钢筋混凝土及土方工程简介-土木外文翻译及甘河工业园西区中电投西宁火电厂2×660MW铁路专用线工程质量创优规划

PAGEPAGE28IntroductiontoreinforcedconcreteandearthworksAbstractAsadesignermustfirstclearthebuildingstructureitselfwasdesignedandintensitylevels,aswellasrelatedissuesin-depthdiscussionandresearch,thispaperdescribesonthereinforcedconcrete,earthworkengineeringknowledge,letmorein-depthunderstandingofthisDiscussesthekey,andtherationalapplicationofknowledgetohelpusdesignmoreexcellentbuildingKeywords:concrete；earthwork；structuralsafety1ReinforcedConcretePlainconcreteisformedfromahardenedmixtureofcement,water,fineaggregate,coarseaggregate(crushedstoneorgravel),air,andoftenotheradmixtures.Theplasticmixisplacedandconsolidatedintheformwork,thencuredtofacilitatetheaccelerationofthechemicalhydrationreactionlfthecement/watermix,resultinginhardenedconcrete.Thefinishedproducthashighcompressivestrength,andlowresistancetotension,suchthatitstensilestrengthisapproximatelyonetenthlfitscompressivestrength.Consequently,tensileandshearreinforcementinthetensileregionsofsectionshastobeprovidedtocompensatefortheweaktensionregionsinthereinforcedconcreteelement.Itisthisdeviationinthecompositionofareinforcesconcretesectionfromthehomogeneityofstandardwoodorsteelsectionsthatrequiresamodifiedapproachtothebasicprinciplesofstructuraldesign.Thetwocomponentsoftheheterogeneousreinforcedconcretesectionaretobesoarrangedandproportionedthatoptimaluseismadeofthematerialsinvolved.Thisispossiblebecauseconcretecaneasilybegivenanydesiredshapebyplacingandcompactingthewetmixtureoftheconstituentingredientsareproperlyproportioned,thefinishedproductbecomesstrong,durable,and,incombinationwiththereinforcingbars,adaptableforuseasmainmembersofanystructuralsystem.Thetechniquesnecessaryforplacingconcretedependonthetypeofmembertobecast:thatis,whetheritisacolumn,abean,awall,aslab,afoundation.amasscolumns,oranextensionofpreviouslyplacedandhardenedconcrete.Forbeams,columns,andwalls,theformsshouldbewelloiledaftercleaningthem,andthereinforcementshouldbeclearedofrustandotherharmfulmaterials.Infoundations,theearthshouldbecompactedandthoroughlymoistenedtoabout6in.indepthtoavoidabsorptionofthemoisturepresentinthewetconcrete.Concreteshouldalwaysbeplacedinhorizontallayerswhicharecompactedbymeansofhighfrequencypower-drivenvibratorsofeithertheimmersionorexternaltype,asthecaserequires,unlessitisplacedbypumping.Itmustbekeptinmind,however,thatovervibrationcanbeharmfulsinceitcouldcausesegregationoftheaggregateandbleedingoftheconcrete.Hydrationofthecementtakesplaceinthepresenceofmoistureattemperaturesabove50°F.Itisnecessarytomaintainsuchaconditioninorderthatthechemicalhydrationreactioncantakeplace.Ifdryingistoorapid,surfacecrackingtakesplace.Thiswouldresultinreductionofconcretestrengthduetocrackingaswellasthefailuretoattainfullchemicalhydration.Itisclearthatalargenumberofparametershavetobedealtwithinproportioningareinforcedconcreteelement,suchasgeometricalwidth,depth,areaofreinforcement,steelstrain,concretestrain,steelstress,andsoon.Consequently,trialandadjustmentisnecessaryinthechoiceofconcretesections,withassumptionsbasedonconditionsatsite,availabilityoftheconstituentmaterials,particulardemandsoftheowners,architecturalandheadroomrequirements,theapplicablecodes,andenvironmentalreinforcedconcreteisoftenasite-constructedcomposite,incontrasttothestandardmill-fabricatedbeamandcolumnsectionsinsteelstructures.Atrialsectionhastobechosenforeachcriticallocationinastructuralsystem.Thetrialsectionhastobeanalyzedtodetermineifitsnominalresistingstrengthisadequatetocarrytheappliedfactoredload.Sincemorethanonetrialisoftennecessarytoarriveattherequiredsection,thefirstdesigninputstepgeneratesintoaseriesoftrial-and-adjustmentanalyses.Thetrial-and–adjustmentproceduresforthechoiceofaconcretesectionleadtotheconvergenceofanalysisanddesign.Henceeverydesignisananalysisonceatrialsectionischosen.Theavailabilityofhandbooks,charts,andpersonalcomputersandprogramssupportsthisapproachasamoreefficient,compact,andspeedyinstructionalmethodcomparedwiththetraditionalapproachoftreatingtheanalysisofreinforcedconcreteseparatelyfrompuredesign.Concretestresstest1TestIntroductionThetensilepropertiesofconcretecanbeenhancedsubstantiallybyincorporatinghighstrengthandsmalldiametershortsteelfibers.whichleadstothesteelfiberreincedconcrete(SFRC).InconventionalSFRC,thesteelfibercontentisusuallywithintherangeofO.2%-2%byvolume.Atsuchalow6hercontent.thetensileresponseofSFRCwouldassumeanonhardeningtype.whichischaracterizedbythewideningofasinglecrack,similartoanunreinforcedconcrete.Thecontributionoffibersisapparentinthepost-crackingresponse,representedbyanincreaseinpost-crackingductilityduetotheworkassociatedwithpulloutoffibersbridgingafailurecrack.However,improvementsinsomeotherproptiesareinsignificant.Moreover,thesofteningsegmentofthestress-straincurveofSFRCwithsuchalowfibercontentunderuniaxialtensiondifficulttobegotwithnormalexperimentalmethods.Manyworkshavebeendonetofmdasuitableandrelativelyeasywaytoanalyzethetensilecharacteristics.Anditwasreportedthatthewholecurvecouldbegotonanonnaltestingmachinewithstiffeningcolponentsadded.Inthisarticle,thestress-strainbehaviorofSFRCunderuniaxialtensionWasanalyzedfordifferentopesoffiber.ThetensilecharacteristicsofSFRCinfluencedbythematrixstrengthandthesteelfibercontentwerestudiedalso.Inaddition,thestress-straincurvesofhighstrengthSFRCwithdifferentfactorswerewellacquired.Themechanismoffiberreinforcedconcretetoenhanceresearch,toobtainsteelfiberreinforcedconcreteintensioncurveofthewholeprocess,usingthemostappropriatemethodofaxialtension,buttomakeStethetestingmethodsimproved,andthetestingmachinemusthaveenoughstiffnesstoensurethetestingprocessstability.Iswellknowninengineeringpractice,process,technologyandeconomicconditionsduetoconstructionconstraints,SFRC-dopedfibervolumeintherateofgenerallynotmorethan2%,whilemostoftheengineeringexalllple,thefiberfractionareabout1%.InthispaperthedesignoftheaxialtensionSFRCmaterialtesting,fiberdosagetotake1%,andusingdifferentopesoffiber-reinforcedforms,wereanalyzed.2ExperimentalContentThespecimensweretestedona601duniversaltestingmachine.Fourhighsteelbarswereaddedtoenhancethestiffuessofthetestingmachine.Inaddition,spherichingeswereusedtoabatetheinitialaxialeccentricityofthespecimens..Itwasensuredthatspecimensshouldbepulledunderuniaxialtensionbyadjustingthefourhighstrengthboltswhichconnectthespecimenstothecrossbeam.Andthedifferencebetweenthetensilestrainsoftheoppositesidesofthespecimenshouldbelessthan15%oftheirmeanvalue.Whenthefibercontentwaslow(0and0.5%byvolume),thecyclicquirethewholestress-strain.2.1MaterialsFour叩pesofsteelfibersshowninTablewerechosenforthistest.Threeofthesefibers(Fl,F2andF3)werehookedendandtheotherone(F4)wassmooth.Threeconcretemixtures,showninTable2,wereinvestigated.WaterreducingagentswereusedinC60andC80mixes(DK-5madebyDalianStructureResearchInstituteandSikamadeinSwitzerlandrespectively).ThecompressivestrengthsoftheseC30,C60,C80mixes28daysusing150mmx150mmx150mmcubes.Averagedresultsfor3specimensaregiveninTable2.OrdinaryPortlandcement(yieldedbyDalianHuanengOnodaCementCompany)of32.5and52.5(accordingtoChinastandard)werechosen.Riversand(modulusoffmenessis2.6)andcrushedlimestoneaggregates(5-20Bin)wereused.TableMatrixStrengthgradeCementuJcSandSandCrushedWaterCompressivecodeOfcementKg/m3rahrahoKg/m3StrnereducingSength(lSO)。Kg/m3MpaC3032.54500.440.36667115537.07C6052.55000.350.336021223DK-567.59C8052.56000.290.315351190Sika82.96SpecimenThetensileSpeCllenwasbondedtosteelpaddingplatesatbothendsbytygoweld.Atotalof110specimensweredividedinto22groupsaccordingtocertainparameters.TheparametersofthesespecimenseshowninTable3.2.3ItemsAttheageof28days.plainconcreteandsteelfiberconcretespeciInensweretestedfortensilestrength,respectively.Thetensilestress-straincveswereacquired.Manyothertensilecharactersofthehighstrengthsteelfiberconcretesuchastensilework,etcwerecalculateda1so.Enhancedclasssteelfiberreinforcedconcretetoughnesscategorythanthestrengthofsteelfiberreinforcedconcreteanaverageof13%;whilecrackingfromthebasictotheαackwidthof0.5mminterval(thecorrespondingstrainofabout2000με)showedtherenerrintegral:tougheningclasssteelfiberreinforcedconcreteenhancedclassthanthefractureenergyofsteelfiberreinforcedconcreteanaverageof20%.omTable3alsoshowsthatmostoftheSFRCfrrstpeakcorrespondstothelimitoftensilestrainvalueandplainconαeterather,inthe100μεaround，indicatingalowrateoffiber-containingincorporationinimprovingtheroleofultiInatetensilestrainofconcretenoteverobvious.letougheningclassSFRCsecondpeakcorrespondstoamuchgreaterstrain,upto1000，Fromthissecondpeakhasgreatlyenhancedtheappeanceoftoughness.DRAXFìberbecauseofthelengthofotherthreekindsoffiberlengthof2tiInesthefracturetoughnessandbetterinthetestcurvecanbeseeninthestrainisattained,theloadcontinuestomaintainahighlevel0fintensity,untilthestrainwhentheloadsoastomaintain10000μεitspeaklevelof50%.3ResultsandDiscussion3.1CrackstressandultinlatetensilestrengthThecracksessandultimatetensilestrengthofdierentspeciInenselistedinTable3.Theadditionofsteelfibersintoconcreteincreaseditscrackstressandultinlatetensilestrength.AndtheratiosofthesetwoparametersofSFRCtothoseofplainconcreue(withthesamemixproportion)aregiveninTable3,too.3.1.1Effectofmatrixstrengthan(1fibertypeFromtable3.ItcanbeseenthattheeffectsofsteelfibersOncrackstressarelittleinfluencedbythematsixstrength.Thatistosay.Whenthematrixstrengthinαeases，theratiosofcrackstressesofSFRC(withthesametypeoffiberscontained)tothoseofplainconcreteoneswiththesamemixproportioneinvariable.However,theconditionforultimatetensilestrengthisdifferent.Whenthematrixstrengthincreases.theseratiosofultimatetensilestrengths(showninTable3)varydissiInilarlyaccordingtothetypeofsteelfiber.Moreover.theincrementsarebiggerthanthoseofcrackstress.TheheighteningefficiencyoffiberFlforultimatetensilestrengthrisesmatrixstrengthincreases.Itisbecausethatthestrengofthiskindoffiberisveryhigh(>1100MPa).Nofiberbrokenwasobservedduringthetestandthehooked-endsofthefiberswerestraightenedwhenthematrixsngthwashigh(C80).Thehigherthematrixsngth.thiskindofsteelfibertakesonitssengtheningeffectmoreefficientlyfortheincreasingofbondstress.ThestrengthsoffibersF2andF3aremid-high(>700MPa).Theyallhavehookedendsandbothoftheirsurfacesarecose.Whenthematrixstrengthwashigh(C80).fiberbreakingoccurredinthetest.AndthisphenomenoniInpairedtheheighteningefficiencyofthesetwokindsofsteelfiber.Sotheyshouldbeusedinmiddlestrengthconcretetoexerttheirstrengtheningeffectmoreefficiently.FiberF4issmooth.anditsbondstresswithmatrixiscomparativelylow.T}1ereitsstrengtheningeffectis1essnotablethanthoseofotherkindsoffiber.Becauseofthelowbondstress.nofiberbrokenwasdaoundduringthetestanditsheighteningefficiencyforultimatetensilestrengthrisesasmatrixstrengthincreases.3.1.2EffectoffibercontentTheeffectoffibercontentonthecrackstressandu1.ultimatetensilestrengthwasinvestigatedforSFRCcontainedfiberF3.Andthefibercontentvariedfrom0.5%to1.5%byvolume(showninTable3).ItcanbeseenonFig.1andFig.2thatasthefibercontentmcreases.ThecrackstressandultimateengthofSFRCimproveobviously.Moreover.therisingtrendsofthecurvesinthesetwofiguresarespendouslysimilar.Inotherwords,theectoffibercontentonthecharactersoftensilestressofSFRCispositiveandconsistent.Table4FiberpefactorsFibercodeatF10.642F20.862F30.794F40.589ThetensilestrengthofSFRCcanbecalculatedwiththelowformula:where，istheultimatetensilestrengthofSFRC;theultimatetensilestrengthofplainconcretewiththesamemixingproportion;a,thefibertypefactor,lIIJ:=ρ'1-;;,PJwhichisshownTable4;.u.isthefibercontentofvolumeandl/distheaspectratioofsteelfibers.3.2Tensilestrainandtoughnesscharacters3.2.1CrackstrainandthestrainatpeaktensileloadThetensilestrainswereacquedbyaveragingthereadingsofthedisplacementsensors:fixedaroundthespecimen.Inaddition,thespecimenswhosedifferencebetweenthetensilestrainsofitsoppositesidesislargerthan15%oftheirmeanvaluewereblankedout.ThecrackstrainorthestrainsatpeaktensileloadofSFRCaremuchbiggerthanthoseofplainconcrete(asshowninTable5).Andtheincrementsgoupasthernatrixstrengthorthefibercontentincreases.Comparedtothatoncrackstrain.theincrescenteffectofsteelfiberonthestrainatpeaktensileloadismoreremarkable.3.2.2TensileworkandtoughnessmodulusThetensileworkwasdefinedastheteaundertheload-displacementcurvefrom0to0.5rain.Moreover,atensiletoughnessmoduluswasintroduced(showninTable5(2)where,fftistheultimatetensilestrengthofSFRC;A,theareaofthecrosssectionofspeclmen.BoththesetwoparameterswerequotedtoevaluatethetoughnesscharactersofSFRCunderuniaxialtension.Thetensiletoughnessmodulusisadimensionlessfactor.Comparedtowhatthetensileworkdoes.itcanavoidtheinfluenceoftheultimatetensilestrengthwhenstudyingthetoughnessofSFRC.Itca11beunderTable5thatthealteringregularitiesofthesetwofactorsalongwiththechangesofmatrixstrengthandfibercontentareapprmate.Therefore,theemphasisofanalysisputonthetoughnessmodulus.TherelationshipbetweenthematrixstrengthandtoughnessmodulusofSFRCwithfourkindsofsteelfiberareshowninFig.3.whosefibercontentsareall1.0%byvolume.togetherwiththatrelationshipofplainconcrete.ThetensiletoughnessofSFRCismuchbetterthanthatofplainconcrete.Thetensiletougheningeffectofsteelfiberisremarkable.Asthematrixstrengthrises.Thebrittlenessofconcreteincreasesobviously,andthenthetensiletoughnessofplainconcretefallsdown.ThisphenomenonwasalsofoundonspecimenscontainingfiberFlandF2.