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土木工程系

班级：

07土本

（1）班

姓　　名：

xxx

指导教师：

xxx

2011年6月8日

DESIGNANDEXECUTIONOFGROUNDINVESTIGATIONFOREARTHWORKS

PAULQUIGLEY,FGS

IrishGeotechnicalServicesLtd

ABSTRACT

Thedesignandexecutionofgroundinvestigationworksforearthworkprojectshasbecomeincreasinglyimportantastheavailabilityofsuitabledisposalareasbecomeslimitedandcostsofimportingengineeringfillincrease.Anoutlineofgroundinvestigationmethodswhichcanaugment‘traditionalinvestigationmethods’particularlyforglacialtill/boulderclaysoilsispresented.Theissueof‘geotechnicalcertification’israisedandrecommendationsoutlinedonitsmeritsforincorporationwithgroundinvestigationsandearthworks.

1.INTRODUCTION

Theinvestigationandre-useevaluationofmanyIrishboulderclaysoilspresentsdifficultiesforboththegeotechnicalengineerandtheroaddesignengineer.Theseglacialtillorboulderclaysoilsaremainlyoflowplasticityandhaveparticlesizesrangingfromclaytoboulders.Mostofourboulderclaysoilscontainvaryingproportionsofsand,gravel,cobblesandbouldersinaclayorsiltmatrix.Theamountoffinesgovernstheirbehaviourandthesiltcontentmakesitveryweathersusceptible.

Moisturecontentscanbehighlyvariablerangingfromaslowas7%forthehardgreyblackDublinboulderclayupto20-25%forMidland,South-WestandNorth-Westlightgreyboulderclaydeposits.Theabilityofboulderclaysoilstotake-infreewateriswellestablishedandpoorplanningofearthworksoftenamplifiesthis.

Thefinesoilconstituentsaregenerallysensitivetosmallincreasesinmoisturecontentwhichoftenleadtolossinstrengthandrenderthesoilsunsuitableforre-useasengineeringfill.Manyofourboulderclaysoils（especiallythosewithintermediatetypesiltsandfinesandmatrix）havebeenrejectedattheselectionstage,butgoodplanningshowsthattheycaninfactfulfilspecificationrequirementsintermsofcompactionandstrength.

Theselectionprocessshouldaimtomaximisetheuseoflocallyavailablesoilsandwithcarefulevaluationitispossibletouseorincorporate‘poorormarginalsoils’withinfillareasandembankments.Fillmaterialneedstobeplacedatamoisturecontentsuchthatitisneithertoowettobestableandtrafficableortoodrytobeproperlycompacted.

Highmoisturecontent/lowstrengthboulderclaysoilscanbesuitableforuseasfillinlowheightembankments（i.e.2to2.5m）butnotsuitablefortraffickingbyearthworkplantwithoutusingageotextileseparatorandgranularfillcappinglayer.Hence,itisvitalthattheearthworkscontractorfullyunderstandsthehandlingpropertiesofthesoils,asformanyprojectsthisiseffectivelygovernedbythetrafficabilityofearthmovingequipment.

2.TRADITIONALGROUNDINVESTIGATIONMETHODS

Forroadprojects,aprincipalaimofthegroundinvestigationistoclassifythesuitabilityofthesoilsinaccordancewithTable6.1fromSeries600oftheNRASpecificationforRoadWorks（SRW）,March2000.Themajorityofcurrentgroundinvestigationsforroadworksincludesacombinationofthefollowingtogivetherequiredgeotechnicaldata:

Trialpits

Cablepercussionboreholes

Dynamicprobing

Rotarycoredrilling

In-situtesting（SPT,variableheadpermeabilitytests,geophysicaletc.）

Laboratorytesting

Theimportanceof‘phasing’thefieldworkoperationscannotbeoverstressed,particularlywhenassessingsoilsuitabilityfromdeepcutareas.Cablepercussionboreholesarenormallysunktoadesireddepthor‘refusal’withdisturbedandundisturbedsamplesrecoveredat1.00mintervalsorchangeofstrata.

Inmanyinstances,cablepercussionboringisunabletopenetratethroughverystiff,hardboulderclaysoilsduetocobble,boulderobstructions.Sampledisturbanceinboreholesshouldbepreventedandlossoffinesiscommon,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,Irishboulderclaysusuallycomprisehighlyvariableproportionsofsands,gravelsandcobblesinasiltorclaymatrix.Lowplasticitysoilswithfinescontentsofaround10to15%oftenpresentthemostdifficulties.BS5930（1999）nowrecognisesthesedifficultiesindescribing‘mixedsoils’–thefinesoilconstituentswhichgoverntheengineeringbehaviournowtakespriorityoverparticlesize.

