土木工程毕业设计外文翻译21913766.docx
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土木工程毕业设计外文翻译21913766.docx
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土木工程毕业设计外文翻译21913766
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DESIGNANDEXECUTIONOFGROUNDINVESTIGATIONFOREARTHWORKS
PAULQUIGLEY,FGS
IrishGeotechnicalServicesLtd
ABSTRACT
Thedesignandexecutionofgroundinvestigationworksforearthworkprojectsoutlineofgroundinvestigationmethodswhichcanaugment‘traditionalinvestigationmethods’particularlyforglacialtillboulderclaysoilsispresented.Theissueof‘geotechnicalcertification’israisedandrecommendationsoutlinedonitsmeritsforincorporationwithgroundinvestigationsandearthworks.
1.INTRODUCTION
Theinvestigationandre-useevaluationofmanyIrishboulderclaysoilspresentsdifficultiesforboththegeotechnicalengineerandtheroaddesignengineer.Theseglacialtillorboulderclaysoilsaremainlyoflowplasticityandvaryingproportionsofsand,gravel,cobblesandbouldersinaclayorsiltmatrix.Theamountoffinesgovernstheirbehaviourandthesiltcontentmakesitveryweathersusceptible.
Moisturecontentscanbeboulderclayupto20-25%forMidland,South-WestandNorth-Westlightgreyboulderclaydeposits.Theabilityofboulderclaysoilstotake-infreewateriswellestablishedandpoorplanningofearthworksoftenamplifiesthis.
Thefinesoilconstituentsaregenerallysensitivetosmallincreasesinmoisturecontentwhichoftenleadtolossinstrengthandrenderthesoilsunsuitableforre-useasengineeringfill.Manyofourboulderclaysoils(especiallythosewithintermediatetypesiltsandfinesandmatrix)rejectedattheselectionstage,butgoodplanningshowsthattheycaninfactfulfilspecificationrequirementsintermsofcompactionandstrength.
Theselectionprocessshouldaimtomaximisetheuseoflocallyavailablesoilsandwithcarefulevaluationitispossibletouseorincorporate‘poorormarginalsoils’withinfillareasandembankments.Fillmaterialneedstobeplacedatamoisturecontentsuchthatitisneithertoowettobestableandtrafficableortoodrytobeproperlycompacted.
HighmoisturecontentlowstrengthboulderclaysoilscanbesuitableforuseasfillinlowistoclassifythesuitabilityofthesoilsinaccordancewithTable6.1fromSeries600oftheNRASpecificationforRoadWorks(SRW),March2000.Themajorityofcurrentgroundinvestigationsforroadworksincludesacombinationofthefollowingtogivetherequiredgeotechnicaldata:
▪Trialpits
▪Cablepercussionboreholes
▪Dynamicprobing
▪Rotarycoredrilling
In-situtesting(SPT,variableassessingsoilsuitabilityfromdeepcutareas.Cablepercussionboreholesarenormallysunktoadesireddepthor‘refusal’withdisturbedandundisturbedsamplesrecoveredat1.00mintervalsorchangeofstrata.
Inmanyinstances,cablepercussionboringisunabletopenetratethroughverystiff,boreholesshouldbepreventedandlossoffinesiscommon,invariablythisleadstoinaccurateclassification.
Trialpitsareconsideredmoreappropriateforrecoveringappropriatesizesamplesandforobservingtheproportionofclaststomatrixandsizesofcobbles,boulders.Detailedandaccuratefielddescriptionsarethereforevitalforcutareasandtrialpitsprovideanopportunitytoexaminethesoilsonalargerscalethanboreholes.Trialpitsalsoprovideaninsightontrenchstabilityandtoobservewateringressanditseffects.
Asuitablyexperiencedgeotechnicalengineerorengineeringgeologistshouldsupervisethetrialpittingworksandrecoveryofsamples.Thecharacteristicsofthesoilsduringtrialpitexcavationshouldbecloselyobservedasthisprovidesinformationonsoilsensitivity,especiallyifwaterfromgranularzonesmigratesintothefinematrixmaterial.Veryoften,theconditionofsoilonthesidesofanexcavationprovidesamoreaccurateassessmentofitsin-situcondition.
3.SOILCLASSIFICATION
SoildescriptionandclassificationshouldbeundertakeninaccordancewithBS5930(1999)andtestedinaccordancewithBS1377(1990).Theengineeringdescriptionofasoilisbasedonitsparticlesizegrading,supplementedbyplasticityforfinesoils.Formanyofourglacialtill,boulderclaysoils(i.e.‘mixedsoils’)difficultiesarisewithdescriptionsandassessingengineeringperformancetests.
Asoutlinedpreviously,Irishboulderclaysusuallycompriseasiltorclaymatrix.Lowplasticitysoilswithfinescontentsofaround10to15%oftenpresentthemostdifficulties.BS5930(1999)nowrecognisesthesedifficultiesindescribing‘mixedsoils’–thefinesoilconstituentswhichgoverntheengineeringbehaviournowtakespriorityoverparticlesize.
Lowplasticityboulderclaysoilsofintermediatepermeability(i.e.Koftheorderof10-5to10-7ms)canoftenbe‘conditioned’bydrainagemeasures.Thisusuallyentailstheinstallationofperimeterdrainsandsumpsatcutareasorborrowpitssoastoreducethemoisturecontent.Hence,withsmallreductioninmoisturecontent,difficultglacialtillsoilscanbecomesuitableasengineeringfill.
