchinacastingbasin.Aprogrammeforcontrolofconcretedensityandconcretedimensionsisrequiredinordertocontroltheweightanddisplacementofthetunnelelements.Thetypicalcastingsequenceisbottom/walls/roof,butsometimesallatonce,in15-20msegments.Thetunnelelementscanbemonolithic,ortheycanbeprovidedwithflexiblejointsbetweentunnelsegmentswithintheelements.Thelatterarrangementminimiseslongitudinalbendingmomentscausedbycompressionofthesubsoilinthepermanentstage,butisunsuitableforrailwaytunnelsinsoftgroundandinseismicregions.Normallythetunnelelementswillbebuoyantandneedtobeballastedpriortofloodingofthecastingbasininordertomakesurethattheyremain‘parked’untiltheyaretobebroughttotheimmersionlocation.Thisballastingisnormallydonewithwatercontainedinpurpose-builtballasttanksinsidethetunnelelement.Pumpsandassociatedpipelinesallowchargingandremovaloftheballast.Anumberofliftingeyesandbollardsmustbeprovidedontheelementroof.Watertight,temporarybulkheadsareinstalledattheendsoftheelement,andrubbergasketsaremountedaroundtheperipheryoftheoneendofthetunnelelement,whileaplanesteelplateisprovidedattheoppositeend.Later,whenthetunnelelementisjoinedtothepreviouslyplacedtunnelelement,thisgasketprovidesawatertightsealbetweenthetwotunnelelements.Asthecastingbasinisfloodedorasthetunnelislaunchedfromthedock,thetunnelelementischeckedforwatertightness,theattentionbeingdirectedprincipallytowardsthetemporarybulkheadsandpipelet-ins.Recentlyasystemofconstructingconcretetubesonfloatingpontoonsisdeveloped.Byremovingtheneedforcastingbasinsonthe

riverorcanalcostsarereduced,andtheprocessismoreenvironmentallyfriendly.Theendsoftheelementarethentemporarilysealedwithbulkheads.

Eachtunnelelementistransportedtothetunnelsite-usuallyfloating,occasionallyonabarge,orassistedbycranes.

Installationoftunnelelementsinthetrench

Fortransportationoftheelementfromthefloodedcastingbasinordocktothetunneltrench,conventionaltowageisnormallyused. Thewarping,whichendswiththetunnelelementbeingmooredforimmersion,isnormallycarriedoutbythecontractor'sorganisationresponsibleforthesubsequentsinkingandjoining,whereastowingnormallyisdonebyexperiencedtowagecompanies.

Theimmersionofthetunnelelementiscarriedoutafterthetunnelelementbasbeenmooredandtheelementhasbeenballastedasnecessarytoprovideadequateloadsintheimmersiontackles.Thetunnelelementisloweredtoitsfinalplaceonthebottomofthedredgedtrench.Onceplaced,theelementsarejoined;firstbybringingrubbergasketatthejointintocontactwiththesteelfaceofthepreviouslyplacedtunnelelement,andthendrainingthejointchamber,therebymobilisingthefullhydrostaticwaterpressureonthetunnelcrosssectionremoteend. Backfill materialisplacedbesideandoverthetunneltofillthetrenchandpermanentlyburythetunnel,asillustratedinthefigures.Approachstructurescanbebuiltonthebanksbefore,afterorconcurrentlywiththeimmersedtunnel,tosuitlocalcircumstances.

Immersedtunnelscanbeplacedimmediatelybeneathawaterway.Incontrast,aboredtunnelisusuallyonlystableifitsroofisatleastitsowndiameterbeneaththewater.Thisallowsimmersedtunnelapproachestobeshorterand/orapproachgradientstobeflatter-anadvantageforalltunnels,butespeciallysoforrailways.

expensive,suchasthesoftalluvialdepositscharacteristicoflargeriverestuaries.Theycanalsobedesignedtodealwiththeforcesandmovementsinearthquakeconditions,asintheexampleillustratedabove,tobeplacedinverysoftgroundinanareapronetosignificantearthquakeactivity.Boredtunnellingisacontinuousprocessinwhichanyproblemintheboringoperationthreatensdelaytothewholeproject.Immersedtunnellingcreatesthreeoperations-dredging,tunnelelementconstructionandtunnelinstallation,whichcantakeplaceconcurrently,thusmoderatingprogrammeriskconsiderably.Partlyforthisreason,animmersedtunnelisgenerallyfastertobuildthanacorrespondingboredtunnel.

ARETHEREANYSPECIALPROBLEMS?

Immersedtunnelsaresometimesperceivedbynewcomerstothetechnologyas"difficult"duetothepresenceofmarineoperations.Inrealitythough,thetechniqueisoftenlessriskythanboredtunnellingandconstructioncanbebettercontrolled.Themarineoperations,thoughunfamiliartomany,posenoparticulardifficulties.Theperceivedproblemsinclude:

DREDGINGDredgingtechnologyhasimprovedconsiderablyinrecentyears,anditisnowpossibletoremoveawidevarietyofmaterialunderwaterwithoutadverseeffectsontheenvironmentofthewaterway.

INTERFERENCEWITHNAVIGATIONInterferencewithnavigation:Onbusywaterways,itissometimesassumedthatconstructionofanimmersedtunnelwouldbeimpracticalasitwouldinterferewithshipping.Infact,suchtunnelshavebeensuccessfullybuiltinsomeexceptionallybusywaterwayswithoutundueproblems.