τ'hepu11ingoutoffiberFlfromconcreteisinfactaprocessofhook-end'sbeingstraightenedandthematrix'sbeingcrushedaroundthehook-end.Whenthehookedendisstraightenedatlast.thetensileloadfallsdownquickly.Thehighertheconcretestrength.thelargertherigidityofthematrixandtheshorterthetimethattheprocessmentionedabovelasts.Thus.thestress-straincurvefa11sdownmorequickly,andthenthetoughnessmodulusdecreases.However,thetougheningeffectoffiberFlisthebestamongthesefourkindsofsteelfiber.TheωpectratiooffiberF2istheleast。andwhenthematrixstrengthishigh,fiberbreakingoccurs.Therefore,thetoughnessmodulusfa11sdowncontinua11yasthematrixstrengthrises.ThetoughnessmoduluioffibersF3andF4risetogetherwiththematrixstrength.Boththetwokindsoffiberaresnippedandtheirsurecesecoarse.Therefore.theqictionisdominantintheproportionsofbondstress.Becausetheictionbetweenfiberandmatrixincreasesalongwiththematrixstrength,andthewholepullingoutofthesekindsofbondstatusisacontinuousprocess,therisingofmatrixstrengthplaysapositiveroleinimprovingthetoughnessofSFRCcontainingthesetwokindsoffiber.ThedifferencebetweenthetwokindsoffibersisthatfiberF3hashookedends,whichmakesfiberF3havebettertougheningeffectsthanfiberF4whenthematrixstrengthiscompativelylow(C30andC60).Whenthematrixstrengthishigh(C80),fiberbreakingimpairesthetougheningeffectoffiberF3.AndthefunctionoffiberF4exceedsthatoffiberF3inreverse.3.3Stress-straincuesofSFRCunderuniaxialtensionThepica1stress--straincurvesofSFRCunderuniaxialtensionshowninFigs.4l1(onecurveischosenforeachgroupofspecimentokeepthegraphsordly).Figs.48expressthevariationofcurvesalongwiththeincreasingofthematrixstrength,andFigs.9一11expressthevariationalongwiththechangeofthefibercontentoffiberF3.Thecurveconsistsofelasticsection.elastic-plasticsectionandfa11ingsection(soeningsection).Pointsofcontraflexureexitinthefallingsectionofthecurve.Itcanbeseenfromthesefiguresthatthematrixstrengthishigher,thestress-straincurves11downfaster,andtherisingofthefibercontentcanmuchimprovethechubbinessofthesecurves.Moreover,thetypeofsteelfiberhassomeeffectontheshapeofthestress-straincurve.ThecurvesoffiberFlaretheplumpiestofthemal1.ThesecondpeakWobservedinthecurvesoffiberFlatthestrainofabout10000ue.ThisphenomenonexpressesagoodtougheningeffectoffiberFl.ThecurvesoffibelsF2andF3areladder一likewhenthematrixstrengthishighbecauseoffiberbreaking.ThecvesoffiberF4alesmoothandlikethoseofplainconcreteinshape.Thatisbecausethepulloutprocessofsmoothsteelfiberisrathergentle.4AnalyticalInvestigationFourkinclsoftypicalsteelfiberconcretetensilestress-straincurvescanbeseen:inaxialtensionconditions,the1%dosageofthesteelfiberisfarshortofstrainhardeningoftheconcretematerialstothepointwheremostoftheexperimentalcurvesareinreachAfterthepeak,thereloadssagsection.However,asdermationincreases,therearetwocurveshaveaclearsecondpeakappeared,whiletheothertwodonot,itisthebasisofthisphenomenoncanbedividedintotwom句orcategoriesofstrengtheningandtougheningofsteelfiberreinforcedconcrete,thereasecondpeakforthetougheningcls，nosecondpeaktoenhancetheclass.Manytensilestress-strainmodelshavebeenbroughtforward一1、2、4、6、9、10，Mostoftheirrmatsesectional.takingthepeakloadasthedivisionalpoint.1nthispaper,theformulaoftherisingsectionandthatofthefallingsectionaredifferent.1ntheformulas:4.1FormulaofrisingsectionThedigitalmodelfortherisingsectionisxy-;where,fibers.