4.ENGINEERINGPERFORMANCETESTINGOFSOILS
Laboratorytestingisverymuchdictatedbytheproposedend-useforthesoils.TheengineeringparameterssetoutinTable6.1pftheNRASRWincludeacombinationofthefollowing:
▪Moisturecontent
▪Particlesizegrading
▪PlasticLimit
▪CBR
▪Compaction(relatingtooptimumMC)
▪Remouldedundrainedshearstrength
Anumberofkeyfactorsshouldbeborneinmindwhenschedulinglaboratorytesting:
▪CompactionCBRMCVtestsarecarriedouton<20mmsizematerial.
▪Moisturecontentvaluesshouldrelateto<20mmsizematerialtoprovideavalidcomparison.
▪Porepressuresarenottakenintoaccountduringcompactionandmayvaryconsiderablybetweenlaboratoryandfield.
▪Preparationmethodsforsoiltestingmustbeclearlystipulatedandagreedwiththedesignatedlaboratory.
Greatcaremustbetakenwhendeterminingmoisturecontentofboulderclaysoils.Ideally,themoisturecontentshouldberelatedtotheparticlesizeand,althoughthisisnotalwayspractical.
Inthemajorityofcases,theMCVwhenusedwithcompactiondataisconsideredtoofferthebestmethodofestablishing(andchecking)thesuitabilitycharacteristicsofaboulderclaysoil.MCVtestingduringtrialpittingisstronglyrecommendedasitprovidesarapidassessmentofthesoilsuitabilitydirectlyafterexcavation.MCVcalibrationcanthenbecarriedoutinthelaboratoryatvariousmoisturecontentincrements.SampledisturbancecanoccurduringtransportationtothelaboratoryandthiscantheresultantMCV’s.
IGSLperformingMCV’sinthefieldonlowplasticityboulderclayswiththosecarriedoutlaterinthelaboratory(2to7days).ManyoftheaforementionedlowplasticityboulderclaysoilsexhibittimedependantbehaviourwithsignificantlydifferentMCV’srecordedatalaterdate–increasedvaluescanbeduetothedrainageofthematerialfollowingsampling,transportationandstoragewhiledilatancyandmigrationofwaterfromgranularlensescanleadtodeteriorationandlowervalues.
Thistypeofinformationisimportanttoboththedesignerandearthworkscontractorasitprovidesanopportunitytounderstandthepropertiesofthesoilswhentestedasoutlinedabove.Itcanalsoillustratetheadvantagesofpre-draininginsomeinstances.Withmixedsoils,faceexcavationmaybenecessarytoacceleratedrainageworks.
CBRtestingofboulderclaysoilsalsoneedscarefulconsideration,mainlywiththepreparationmethodemployed.Designengineersneedtobeawareofthis,asitcanorderofmagnitudedifferenceinresults.Staticcompactionofboulderclaysoilsisadvisedascompactionwiththe2.5or4.5kgrammeroftenleadstoresult.Also,curingofcompactedboulderclaysamplesisimportantasthisallowsexcessporewaterpressurestodissipate.
5.ENGINEERINGCLASSIFICATIONOFSOILS
InaccordancewiththeNRASRW,generalcohesivefilliscategorisedinTable6.1asfollows:
▪2AWetcohesive
▪2BDrycohesive
▪2CStonycohesive
▪2DSiltycohesive
Thematerialpropertiesrequiredforacceptabilityaregivenandthedesignengineerthendeterminestheupperandlowerboundlimitsonthebasisofthelaboratoryclassificationandengineeringperformancetests.IrishboulderclaysoilsarepredominantlyClass2C.
Clause612oftheSRWsetsoutcompactionmethods.Twoproceduresareavailable:
▪MethodCompaction
▪End-ProductCompaction
Endproductcompactionisconsideredmorepractical,especiallywhengoodcompactioncontroldatabecomesavailableduringtheearlystagesofanearthworkscontract.AminimumTargetDryDensity(TDD)isconsideredveryusefulforthecontractortoworkwithasameansofcheckingcompactionquality.Oncethematerialapprovedandmeetstheacceptabilitylimits,thenin-situdensitycanbemeasured,preferablybynucleargaugeorsandreplacementtestswherethestonecontentislow.
Asplacingandcompactionofthefillprogresses,thein-situTDDcanbecheckedandnon-conformingareasquicklyrecognisedandcorrectiveactiontaken.Thisprocessrequiresthedesignengineertoreviewthefielddensitieswiththelaboratorycompactionplotsandevaluateactualwith‘theoreticaldensities’.
6.SUPPLEMENTARYGROUNDINVESTIGATIONMETHODSFOREARTHWORKS
ThemoretraditionalmethodsandproceduresoutlinedinSection2.Thefollowingareexamplesofmethodswhicharebelievedtoenhancegroundinvestigationworksforroadprojects:
▪Phasingthegroundinvestigationworks,particularlythelaboratorytesting
▪Excavation&samplingindeeptrialpits
▪Largediametertrialsonpotentiallysuitablecutmaterial
6.1PHASING
Phasinggroundinvestigationworksformanylargeprojectsadvocatedformanyyears–thisisparticularlytrueforroadprojectswheresignificantamountsofgeotechnical
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