Geotechnical Report.docx
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Geotechnical Report.docx
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GeotechnicalReport
1.PURPOSEANDSCOPE
ThisreportispartofthedeliverablesthataretobesubmittedfortheworkofSection-1oftheAddis-AdamaTollMotorway.ThereportexpoundstheGeotechnicalInvestigationandAnalysisthatwerecarriedoutatthebridge,overbridge,andculvertlocationsandatdeepcutsectionsfoundbetweenkm3+500and17+000.
Inthissectionoftheroad,twentyboreholesweredrilledaspartofthegeotechnicalinvestigation.Outofthese,thirteenboreholesweredrilledatoverbridgeandbridgefoundationspositions,sixatheavycutsections,andoneatboxculvertfoundationposition.
Thegeotechnicalinvestigationwascarriedouttoexploreandassesstheengineeringpropertiesofmaterialsofsubsurfacestrata.Itwasundertakenbyrunningrotarycoredrilling,recoveringsamples(bothdisturbedandundisturbed)andconductingin-situandex-situ(laboratory)tests.
ThescopeoftheGeotechnicalAnalysisextendstothedeterminationsuitablefootingdepth,safebearingpressures,anticipatedsettlementandrecommendationofstableslopes.
ThedetailsoftheGeotechnicalstudiestogetherwiththedeskstudiesandfieldreconnaissancewerediscussedinthesubsequentsectionsofthereport.
2.GEOLOGYANDGEOMORPHOLOGY
TheproposedalignmentoftheAddis–AdamaTollRoadtraversesvarioustypesofgeologicalformations.ThegeologyoftheprojectstretchisdominatedbyvolcanicrockswithinterruptionsofAlluvialandLacustrinedepositsatsomeshortstretch.Themaingeologicalformationsalongtheroadalignmentarecomposedof:
Nn=NazretSeries:
Ignimbrite,unweldedtuffs,ashflows,rhyoliticflows,domesandtrachyte;
NQtb=BishoftuFormation:
Alkalinebasaltandtrachyte;
Qb=Alkalinebasaltandtrachyte;
Q=AlluvialbasaltandLacustrinedeposits:
sand,siltandclay;
Nc=Trachyte,Trachy-basalt,peralkalinerhyolitewithsubordinatealkalinebasalt.
Thetopographyoftheprojectrouteischaracterizedbyrollingandflatterrainanddendriticdrainagepattern.
ThegeologicformationoftheprojectareaasshownintheGeologicalMapofEthiopia1995ed.isshowninthefollowingplate.
Plate#1GeologicalMapoftheProjectStretch.
3.FIELDEXPLORATION
Thesubsurfacematerialsatthebridge,overbridge,andculvertlocationsandonheavycutsectionswereexaminedbydrillingboreholesbyrotarycoredrillingmethod.Twotypesofdrillbitsandsamplerswereusedduringthedrillingoperation.Theseare:
1.Drillbitsandsamplerswithdiameterof13cmforshallowdrillingupto1m;and
2.Drillbitsandsamplerswithdiameterof10cmfordeepdrillingbeyond1m.
Thedrillingwascarriedoutwithoutcasing.Insteadofcasing,asthedrilladvancesdeeper,bentoniteslurrywasusedtopreventtheoverlyingstratafromcollapseandkeeptheboreholeopen.
Thematerialsfromeachofdistinctstrataweresampledforlaboratoryinvestigation.Basedonthematerialtypesandconsistency,samplingoftherecoveredsubsurfacematerialswasundertakeninthreeways.Forthefirmtohardcohesivesoilsandfornon-cohesivesoils,disturbed(representative)sampleswerecollected;andforsoftcohesivesoilsundisturbedsampleswererecovered.Ontheotherhand,forrocks(ofvariousdegreesofweathering),rockcoresweresampled.
In-situtests,namelySPT(StandardPenetrationTest)andCPT(DynamicConePenetrationTest),wereconductedatanaverageintervalof2m.TheStandardPenetrationTestwasdoneonfineandmediumgrainedsoils.Thetestwascarriedoutwithoutliner.TheDynamicConePenetrationTest,ontheotherhand,ranongranularsoilsonwhichitisdifficulttoconductSPT.
Whenarockstratumwasencountered,itwasproperlydescribedafterthoroughscrutinyofitstype,levelofweatheringanddiscontinuity.Besides,theparameterslikeRockQualityDesignation(RQD),discontinuityspacingandjointspacing,whichareessentialforrockmassratingwererecordedbytakingappropriatemeasurementoftherecoveredrockcores.
Allofthefieldproceduresandobservationswereloggedinawell-organizedformat.Theformatcontainseight(8)columnsforkeepingarecordofthedownwardrunlength,recoveryratio,materialdescription,theSPTandCPTresults,thesamplelocations,thelaboratorytestprograms;andfortakinganyotheradditionalremarks.
4.GROUNDWATERMONITORING
Groundwaterwasnotencounteredinanyoftheboreholesduringdrillingandaftershortdelaysofthedrillingoperation.AspertheinformationobtainedfromCCCC(theClient),groundwatermonitoringwascarriedoutbeforecommencementofthedrillingoperationanditwasreportedthatthegroundwaterlevelwasfoundatdeeperdepths,around90mbelowthenaturalgroundlevel,aftertherainyseason.