(1)eparametersrelatedtothecharactersofmatrixandsteelTheboundaryconditionsareasllowing:1)X，Y=O;2)X=O,Itcanbedrawnfromtheboundary'.Formula(4)canbesimplified(2)AndthevalueofQIcanbecalculated企omexperimentaldataas:where,Eoistheorigintangentmodulus;Ep,secantmodulusatpeakload(thefrrstpeak)Thus,FormulaCallbeinvertedas:xy=63Fi11.67(3)4.2FormulaoffallingsectionThedigitalmodelforthefallingsectionis:且y=-α2(x-i)'+:c(4)where,(l~andþJarepametersrelatedtothecharactersofmatrixandsteelfibers.τ'hevalueofß~ischosenas1.7intheformulaoffallingsection9、10.theboundaryconditionX=1,y=1issatisfiedinherently.Inaddition.thevalueofacouldberegressedwiththemethodofleastsquaresas:(9)itcanbeseenontheexpressionthattheeffectsofthematrixsengthandfibercontentonthecue'sfallingrateareopposite.5.ComparisonofPredictionsandExperirnentalResultsThecomparisonofpredictionsandexperimentalresultsforstress-straincurvesareshowninFig.12(takethecurvesofF3--6010asanexample).Thetheoreticalcurveandtheexperimentalonesfitwel1.6.Conclusionsa)enthematrixstrengthincreases,theratiosofcrackstressesofSFRC(withthesametypeoffiber)tothoseofplainconcreteoneswiththesaii3emproportioneinvariable.Theseratiosofultirnatetensilestrengthsvarydissimilarlyaccordingtothetypeofsteelfiber.Moreover,theincrementsalebiggerthanthoseofcracksessandareinfluencedbyfibertype.b)Asthefibercontentincreases.thecrackstressandultimatetensilestrengthofSFRCirnproveobviouslyandtheeffectofthefibercontentonthecharactersoftensilestrengthofSFRCispositiveandconsistent.c)ThecrackstrainorthestrainsatpeaktensileloadOfSFRCaremuchbiggerthantheseofplainconcrete.Inaddition,theincrementsgoupasthematrixstrengthorthefibercontentmcreases.d)AtensiletoughnessmoduluswasintroducedωevaluatethetoughnesscharactersofSFRCunderuniaxialtension.ThetensiletoughnessofSFRCismuchbetterthanthatofplainconcrete.Inaddition.isinfluencedbythematrixstrengthandcharactersofsteelfiber.e)Thematrixstrengthishigher,thestress-straincurvesfalldownfaster.Otherwise,therisingofthefibercontentcanmuchirnprovethechubbinessofthesecurves.Moreover.thetypeofsteelfiberhassomeeffectontheshapeofthestress-straincurve.。Theformulaofthetensilestress-straincveofSFRCwasregressed.Thetheoreticalcurveandtheexperimentalonesfitwel1.T}1ismodelmaybehelpfulinthefurtherresearchofSFRCunderuniaxialtension.2EarthworkBecauseearthmovingmethodsandcostschangemorequicklythanthoseinanyotherbranchofcivilengineering,thisisafieldwheretherearerealopportunitiesfortheenthusiast.In1935mostofthemethodsnowinuseforcarryingandexcavatingearthwithrubber-tyredequipmentdidnotexist.Mostearthwasmovedbynarrowrailtrack,nowrelativelyrare,andthemainmethodsofexcavation,withfaceshovel,backacter,ordraglineorgrab,thoughtheyarestillwidelyusedareonlyafewofthemanycurrentmethods.Tokeephisknowledgeofearthmovingequipmentuptodateanengineermustthereforespendtinestudyingmodernmachines.Generallytheonlyreliableup-to-dateinformationonexcavators,loadersandtransportisobtainablefromthemakers.Earthworksorearthmovingmeanscuttingintogroundwhereitssurfaceistoohigh(cuts),anddumpingtheearthinotherplaceswherethesurfaceistoolow(fills).Toreduceearthworkcosts,thevolumeofthefillsshouldbeequaltothevolumeofthecutsandwhereverpossiblethecutsshouldbeplacedneartofillsofequalvolumesoastoreducetransportanddoublehandlingofthefill.Thisworkofearthworkdesignfallsontheengineerwholaysouttheroadsinceitisthelayoutoftheearthworkmorethananythingelsewhichdecidesitscheapness.Fromtheavailablemapsahdlevels,theengineeringmusttrytoreachasmanydecisionsaspossibleinthedrawingofficebydrawingcrosssectionsoftheearthwork.Onthesitewhenfurtherinformationbecomesavailablehecanmakechangesinjissectionsandlayout,butthedrawinglfficeworkwillnothavebeenlost.Itwillhavehelpedhimtoreachthebestsolutionintheshortesttime.Thecheapestwayofmovingearthistotakeitdirectlyoutofthecutanddropitasfillwiththesamemachine.Thisisnotalwayspossible,butwhenitcanbedoneitisideal,beingbothquickandcheap.Draglines,bulldozersandfaceshovelsandothis.Thelargestradiusisobtainedwiththedragline,andthelargesttonnageofearthismovedbythebulldozer,thoughonlyovershortdistances.Thedisadvantagesofthedraglinearethatitmustdigbelowitself,itcannotdigwithforceintocompactedmaterial,itcannotdigonsteepslopws,anditsdumpinganddiggingarenotaccurate.Faceshovelsarebetweenbulldozersanddraglines,havingalargerradiusofactionthanbulldozersbutlessthandraglines.Theyareanletodigintoaverticalclifffaceinawaywhichwouldbedangeroustorabulldozeroperatorandimpossibleforadragline.Eachpieceofequipmentshouldbeleveloftheirtracksandfordeepdigsincompactmaterialabackacterismostuseful,butitsdumpingradiusisconsiderablylessthanthatofthesameescavatorfittedwithafaceshovel.Rubber-tyredbowlscrapersareindispensableforfairlyleveldiggingwherethedistanceoftransportistoomuchtoradraglineorfaceshovel.Theycandigthematerialdeeply(butonlybelowthemselves)toafairlyflatsurface,carryithundredsofmetersifneedbe,thendropitandlevelitroughlyduringthedumping.Forharddiggingitisoftenfoundeconomicaltokeepapushertractor(wheeledortracked)onthediggingsite,topusheachscraperasitreturnstodig.Assoonasthescraperisfull,thepushertractorreturnstothebeginningofthedigtoheoptohelpthenestscraper.Bowlscrapersareoftenextremelypowerfulmachines;manymakersbuildscrapersof8cubicmetersstruckcapacity,whichcarry10m³heaped.Thelargestself-propelledscrapersareof19m³struckcapacity(25m³heaped)andtheyaredrivenbyatractorengineof430horse-powers.Dumpersareprobablythecommonestrubber-tyredtransportsincetheycanalsoconvenientlybeusedforcarryingconcreteorotherbuildingmaterials.Dumpershavetheearthcontaineroverthefrontaxleonlargerubber-tyredwheels,andthecontainertipsforwardsonmosttypes,thoughinarticulateddumpersthedirectionoftipcanbewidelyvaried.Thesmallestdumpershaveacapacityofabout0.5m³,andthelargeststandardtypesareofabout4.5m³.Specialtypesincludetheself-loadingdumperofupto4m³andthearticulatedtypeofabout0.5m³.Thedistinctionbetweendumpersanddumptrucksmustberemembered.dumperstipforwardsandthedriversitsbehindtheload.Dumptrucksareheavy,strengthenedtippinglorries,thedrivertravelsinfrontlftheloadandtheloadisdumpedbehindhim,sotheyaresometimescalledrear-dumptrucks.3SafetyofStr