WATERTIGHTNESSItisoftenassumedthattheprocessofbuildingatunnelinwater,ratherthanboringthroughthegroundbeneathitwillincreasethelikelihoodofleakage.Infact,immersedtunnelsarenearlyalwaysmuchdrierthanboredtunnels,duetotheabove-groundconstructionoftheelements.Underwaterjointsdependonrobustrubbersealswhichhaveprovedeffectiveindozensoftunnelstodate.

ANEWDEVELOPMENT:THESUBMERGEDFLOATINGTUNNEL

Traditionalimmersedtunnellingresultsinatunnelburiedbeneaththewaterwaywhichittraverses.Anewdevelopment-thesubmergedfloatingtunnel-consistsofsuspendingatunnelwithinthewaterway,eitherbytetheringabuoyanttunnelsectiontothebedofthewaterway,orbysuspendingaheavier-than-watertunnelsectionfrompontoons.Thistechniquehasnotyetbeenrealised,butoneproject,inNorway,iscurrentlyinthedesignphase.Thesubmergedfloatingtunnelallowsconstructionofatunnelwithashallowalignmentinextremelydeepwater,wherealternativesaretechnicallydifficultorprohibitivelyexpensive.Likelyapplicationsincludefjords,deep,narrowseachannels,anddeeplakes.

DesignAspectsofImmersedTunnelsThestartingpointofanimmersedtunneldesignisrequiredcross-sectionalareai.e.the‘hollowspace’.Thetunnelmusthavethesamenumberoftrafficlanesastheroad.Dimensionalrequirementsvaryfromcountrytocountry;generallyspeakingthelanesshouldbe3,5mwidewithheadroomabove,dependingonlocalregulations(e.g.4.5mforHolland).Thereshouldalsobeaclearancefromthecarriagewaytothewallsof0.8to1.0m,forbrokendowncars.Theclearancewillalsoreducethe‘walleffect’;driversshyingawayfromthewalltherebyreducingthecapacityoftheroad.Abovetheheadroomthereshouldbeadequateroomforventilationboosterfans,luminariesandsignalequipment.Inthedual-carriagewaytunnelsthereisoftenaservicegalleryforcableslocatedbetweenthetraffictubes.DesignforfloatingAfterconstruction,theelementsarefloatedtotheirfinalposition.Theelementisthenmadeheavierthanitsdisplacementbymeansoftemporaryballast(oftenwater),afterbeingtemporarilysupportedbytheimmersionrigs.Atalaterstagethisballastisreplacedbydefinitiveballastintheshapeofnon-reinforcedconcretebelowthefuturecarriagewayorexternally,orothersecondaryinteriorstructuralconcrete.Bythistimetheimmersionequipmentandbulkheadswillhavetoberemoved.Theelementmustnowweighsufficientlymorethanitsbuoyancytoremaininplace.Thepressureheadofthegroundwaterbelowthetunnelbasemaylagbehindthewaterlevelintheriver.Atlowtidethismayresultinanadditionalupwardforce.Tocompensateforeffectsofthiskind,thedesigncriterionoftenadoptedatthisstageisthattheweightofthetunnelmustexceedthewaterdisplacementbyanabsoluteminimummarginagainstflotationwhenallremovableitemsandbackfillareremoved.Thisfloationmarginmaybeintherangeof1.075,butisdeterminedonaprojectbasis.Thesafetymarginislaterincreasedbecausethesidesandtopofthedredgedtrenchintowhichthetunnelwasplacedisthenbackfilled.Thisresultsinthefirstplaceinaloadontheroof;whereasfrictiononthewallsisignored.Erosionprotectionisplacedtocontinuouslymaintaina1.15or1.2factorofsafetyagainstflotation,dependingupontheclient’srequirements,andsafetyagainstsinkingshipsanddroppinganchors.Atthetransportstage,Weight=0.99·maximumwaterdisplacementor2.46S+3.0=0.99(B+H+S) (1)Avalueof2.46willbetakenasthespecificweightofreinforcedconcreteintheflotationstage,andadensityof2.42inthefinalstage.Inthefinalphaseacounter-flotationmarginshouldapply,assumethismargintobe7,5percent,hence:Weight=1.075waterdisplacement,or2.42S+2.25B=1.075(B+H+S) (2)Casestudy:ØresundLinkStraitcrossing

TheØresundlinkconnectsCopenhagenonZealand,DenmarktoMalmöinSwedenthusestablishingalandtrafficcorridorfromScandinaviatothecontinent.Thelinkcomprisesa3.5kmimmersedtunnel,4kmartificialisland,and8kmbridge,includinga490mspancable-stayedbridge.Thelinkincorporatesapproximatelyonemillionm³ofconcrete,ofwhichmorethantwothirdsconstitutetheimmersedtunnel.TheØresundTunnelwasmotivatedbythefactthatoneofthemainshippinglaneisveryclosetotheCopenhageninternationalairport,makingahighbridgeoverthenearestnavigationalchannelunfeasible.Thetunnelcross-sectionaccommodatestwotubesforthetwo-trackrailwayandtwotubesforthefour-lanemotorway.Acentralinstallationgallerybetweenthemotorwaytubesdoublesasasafeandsmoke-freeescaperouteincaseofemergency.Theimmersedpartofthetunnelconsists