Lesson Fourdam解读Word文档格式.docx
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Lesson Fourdam解读Word文档格式.docx
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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.Thechiefconstituentofconcrete,cement,shrinksasitsetsandhardens,duetowaterabsorptioninthecrystallinestructure,toevaporationofwatertotheatmosphere,andtocoolingfromthehighertemperaturesreachedwhenthechemicalreactionsinthecementareinprogressduringhydration.Becauseofthelargevolumeofconcreteinadam,shrinkagepresentsaseriouscrackinghazard.
Variousexpedientsareusedtoovercometheproblem.Concreteisusuallycastinseparateblocksoflimitedheight.Gapsmaybelefttopermitheatlossesandfilledinlater.Low-heatcementsmaybeused;
thesearespeciallyblendedsothatratesofheatevolutionareretarded.Cementcontentcanbesafelyreducedintheinteriorconcreteinthedam,inwhichstrengthandresistancetoclimaticandchemicaldeteriorationarelessimportant.Thecementcontent,andthereforetheheatcausedbyhydrating,canalsobereducedbyusingaggregate(theothermajorconstituentofconcrete)oflargerstones.Anotherexpedientistouseotherfine-grainedmaterials,suchasflyash(pulverizedfuel),asfiller,reducingthetotalcementvolumeintheconcrete.Anotheristousecertainadditives,surface-activeagents,andair-entrainingagentsthatpermitusingalowerwater-to-cementratioinmixingtheconcrete.Techniquesusedtospeedthecoolingprocessincludereplacingsomeofthewaterinthemixbyice,circulatingwaterthroughpipeslaidintheconcrete,andextractingexcesswaterfromsurfacesbyvacuum.
Weaknessesofearthandrockfill.Soilsandrockfragmentslackthestrengthofconcrete,aremuchmorepermeable,andpossesslessresistancetodeteriorationanddisturbancebyflowingwater.Thesedisadvantagesarecompensatedforbyamuchlowercostandbytheabilityofearthfilltoadapttodeformationcausedbymovementsinthedamfoundation.Thisassumes,ofcourse,sufficientusablesoilavailableclosetothedamsite.Inbaremountaincountryitmaybenecessarytoquarryrockandconstructarockfillratherthananearthfilldam.Earthfillisofcoursemoreeconomical,andoftenasuitableborrowareacanbefoundclosetothesite.
Soilconsistsofsolidparticleswithwaterandairinbetween.Whenthesoiliscompressedbyloading,asoccursindamconstruction,somedrainageofairandwatertakesplace,causinganincreaseinpressuresbetweenthesolidparticles.Whenthereisahighrateofseepage,thesoiltendstodevelopdifferentialpressuresandreachaconditioncalledquick,inwhichitbehavesasafluid.Evenifitdoesnotreachthiscondition,thereisoftensomeweakeningofitsstructure,andstepsmustbetakentocounterthis.
Theearthquakeproblem.Manylargedamshavebeenbuiltintheseismicallyactiveregionsoftheworld,includingJapan,thewesternUnitedStates,NewZealand,theHimalayas,andtheMiddleEast.In1968,theTokachiearthquakedamaged93damsinHonshu,themainJapaneseIsland;
allwereembankmentdamsofrelativelysmallheight.
Despiteagreatdealofworkonthedistributionofseismicactivity,themeasurementofstronggroundmotions,andtheresponseofdamstosuchmotions,earthquakedesignofdamsremainsimprecise.Thecharacteristicsofstronggroundmotionsatagivensitecannotbepredicted,andalltypesofdamspossesssomedegreeoffreedom,imperfectelasticity,andimprecisedamping.Nevertheless,thedigitalcomputerandmodeltestinghavegivenpromiseofconsiderableprogress.Itisnowpossibletocalculatetheresponseofaconcretedamtoanyspecifiedgroundmotion;
thishasbeendonefortheTang-eSoleymanDaminIranandtheHendrikVerwoerdDaminSouthAfrica.
Therehasalsobeenconsiderableadvanceinthetheoreticalestimationoftheeffectsofgroundmotiononembankmentdams.
NewWordsandExpressions
1.estuary['
estjuəri]n.河口,港湾,入海口
2.auxiliary[ɔ:
ɡ'
ziljəri]a.附属的,辅助的
3.surplus['
sə:
plʌs]n.剩余,过剩;
a.剩余的,过剩的
4.monolithic[,mɔnəu'
liθik]a.整体式的,整体浇铸的;
巨石般的
5.deadweight['
dedweit]n.自重,载重量,重负
6.buttress['
bʌtris]n.支墩,扶壁,支撑物
7.configuration[kən,fiɡju'
reiʃən]n.外形,构造,结构
8.strata['
strɑ:
tə]n.(stratum的复数)地层,岩层
9.shaft[ʃɑ:
ft]n.轴;
杆状物
10.elasticity[,elæ
s'
tisəti]n.弹性,弹力,灵活性
11.seismic['
saizmik]a.地震的,因地震而引起的
12.expedient[ik'
spi:
diənt]n.手段,方法;
权宜之计;
adj.权宜的;
方便的,有利的
13.blend[blend]vt.掺和,混和,调和
14.retard[ri'
tɑ:
d]vt.延迟,使减速,阻止,妨碍
15.hydrate['
haidreit]vi.水化,水合;
n.水化物
16.fragment['
fræ
ɡmənt]n.碎片,片断或不完整部分
17.pulverize['
pʌlvəraiz]vt.成粉末,碾磨,粉碎
18.rockfill['
rɔkfil]n.堆石,填石
19.peakdischarge洪峰流量
20.embankmentdam土石坝,堤坝
21.simultaneousequations联立方程
22.finiteelementanalysis有限元分析
23.crystallinestructure晶体结构
24.surface-activeagent表面活性剂
25.air-entrainingagent加气剂
26.water-to-cementratio水灰比
大坝
大坝是横跨在江河港口的一种用来蓄水的结构。
修建大坝的目的是用来满足居民供水、灌溉、工业用水的需求;
也用来减小洪峰;
也可以用来存蓄可利用水来发电;
也可以增高江河水深以提高水上交通。
同时也可以提供一种可以娱乐的湖。
在大坝上的一些辅助工作包括溢洪道、闸门、用来控制从大坝向下游泻出的
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