路桥工程专业英语第三课

第三课：TheSubgradeDesignandConstructionTechnology路基设计与施工技术Text:TheSubgradeDesignReadingMaterial:HighwaySubgradeConstructionTextThesubgradeiscomprisedoftheuppermostmaterialsplacedintheroadbedembankmentorthesoilremainingatthebaseofacut.Thesubgradesoilisofenreferredtoasthefoundationorroadbedsoil.Thisfoundationcomponentisusuallyconstructedofnativeinorganicsoilofencombinationwithimportedsoilsfromselectborrowsources,andwouldbecompactedtoaspecifieddensityandmoisturecontent.1SoilClassificationThebasiccomponentsofsoilsaredifferentiatedonthebasisofgrainsizeasfollows:Cobbles﹥75mmSilt0.075mmto0.002mmGravel75mmto5mmClay﹤0.002mmSand5mmto0.075mmFinegrainedsoilsaredefinedasmaterialshavingmorethan50percentofthedrymasssmallerthanthe0.075mmparticalssize.Althoughsizelimitationsarearbitrary,suchlimitationsallowstandardizationbydefinition.Itisnecessarytounderstandaswellthatplasticityisanextremelyimportantpropertytodifferentiatebetweensiltandclay,andtopredictbehaviour.Thepavementdesignengineerismostinterestedinthestrengthofthesoilandtheextenttowhichthisstrengthvarieswithclimate,environmentandeffects.Atypicalsoilprofileforhighwaydesignpurposesconsistsofthreehorizons.Thesurfaceor“A”horizonmaterialswillconsistoforganicsoils,followedby“B”horizonwhichisasemi-weatheredzonefollowedby“C”horizonwhichrepresentstheparentmaterialtype.Forallnewconstructionitisveryimportantthatin-situmoisturecontents,Atterberglimitsandgrainsizeanalysisofsubgradesoilmaterialsbedeterminedtoassesssubgradesoilcharacteristicsandtoinferresilientmodulus(MR)values.2SubgradeStrengthEvaluationThecharacteristicmaterialpropertyofsubgradesoilsusedforpavementdesignistheresilientmodulus(MR).Theresilientmodulusisdefinedasbeingameasureoftheelasticpropertyofasoilrecognizingselectednon-linearcharacteristics.MethodsforthedeterminationofMRaredescribedinAASHTOT294-92testmethod.Formanyyears,standardCaliforniaBearingRatio(CBR)testswereutilizedtomeasurethesubgradestrengthparameterasadesigninput.Forroadbedmaterials,theAASHTOGuide[AASHTO93]recommendsthattheresilientmodulusbeestablishedbasedonlaboratorytestingofrepresentativesamplesinstressandmoistureconditionssimulatingtheprimarymoistureseasons.Alternatively,theseasonalresilientmodulusvaluesmaybedeterminedbasedoncorrelationswithsoilproperties.Sincetheresilientmodulustestequipmentiscurrentlynotpresentinmanylaboratories,researchershavedevelopedcorrelationstoconvertingCBRvaluestoapproximateMRvalues.ThecorrelationconsideredreasonableforfinegrainedsoilswithsoakedCBRof10orlessis:MR(MPa)=10.3×(CBR)[AASHTO93]Since1991,AT&UhasusedtheFallingWeightDeflectometer(FWD)toobtaindeflectiondata.ForthepurposesofthisManual,thebackcalculatedsubgrademodulusisusedtorepresentthein-situsubgraderesilientmoduluswhichinturnisaninputforthedesignoffinalstagepavementsandoverlays.Forthedesignofnewconstructionpavementstructures,thesubgraderesilientmodulusisestimatedusinganexistingrepresentativeroadwaylocatednearthenewproject,withsimilarsubgradesoilsanddrainageconditions,asaprototype.TheprototypecanbetestedwiththeFWDandthedeflectiondataanalyzedwithcomputerprogramtodeterminethebackcalculatedsubgrademodulus.Thisvaluecanthenbeusedasanapproximationofthestrengthofthesubgradematerialsthatwouldexistinthenewsubgrade.3SeasonalVariationsOneofthemostcriticalconditionsthatdevelopsinaseasonalfrostareasuchasAlbertaistheweakeningofthesubgradeduringthespringthawperiod.Thisweakeningresultsfromthemeltingoficesegregationwithinthesubgradesoilsand,toalesserextent,duetohighermoisturecontentsduringthisperiodassociatedwithreduceddrainage.Seasonalvariationsofsubgradestrengthisadifficultfactortomodel.Thetaskisoneofdeterminingtheextenttowhichthesubgradestrengthisreducedduringorimmediatelyfollowingthethawperiod.Nevertheless,theseasonalvariationsmodelisimportanttothestructuraldesignofpavements.AstudyofsubgradestrengthseasonalvariationswasconductedbyAT&Ufrom1989to1994.ThisstudywasbasedonFWDdeflectiontestingofseveralpavementtestsectionsandsubsequentanalysisusingtheELMODcomputerprogram.Althoughthestrengthreductionswerecontingentuponseveralfactors,formanysubgradesthereductionwasupto50percentofthesummerstrength.Pavementswithcementstabilizedbasecoursesgenerallyindicatedlessreductioninsubgradestrength.Forfull-depthasphaltconcretepavementsthereductionwassimilartothatofgranularbasepavementsbuttheperiodofminimumsubgradestrengthoccurredinthelatespringtoearlysummerratherthanearlyspringasfortheotherbasetypes.Asaresultofthisresearch,subgradeseasonalvariationfactorsweredevelopedthatwereusedintheELMODanalysis.TheAASHTOGuide[AASHTO93]outlinesguidelinesfordeterminingtheseasonalvariationofthesubgrademodulusbasedeitheruponlaboratoryresilientmodulustestingorfrombackcaculatedmoduldeterminedfromFWDdeflectiondata.Aprocedureisdescribedwhichallowstheeffectiveroadbedsoilresilientmodulustobedeterminedbasedontheestimatedrelativedamagethatcorrespondstotheseasonallyadjustedsubgrademodulusforeachmonthoftheyear.ForFWDtestingperformedduringthemonthsofJunetoOctober,thecalculatedEffectiveRoadbedSoilresilientModuluswasapproximately10perc