土木工程类专业英文文献及翻译.docx

土木工程类专业英文文献及翻译.docx

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土木工程类专业英文文献及翻译

土木工程类专业英文文献及翻译

PAVEMENTPROBLEMSCAUSED

BYCOLLAPSIBLESUBGRADES

BySandraL.Houston,1AssociateMember,ASCE

（ReviewedbytheHighwayDivision）

ABSTRACT:

Problemsubgradematerialsconsistingofcollapsiblesoilsarecom-

moninaridenvironments,whichhaveclimaticconditionsanddepositionaland

weatheringprocessesfavorabletotheirformation.Includedhereinisadiscussion

ofpredictivetechniquesthatusecommonlyavailablelaboratoryequipmentand

testingmethodsforobtainingreliableestimatesofthevolumechangeforthese

problemsoils.Amethodforpredictingrelevantstressesandcorrespondingcollapse

strainsfortypicalpavementsubgradesispresented.Relativelysimplemethodsof

evaluatingpotentialvolumechange,basedonresultsoffamiliarlaboratorytests,

areused.

INTRODUCTION

Whenasoilisgivenfreeaccesstowater,itmaydecreaseinvolume,

increaseinvolume,ordonothing.Asoilthatincreasesinvolumeiscalled

aswellingorexpansivesoil,andasoilthatdecreasesinvolumeiscalleda

collapsiblesoil.Theamountofvolumechangethatoccursdependsonthe

soiltypeandstructure,theinitialsoildensity,theimposedstressstate,and

thedegreeandextentofwetting.Subgradematerialscomprisedofsoilsthat

changevolumeuponwettinghavecauseddistresstohighwayssincethebe-

ginningoftheprofessionalpracticeandhavecostmanymillionsofdollars

inroadwayrepairs.Thepredictionofthevolumechangesthatmayoccurin

thefieldisthefirststepinmakinganeconomicdecisionfordealingwith

theseproblemsubgradematerials.

Eachprojectwillhavedifferentdesignconsiderations,economiccon-

straints,andriskfactorsthatwillhavetobetakenintoaccount.However,

withareliablemethodformakingvolumechangepredictions,thebestdesign

relativetothesubgradesoilsbecomesamatterofeconomiccomparison,and

amuchmorerationaldesignapproachmaybemade.Forexample,typical

techniquesfordealingwithexpansiveclaysinclude:

（1）Insitutreatments

withsubstancessuchaslime,cement,orfly-ash;

（2）seepagebarriersand/

ordrainagesystems;or（3）acomputingoftheserviceabilitylossandamod-

ificationofthedesignto"accept"theanticipatedexpansion.Inordertomake

themosteconomicaldecision,theamountofvolumechange（especiallynon-

uniformvolumechange）mustbeaccuratelyestimated,andthedegreeofroad

roughnessevaluatedfromthesedata.Similarly,alternativedesigntechniques

areavailableforanyroadwayproblem.

Theemphasisherewillbeplacedonpresentingeconomicalandsimple

methodsfor:

（1）Determiningwhetherthesubgradematerialsarecollapsible;

and

（2）estimatingtheamountofvolumechangethatislikelytooccurinthe

'Asst.Prof.,Ctr.forAdvancedRes.inTransp.,ArizonaStateUniv.,Tempe,AZ

85287.

Note.DiscussionopenuntilApril1,1989.Toextendtheclosingdateonemonth,

awrittenrequestmustbefiledwiththeASCEManagerofJournals.Themanuscript

forthispaperwassubmittedforreviewandpossiblepublicationonFebruary3,1988.

ThispaperispartoftheJournalofTransportation.Engineering,Vol.114,No.6,

November,1988.ASCE,ISSN0733-947X/88/0006-0673/$1.00+$.15perpage.

PaperNo.22902.

673

fieldforthecollapsiblesoils.Thenthisinformationwillplacetheengineer

inapositiontomakearationaldesigndecision.Collapsiblesoilsarefre-

quentlyencounteredinanaridclimate.Thedepositionalprocessandfor-

mationofthesesoils,andmethodsforidentificationandevaluationofthe

amountofvolumechangethatmayoccur,willbediscussedinthefollowing

sections.

COLLAPSIBLESOILS

FormationofCollapsibleSoils

Collapsiblesoilshavehighvoidratiosandlowdensitiesandaretypically

cohesionlessoronlyslightlycohesive.Inanaridclimate,evaporationgreatly

exceedsrainfall.Consequently,onlythenear-surfacesoilsbecomewetted

fromnormalrainfall.Itisthecombinationofthedepositionalprocessand

theclimateconditionsthatleadstotheformationofthecollapsiblesoil.

Althoughcollapsiblesoilsexistinnondesertregions,thedryenvironmentin

whichevaporationexceedsprecipitationisveryfavorablefortheformation

ofthecollapsiblestructure.

