Lesson FourdamWord下载.docx
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Lesson FourdamWord下载.docx
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Adamisastructurebuiltacrossastream,river,orestuarytoretainwater.Itspurposesaretomeetdemandsforwaterforhumanconsumption,irrigation,orindustry;
toreducepeakdischargeoffloodwater;
toincreaseavailablewaterstoredforgeneratinghydroelectricpower;
ortoincreasethedepthofwaterinariversoastoimprovenavigation.Anincidentalpurposecanbetoprovidealakeforrecreation.
Auxiliaryworksatadammayincludespillways,gates,orvalvestocontrolthedischargeofsurpluswaterdownstreamfromthedam;
anintakestructureconductingwatertoapowerstationortocanals,tunnels,orpipelinesformoredistantuse;
provisionforevacuatingsiltcarriedintothereservoir;
andmeansforpermittingshipsorfishtopassthedam.Adamthereforeisthecentralstructureinamultipurposeschemeaimingattheconservationofwaterresources.Themultipurposedamholdsspecialimportanceintheunderdevelopedcountries,whereasmallnationmayreapenormousbenefitsinagricultureandindustryfromasingledam.
Damsfallintoseveraldistinctclasses,byprofileandbybuildingmaterial.Thedecisionastowhichtypeofdambebuilddependslargelyonthefoundationconditionsinthevalleyandtheconstructionmaterialsavailable.Basically,thechoiceofmaterialsnowliesbetweenconcrete,soils,androckfill.Thoughanumberofdamswerebuiltinthepastofjointedmasonry,thispracticeisnowlargelyobsolete.Themonolithicformofconcretedamspermitsgreatervariationsinprofile,accordingtotheextentwaterpressureisresistedbythedeadweightofthestructure,istransferredlaterallytobuttresses,oriscarriedbyhorizontalarchingacrossthevalleytoabutmentsformedbythesidesofthevalley.
BasicProblemsinDamDesign
Mostmoderndamscontinuetobeoftwobasictypes:
masonry(concrete)andembankment(earthfill).Masonrydamsaretypicallyusedtoblockstreamsrunningthroughnarrowgorges,asinmountainousterrain;
thoughsuchdamsmaybeveryhigh,thetotalamountofmaterialrequiredislimited.Embankmentdamsarepreferredtocontrolbroadstreams,whereonlyaverylargebarrier,requiringagreatvolumeofmaterial,willsuffice.Thechoiceofmasonryorembankmentandtheprecisedesigndependonthegeologyandconfigurationofthesite,thefunctionsofthedam,andcostfactors.
Siteinvestigationandtesting.Investigationofasiteforadamincludessinkingtrialboringstodeterminethestrata.Theboringsaresupplementedbyshaftsandtunnelswhich,becauseoftheircost,mustbeusedassparinglyaspossible.Intheshaftsandtunnels,testscanbemadetomeasurestrength,elasticity,permeability,andprevailingstressesinstrata,withparticularattentiongiventothepropertiesofthinpartings,orwalls,betweenthemoremassivebeds.Thepresenceingroundwaterofchemicalsolutionsharmfultothematerialstobeusedintheconstructionofthedammustbeassessed.Sourcesofconstructionmaterialsneedexploration.Asdamscontinuetoincreaseinheight,thestudyoffoundationconditionsbecomesincreasinglycritical.
Modeltestsplayamajorpartinthestructural,seismic,andhydraulicdesignofdams.Structuralmodelsareparticularlyusefulinanalysisofarchdamsandinverifyinganalyticalstresscalculations.Variousmaterialshavebeenusedformodeltests;
onsomeearlytestsforHooverDam,rubberwasemployed.Theneedforaccuratereproductionofstresspatternsincomplexmodelsismetbyusingmaterialoflowelasticity.Inasense,damsthemselvesaremodelsforfuturedesign.Theinstrumentsbuiltintothemtorecordmovementsunderload,strainswithinmaterialsafterconstruction,temperatureandpressurechanges,andotherfactorsareinstalledprimarilytostudytheperformanceofthestructureandtowarnofpossibleemergencies,buttheirvalueinconfirmingdesignassumptionsisimportant.
Thedigitalcomputerhaspermittedconsiderableadvanceinanalyticalmethodsofdesign.ItsabilitytohandleagreatvolumeofdataandtosolvelargesetsofsimultaneousequationscontainingmanyvariableshasmadepracticablethemethodofFiniteElementAnalysis.Inthismethod,acomplicatedstructureisdividedintoanumberofseparateequilibriumconditions,andstrainsarerenderedcompatible,thusleadingtoacompleteanalysisofstressandstraindistributionthroughoutthestructure.
Problemsofmaterials.Eachofthetwobasicdammaterials,concreteandearthorrockfill,hasaweaknessthatmustbeovercomebytheproperdesignofthedam.
Weaknessesofconcrete.Concreteisweakintensilestrength;
thatis,itcanbepulledaparteasily.Concretedamsmustthereforebedesignedtoplaceminimumtensilestrainonthedamandtomakeuseofconcrete’sgreatcompressivestrengthorabilitytosupportverticalloads.Thechiefc
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