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土木工程类专业英文文献及翻译PAVEMENTPROBLEMSCAUSEDBYCOLLAPSIBLESUBGRADESBySandraL.Houston,1AssociateMember,ASCE(ReviewedbytheHighwayDivision)ABSTRACT:Problemsubgradematerialsconsistingofcollapsiblesoilsarecom-moninaridenvironments,whichhaveclimaticconditionsanddepositionalandweatheringprocessesfavorabletotheirformation.Includedhereinisadiscussionofpredictivetechniquesthatusecommonlyavailablelaboratoryequipmentandtestingmethodsforobtainingreliableestimatesofthevolumechangefortheseproblemsoils.Amethodforpredictingrelevantstressesandcorrespondingcollapsestrainsfortypicalpavementsubgradesispresented.Relativelysimplemethodsofevaluatingpotentialvolumechange,basedonresultsoffamiliarlaboratorytests,areused.INTRODUCTIONWhenasoilisgivenfreeaccesstowater,itmaydecreaseinvolume,increaseinvolume,ordonothing.Asoilthatincreasesinvolumeiscalledaswellingorexpansivesoil,andasoilthatdecreasesinvolumeiscalledacollapsiblesoil.Theamountofvolumechangethatoccursdependsonthesoiltypeandstructure,theinitialsoildensity,theimposedstressstate,andthedegreeandextentofwetting.Subgradematerialscomprisedofsoilsthatchangevolumeuponwettinghavecauseddistresstohighwayssincethebe-ginningoftheprofessionalpracticeandhavecostmanymillionsofdollarsinroadwayrepairs.Thepredictionofthevolumechangesthatmayoccurinthefieldisthefirststepinmakinganeconomicdecisionfordealingwiththeseproblemsubgradematerials.Eachprojectwillhavedifferentdesignconsiderations,economiccon-straints,andriskfactorsthatwillhavetobetakenintoaccount.However,withareliablemethodformakingvolumechangepredictions,thebestdesignrelativetothesubgradesoilsbecomesamatterofeconomiccomparison,andamuchmorerationaldesignapproachmaybemade.Forexample,typicaltechniquesfordealingwithexpansiveclaysinclude:(1)Insitutreatmentswithsubstancessuchaslime,cement,orfly-ash;(2)seepagebarriersand/ordrainagesystems;or(3)acomputingoftheserviceabilitylossandamod-ificationofthedesigntoaccepttheanticipatedexpansion.Inordertomakethemosteconomicaldecision,theamountofvolumechange(especiallynon-uniformvolumechange)mustbeaccuratelyestimated,andthedegreeofroadroughnessevaluatedfromthesedata.Similarly,alternativedesigntechniquesareavailableforanyroadwayproblem.Theemphasisherewillbeplacedonpresentingeconomicalandsimplemethodsfor:(1)Determiningwhetherthesubgradematerialsarecollapsible;and(2)estimatingtheamountofvolumechangethatislikelytooccurinthe'Asst.Prof.,Ctr.forAdvancedRes.inTransp.,ArizonaStateUniv.,Tempe,AZ85287.Note.DiscussionopenuntilApril1,1989.Toextendtheclosingdateonemonth,awrittenrequestmustbefiledwiththeASCEManagerofJournals.ThemanuscriptforthispaperwassubmittedforreviewandpossiblepublicationonFebruary3,1988.ThispaperispartoftheJournalofTransportation.Engineering,Vol.114,No.6,November,1988.ASCE,ISSN0733-947X/88/0006-0673/$1.00+$.15perpage.PaperNo.22902.673fieldforthecollapsiblesoils.Thenthisinformationwillplacetheengineerinapositiontomakearationaldesigndecision.Collapsiblesoilsarefre-quentlyencounteredinanaridclimate.Thedepositionalprocessandfor-mationofthesesoils,andmethodsforidentificationandevaluationoftheamountofvolumechangethatmayoccur,willbediscussedinthefollowingsections.COLLAPSIBLESOILSFormationofCollapsibleSoilsCollapsiblesoilshavehighvoidratiosandlowdensitiesandaretypicallycohesionlessoronlyslightlycohesive.Inanaridclimate,evaporationgreatlyexceedsrainfall.Consequently,onlythenear-surfacesoilsbecomewettedfromnormalrainfall.Itisthecombinationofthedepositionalprocessandtheclimateconditionsthatleadstotheformationofthecollapsiblesoil.Althoughcollapsiblesoilsexistinnondesertregions,thedryenvironmentinwhichevaporationexceedsprecipitationisveryfavorablefortheformationofthecollapsiblestructure.Asthesoildriesbyevaporation,capillarytensioncausestheremainingwatertowithdrawintothesoilgraininterfaces,bringingwithitsolublesalts,clay,andsiltparticles.Asthesoilcontinuestodry,thesesalts,clays,andsiltscomeoutofsolution,andtack-weldthelargergrainstogether.Thisleadstoasoilstructurethathashighapparentstrengthatitslow,naturalwatercontent.However,collapseofthecementedstructuremayoccuruponwettingbecausethebondingmaterialweakensandsoftens,andthesoilisunstableatanystresslevelthatexceedsthatatwhichthesoilhadbeenpreviouslywetted.Thus,iftheamountofwatermadeavailabletothesoilisincreasedabovethatwhichnaturallyexists,collapsecanoccuratfairlylowlevelsofstress,equivalentonlytooverburdensoilpressure.Additionalloads,suchastrafficloadingorthepresenceofabridgestructure,addtothecollapse,especiallyofshallowcollapsiblesoil.Thetriggeringmechanismforcollapse,however,istheadditionofwater.HighwayProblemsResultingfromCollapsibleSoilsNonuniformcollapsecanresultfromeitheranonhomogeneoussubgradedepositinwhichdifferingdegreesofcollapsepotentialexistand/orfromnonuniformwettingofsubgradematerials.Whendifferentialcollapseofsubgradesoilsoccurs,theresultisarough,wavysurface,andpotentiallymanymilesofextensivelydamagedhighway.Therehavebeenseveralre-portedcasesforwhichdifferentialcollapsehasbeencitedasthecauseofroadwayorhighwaybridgedistress.AfewoftheseintheArizonaandNewMexicore