Asthesoildriesbyevaporation,capillarytensioncausestheremaining

watertowithdrawintothesoilgraininterfaces,bringingwithitsolublesalts,

clay,andsiltparticles.Asthesoilcontinuestodry,thesesalts,clays,and

siltscomeoutofsolution,and"tack-weld"thelargergrainstogether.This

leadstoasoilstructurethathashighapparentstrengthatitslow,natural

watercontent.However,collapseofthe"cemented"structuremayoccur

uponwettingbecausethebondingmaterialweakensandsoftens,andthesoil

isunstableatanystresslevelthatexceedsthatatwhichthesoilhadbeen

previouslywetted.Thus,iftheamountofwatermadeavailabletothesoil

isincreasedabovethatwhichnaturallyexists,collapsecanoccuratfairly

lowlevelsofstress,equivalentonlytooverburdensoilpressure.Additional

loads,suchastrafficloadingorthepresenceofabridgestructure,addto

thecollapse,especiallyofshallowcollapsiblesoil.Thetriggeringmechanism

forcollapse,however,istheadditionofwater.

HighwayProblemsResultingfromCollapsibleSoils

Nonuniformcollapsecanresultfromeitheranonhomogeneoussubgrade

depositinwhichdifferingdegreesofcollapsepotentialexistand/orfrom

nonuniformwettingofsubgradematerials.Whendifferentialcollapseof

subgradesoilsoccurs,theresultisarough,wavysurface,andpotentially

manymilesofextensivelydamagedhighway.Therehavebeenseveralre-

portedcasesforwhichdifferentialcollapsehasbeencitedasthecauseof

roadwayorhighwaybridgedistress.AfewoftheseintheArizonaandNew

Mexicoregionincludesectionsof1-10nearBenson,Arizona,andsections

of1-25inthevicinityofAlgadonas,NewMexico（Lovelaceetal.1982;

Russman1987）.Inadditiontotheexcessivewavinessoftheroadwaysur-

face,bridgefoundationsfailures,suchastheSteinsPassHighwaybridge,

1-10,inArizona,havefrequentlybeenidentifiedwithcollapseoffoundation

soils.

IdentificationofCollapsibleSoils

Therehavebeenmanytechniquesproposedforidentifyingacollapsible

soilproblem.Thesemethodsrangefromqualitativeindextestsconductedon

674

disturbedsamples,toresponsetowettingtestsconductedonrelativelyun-

disturbedsamples,toinsitumeausrementtechniques.Inallcases,theen-

gineermustfirstknowifthesoilsmaybecomewettedtoawatercontent

abovetheirnaturalmoisturestate,andifso,whattheextentofthepotential

wettedzonewillbe.Mostmethodsforidentifyingcollapsiblesoilsareonly

qualitativeinnature,providingnoinformationonthemagnitudeofthecol-

lapsestrainpotential.Thesequalitativemethodsarebasedonvariousfunc-

tionsofdrydensity,moisturecontent,voidratio,specificgravity,andAt-

terberglimits.

Insitumeasurementmethodsappearpromisinginsomecases,inthatmany

researchersfeelthatsampledisturbanceisgreatlyreduced,andthatamore

nearlyquantitativemeasureofcollapsepotentialisobtainable.However,

insitutestmethodsforcollapsiblesoilstypicallysufferfromthedeficien-

cyofanunknownextentanddegreeofwettingduringthefieldtest.This

makesaquantitativemeasurementdifficultbecausethezoneofmaterial

beinginfluencedisnotwell-known,and,therefore,theactualstrains,in-

ducedbytheadditionofstressandwater,arenotwell-known.Inaddition,

thedegreeofsaturationachievedinthefieldtestisvariableandusually

unknown.

Basedonrecentlyconductedresearch,itappearsthatthemostreliable

methodforidentifyingacollapsiblesoilproblemistoobtainthebestquality

undisturbedsamplepossibleandtosubjectthissampletoaresponsetowet-

tingtestinthelaboratory.Theresultsofasimpleoedometertestwillindicate

whetherthesoiliscollapsibleand,atthesametime,giveadirectmeasure

oftheamountofcollapsestrainpotentialthatmayoccurinthefield.Potential

problemsassociatedwiththedirectsamplingmethodincludesampledistur-

banceandthepossibilitythatthedegreeofsaturationachievedinthefield

willbelessthanthatachievedinthelaboratorytest.

Thequalityofanundisturbedsampleisrelatedmoststronglytothearea

ratioofthetubethatisusedforsamplecollection.Thearearatioisameasure

oftheratioofthecross-sectionalareaofthesamplecollectedtothecross-

sectionalareaofthesampletube.Athin-walledtubesamplerbydefinition

hasanarearatioofabout10-15%.Althoughundisturbedsamplesarebest

obtainedthroughtheuseofthin-walledtubesamplers,itfrequentlyoccurs

thatthesestiff,cementedcollapsiblesoils,especiallythosecontaininggravel,

cannotbesampledunlessatubewithamuchthickerwallisused.Samplers

havinganarearatioasgreatas56%arecommonlyusedforArizonacol-

lapsiblesoils.Further,itmaytakeconsiderablehammeringofthetubeto

drivethesample.Theresultis,ofcourse,somedegreeofsampledistur-

bance,broken.bonds,densification,andacorrespondinglyreducedcollapse

measureduponlaboratorytesting.However,forcollapsiblesoils,whichare

compressivebydefinition,theinsertionofthesampletubeleadstolocal

shearfailureatthebaseofthecuttingedge,and,therefore,thereisless

sampledisturbancethanwouldbeexpectedforsoilsthatexhibitgeneralshear

failure（i.e.,saturatedclaysordilativesoils）.Resultsofanongoingstudy

ofsampledisturbanceforcollapsiblesoilsindicatethatblocksamp