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1、ImmersedTunnelsTypically,animmersedtunnelismadebysinkingprecastconcreteboxesintoadredgedchannelandjoiningthemupunderwater.Tunnelsectionsinconvenientlengths,usually90to150meters,areplacedintoapre-dredgedtrench,joined,connectedandprotectedbybackfillingtheexcavation.Thesecti
2、onsmaybefabricatedinshipyards,indrydocks,orintemporaryconstructionbasinservingasdrydocks.Immersedtunnelsare,moreadvantegousasasubaquoussolutioninsoftsoilsincreasinglyusedalternativetotraditionallyusedshieldtunnelling,withouthavingtherisksassociatedwithpressurechambersandi
3、nrushofwater.alsosuitableinwaterdeeperthanitispossiblewiththeshieldmethod,whichessentiallyisrestrictedtolessthan30mofwater(concerningthemaximumairpressureatwhichworkerscansafelywork).Advantegousasthereislesslossinheightthanwithtunnellingdeeplyundertheriverbed,andthetunnel
4、maythereforebeshorteroverall.Theworld'slongestanddeepestapplicationtodateisthetwin-tubesubwaycrossingofSanFranciscoBay,constructedbetween1966and1971withalengthof3.6miles(5,8km)inamaximumwaterdepthof41m.The100mlong,15mwidesectionswereconstructedofsteelplateandlaunchedbyshi
5、pbuildingprocedures.Thefirsttunneloftheconcretetypewasconstructedin1940inRotterdamundertheMaasestuary.Themostprofoundeffectofanimmersedtunnelontheenvironmentconcernstheelementitismeanttobypass—water.Theinfluenceofthetunnelonthegroundwaterandthesurfacewaterintheareaplaysap
6、redominantroleinthetunneldesignandconstructionmethods.Anaspectofmorerecentconcernaffectingconstructionisthepossiblepresenceofcontaminatedsoilsthatmustberemovedforthetunneltrench.Waysofremovingthesesoilsandtransportingthemtodepositoriesthatareespeciallyequippedtoreceivethe
7、mareenvironmentalproblemsrequiringnoveltechniquesandqualitycontrolprocedures.Themoretraditionalenvironmentalaspectsarethoseencounteredonanyconstructionjob:noise,dirt,andtraffichindrance.Thetopofthetunnelshouldbeprotectedbyadequateprotectivebackfill,extendingabout30moneach
8、sideofthestructureandconfinedwithindykesorbunds.Thefillmustbeprotectedagainsterosionbycurrentswi