5.LABORATORYTESTS
Thesoilsamplescollectedfromeachoftheboreholesweresubjectedtothefollowinglaboratorytests:
▪Atterberglimits(Liquidlimit,plasticlimitandplasticityindex);
▪Particlesizeanalysis;
▪Unitweight;
▪Specificgravityand
▪BasedontheaboveteststhesoilisclassifiedbyUSCSSystem.
Ontheotherhand,thesoundrockcoresamplesweretestedfortheirUniaxialCompressiveStrengthandUnitweightandtheweatheredrockcoresweretestedfortheirunitweights.
Theresultsofthelaboratorytestsareappendedattheendofthereport.
6.FOUNDATIONANDSLOPESTABILITYANALYSIS
6.1BearingCapacityDetermination
TheallowablebearingcapacitiesofthefoundationmaterialsweredeterminedusingMeyerhofandTerzaghibearingcapacityequations.
Forcohesiveandnon-cohesivesoils,Meyerhof’sequations,basedonSPTresults,wereusedforthedeterminationofallowablebearingcapacityfor25mmsettlement.TheN-valuesoftheSPTresultswerestandardizedtoN70valuesbyadoptingenergyratioandfieldprocedurecorrectionsinthefollowingmanner.
Whereηi=adjustmentfactorsfrom(andcomputedasshown)Table6-1;
N70=adjustedNformachineefficiencyof70%;
CN=adjustmentforeffectiveoverburdenpressurep’o(kPa)computed[LiaoandWhiteman(1986)]as
Table6-1SPTFieldProcedureCorrectionFactors
Hammerforη1
AverageEnergyRatio,Er
Donut
Safety
Country
Rope-Pulley
Trip
Rope-Pulley
Trip/Auto
UnitedStates
45
-
70-80
80-100
Japan
67
78
-
-
UnitedKingdom
-
-
50
60
China
50
60
-
-
RodLengthCorrection,η2
Length>10m
η2=1.00
6-10m
=0.95
4-6m
=0.85
0-4m
=0.75
SamplerCorrection,η3
Withoutliner
η3=1.00
Withliner:
Densesand,clay
=0.80
Loosesand
=0.90
BoreholeDiameterCorrection,η4
Holediameter:
60-120mm
η4=1.00
150mm
=1.05
200mm
=1.15
TheN-valuesofthedynamicconepenetrationtestwereconvertedtoSPTN-valuesusingthefollowingrelations.
whereNcorr=SPTN-valuecorrectedforoverburdenpressure
TheadjustedN-valuesoftheSPTwerethenusedasinputparametersinthefollowingMeyerehof’sbearingcapacityformula.
N55
N70
F1
0.05
0.04
F2
0.08
0.06
F3
0.03
0.03
F4
1.20
1.20
B≤F4
B>F4
Theallowablebearingcapacitycomputedbysuchequationsshallbeadjustedbythefollowingrelationiftheactualsettlementisdifferentfrom25mm.
Forclayeysoils,theallowablebearingcapacitieswerealsodeterminedbySkempton’sFormuladevelopedforSaturatedClays.Tousethisformula,theunconfinedcompressivestrengthofthesoilisdeterminedbasedonSPTN-valueas:
whereCu=undrainedshearstrength(kPa)
ThisvalueofCu=C=cohesionalresistanceofthesoilisusedasinputinthefollowingSkemptonformula.
whereqult=ultimatebearingcapacity
Nc=thebearingcapacityfactorreadfromSkempton’schart
γ=unitweightofsoiland
Df=depthoffooting
Theallowablebearingcapacitieswerethendeterminedusingfactorofsafetyof,FS=3,onthenetultimatebearingcapacityofthesoil.ThevaluesdeterminedbythisequationareonlyusedforcomparisonwiththeallowablebearingcapacitiesdeterminedbyMeyerhof’sequations.
ThebearingcapacitiesoftherockyfoundationsweredeterminedusingTerzaghi’sbearingcapacityequation:
where
;
;
;
c=cohesionalresistance
ф=angleoffrictionalresistance
=effectiveoverburdenpressure
B=widthoffooting
γ=Unitweightoftherockand
scandsγ=shapefactors
TherockmasswasratedusingBeiniawski(1989)methodofrockmassratingsystem.Aftertherockmassclasswasdeterminedinsuchafashion,theapproximatevaluesofcohesionandangleoffrictionwerereadfromthesametableandusedintheTerzaghiequationsfordeterminationoftheultimatebearingresistanceoftherockmass.Theallowablebearingpressureswerecomputedtakingafactorofsafetyof,FS=5.
Thebearingcapacitiescomputedforeachofthefoundationmaterialsareappendedattheendofthereport.
Beiniawski-1989
6.2SettlementComputation
Thesettlementanticipatedateachfoundationwasestimatedinthreewaysbasedonthetypeoffoundationmaterials.
Forcohesivesoils,thesettlementwasestimatedusingthechartpresentedinEBCS-7,1995,whichrelatesthetotalsettlementwiththeSPTN-valuescorrectedforoverburdenbythefollowingformula.
Where
=effectiveoverburdenpressure(MPa)
N=FieldSPTN-value
Chart-1:
ChartpresentedbyEBCS-7-1995forestimationoftotalsettlement.
ThetotalsettlementofthecohesionlesssoilontheotherhandiscomputedusingMeyerhof’sformula,whichwasdevelopedfordeterminationtotalsettlementbasedonSPTN-valueadjustedfor60%ofmachineefficiency.Theseequationsare:
forB≤1.2m
forB>1.2m
whereρ=totalsettlement(mm);
q=bearingpressure(kPa);
p’o=verticaleffectivestr
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