Akeyparameter（whichisoftenunderestimated）inclassifyingandunderstandingthesesoilsispermeability（K）.Inspectionoftheparticlesizegradingswillindicatemagnitudeofpermeability.Wherepossible,triaxialcelltestsshouldbecarriedoutoneitherundisturbedsamples（U100’s）orgoodqualitycoresamplestoevaluatethedrainagecharacteristicsofthesoilsaccurately.

Lowplasticityboulderclaysoilsofintermediatepermeability（i.e.Koftheorderof10-5to10-7m/s）canoftenbe‘conditioned’bydrainagemeasures.Thisusuallyentailstheinstallationofperimeterdrainsandsumpsatcutareasorborrowpitssoastoreducethemoisturecontent.Hence,withsmallreductioninmoisturecontent,difficultglacialtillsoilscanbecomesuitableasengineeringfill.

4.ENGINEERINGPERFORMANCETESTINGOFSOILS

Laboratorytestingisverymuchdictatedbytheproposedend-useforthesoils.TheengineeringparameterssetoutinTable6.1pftheNRASRWincludeacombinationofthefollowing:

Moisturecontent

Particlesizegrading

PlasticLimit

CBR

Compaction（relatingtooptimumMC）

Remouldedundrainedshearstrength

Anumberofkeyfactorsshouldbeborneinmindwhenschedulinglaboratorytesting:

Compaction/CBR/MCVtestsarecarriedouton<

20mmsizematerial.

Moisturecontentvaluesshouldrelateto<

20mmsizematerialtoprovideavalidcomparison.

Porepressuresarenottakenintoaccountduringcompactionandmayvaryconsiderablybetweenlaboratoryandfield.

Preparationmethodsforsoiltestingmustbeclearlystipulatedandagreedwiththedesignatedlaboratory.

Greatcaremustbetakenwhendeterminingmoisturecontentofboulderclaysoils.Ideally,themoisturecontentshouldberelatedtotheparticlesizeandhaveacorrespondinggradinganalysisfordirectcomparison,althoughthisisnotalwayspractical.

Inthemajorityofcases,theMCVwhenusedwithcompactiondataisconsideredtoofferthebestmethodofestablishing（andchecking）thesuitabilitycharacteristicsofaboulderclaysoil.MCVtestingduringtrialpittingisstronglyrecommendedasitprovidesarapidassessmentofthesoilsuitabilitydirectlyafterexcavation.MCVcalibrationcanthenbecarriedoutinthelaboratoryatvariousmoisturecontentincrements.SampledisturbancecanoccurduringtransportationtothelaboratoryandthiscanhaveasignificantimpactontheresultantMCV’s.

IGSLhasfoundlargediscrepancieswhenperformingMCV’sinthefieldonlowplasticityboulderclayswiththosecarriedoutlaterinthelaboratory（2to7days）.ManyoftheaforementionedlowplasticityboulderclaysoilsexhibittimedependantbehaviourwithsignificantlydifferentMCV’srecordedatalaterdate–increasedvaluescanbeduetothedrainageofthematerialfollowingsampling,transportationandstoragewhiledilatancyandmigrationofwaterfromgranularlensescanleadtodeteriorationandlowervalues.

Thistypeofinformationisimportanttoboththedesignerandearthworkscontractorasitprovidesanopportunitytounderstandthepropertiesofthesoilswhentestedasoutlinedabove.Itcanalsoillustratetheadvantagesofpre-draininginsomeinstances.Withmixedsoils,faceexcavationmaybenecessarytoacceleratedrainageworks.

CBRtestingofboulderclaysoilsalsoneedscarefulconsideration,mainlywiththepreparationmethodemployed.Designengineersneedtobeawareofthis,asitcanhaveanorderofmagnitudedifferenceinresults.Staticcompactionofboulderclaysoilsisadvisedascompactionwiththe2.5or4.5kgrammeroftenleadstohighexcessporepressuresbeinggenerated–henceverylowCBRvaluescanresult.Also,curingofcompactedboulderclaysamplesisimportantasthisallowsexcessporewaterpressurestodissipate.

5.ENGINEERINGCLASSIFICATIONOFSOILS

InaccordancewiththeNRASRW,generalcohesivefilliscategorisedinTable6.1asfollows:

2AWetcohesive

2BDrycohesive

2CStonycohesive

2DSiltycohesive

Thematerialpropertiesrequiredforacceptabilityaregivenandthedesigne