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1、DESIGNEXECUTIONOFGROUNDINVESTIGATIONFEARTHWKSPAULQUIGLEYFGSIrishGeotechnicalServicesLtdABSTRACTThedesignexecutionofgroundinvestigationwksfearthwkprojectshasbecomeincreasinglyimptantastheavailabilityofsuitabledisposalarea
2、sbecomeslimitedcostsofimptingengineeringfillincrease.Anoutlineofgroundinvestigationmethodswhichcanaugment‘traditionalinvestigationmethods’particularlyfglacialtillboulderclaysoilsispresented.Theissueof‘geotechnicalcertifi
3、cation’israisedrecommendationsoutlinedonitsmeritsfincpationwithgroundinvestigationsearthwks.1.INTRODUCTIONTheinvestigationreuseevaluationofmanyIrishboulderclaysoilspresentsdifficultiesfboththegeotechnicalengineertheroadd
4、esignengineer.Theseglacialtillboulderclaysoilsaremainlyoflowplasticityhaveparticlesizesrangingfromclaytoboulders.Mostofourboulderclaysoilscontainvaryingproptionsofsgravelcobblesbouldersinaclaysiltmatrix.Theamountoffinesg
5、overnstheirbehaviourthesiltcontentmakesitveryweathersusceptible.Moisturecontentscanbehighlyvariablerangingfromaslowas7%fthehardgreyblackDublinboulderclayupto2025%flSouthWestNthWestlightgreyboulderclaydeposits.Theabilityo
6、fboulderclaysoilstotakeinfreewateriswellestablishedpoplanningofearthwksoftenamplifiesthis.Thefinesoilconstituentsaregenerallysensitivetosmallincreasesinmoisturecontentwhichoftenleadtolossinstrengthrenderthesoilsunsuitabl
7、efreuseasengineeringfill.Manyofourboulderclaysoils(especiallythosewithintermediatetypesiltsfinesmatrix)havebeenrejectedattheionstagebutgoodplanningshowsthattheycaninfactfulfilspecificationrequirementsintermsofcompactions
8、trength.Theionprocessshouldaimtomaximisetheuseoflocallyavailablesoilswithcarefulevaluationitispossibletouseincpate‘pomarginalsoils’withinfillareasembankments.Fillmaterialneedstobeplacedatamoisturecontentsuchthatitisneith
9、ertoowettobestabletrafficabletoodrytobeproperlycompacted.Highmoisturecontentlowstrengthboulderclaysoilscanbesuitablefuseasfillinlowheightembankments(i.e.2to2.5m)butnotsuitableftraffickingbyearthwkplantwithoutusingageotex
10、tileseparatgranularfillcappinglayer.Henceitisvitalthattheearthwkscontractfullyunderststhehlingpropertiesofthesoilsasfmanyprojectsthisiseffectivelygovernedbythetrafficabilityofearthmovingequipment.2.TRADITIONALGROUNDINVES
11、TIGATIONMETHODSWherepossibletriaxialcelltestsshouldbecarriedoutoneitherundisturbedsamples(U100’s)goodqualitycesamplestoevaluatethedrainageacteristicsofthesoilsaccurately.Lowplasticityboulderclaysoilsofintermediatepermeab
12、ility(i.e.Kofthederof105to107ms)canoftenbe‘conditioned’bydrainagemeasures.Thisusuallyentailstheinstallationofperimeterdrainssumpsatcutareasbrowpitssoastoreducethemoisturecontent.Hencewithsmallreductioninmoisturecontentdi
13、fficultglacialtillsoilscanbecomesuitableasengineeringfill.4.ENGINEERINGPERFMANCETESTINGOFSOILSLabatytestingisverymuchdictatedbytheproposedendusefthesoils.TheengineeringparameterssetoutinTable6.1pftheNRASRWincludeacombina
14、tionofthefollowing:?Moisturecontent?Particlesizegrading?PlasticLimit?CBR?Compaction(relatingtooptimumMC)?RemouldedundrainedshearstrengthAnumberofkeyfactsshouldbebneinmindwhenschedulinglabatytesting:?CompactionCBRMCVtests
15、arecarriedouton20mmsizematerial.?Moisturecontentvaluesshouldrelateto20mmsizematerialtoprovideavalidcomparison.?Pepressuresarenottakenintoaccountduringcompactionmayvaryconsiderablybetweenlabatyfield.?Preparationmethodsfso
16、iltestingmustbeclearlystipulatedagreedwiththedesignatedlabaty.Greatcaremustbetakenwhendeterminingmoisturecontentofboulderclaysoils.Ideallythemoisturecontentshouldberelatedtotheparticlesizehaveacrespondinggradinganalysisf
17、directcomparisonalthoughthisisnotalwayspractical.InthemajityofcasestheMCVwhenusedwithcompactiondataisconsideredtoofferthebestmethodofestablishing(checking)thesuitabilityacteristicsofaboulderclaysoil.MCVtestingduringtrial
18、pittingisstronglyrecommendedasitprovidesarapidassessmentofthesoilsuitabilitydirectlyafterexcavation.MCVcalibrationcanthenbecarriedoutinthelabatyatvariousmoisturecontentincrements.Sampledisturbancecanoccurduringtransptati
19、ontothelabatythiscanhaveasignificantimpactontheresultantMCV’s.IGSLhasfoundlargediscrepancieswhenperfmingMCV’sinthefieldonlowplasticityboulderclayswiththosecarriedoutlaterinthelabaty(2to7days).Manyoftheafementionedlowplas
20、ticityboulderclaysoilsexhibittimedependantbehaviourwithsignificantlydifferentMCV’srecdedatalaterdate–increasedvaluescanbeduetothedrainageofthematerialfollowingsamplingtransptationstagewhiledilatancymigrationofwaterfromgr
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