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RockBlastingTechnique - I
By:Dr.Ir.SinggihSaptono,MT.Dr.Ir.S.Koesnaryo,M.Sc.
Dr.Ir.BarlianDwinagara,MT.Ir.R.Hariyanto,MT.
HANDOUT
MiningEngineeringDepartment– FacultyofTechnologyMineralUniversitasPembangunanNasionalVeteranYogyakarta
February,2016
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MechanicsmRockBreakageLesson4
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BasicConcept
• Duringthedetonationofanexplosivechargeinsiderock,theconditionpresentedarecharacterizedbytwophasesofaction:
• 1st.phase.Astrongthimpactisproducedbytheshockwavelinkedtothestrainenergy,duringashortperiodoftime.
• 2nd.phase.Thegasesproducedbehidthedetonatinfrontcomeintoaction,athightemperatureandpressude,carryingtheThermodynamicorBubbleEnergy.
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tRockBreakageMechanismsIn the fragmentation of rocks with explosives at least eigth breakagemechanisms are involved, with more or less reponsablity, but it allexert influence upon the results of the blastings
1.Crushingofrock.2.RadialFracturing.3.Reflectionbreakageorspalling.4.Gasextentionfractures.5.Fracturingbyrelease-of-load.6.Fracturingalongboundaryofmoduluscontrastofshear
fracturing.7.Breakagebyflexion8.Fracturingbyin-flightcollisins.
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1.Crushingofrock
• Inthefirstinstantsofdetonations,thepressureinfrontofthestrainwave,whichexpandsincylindricalformreachesvaluesthatwellexceedthedynamiccompressivestrengthoftherock,provokingthedestructionofitsintercrystallineandintergranullarstructure.• Thethicknessofthesocalledcrushedzoneincreaseswithdetonationpressureoftheexplosiveandwththecouplingbetweenthechargeandtheblastholewall.Highstrengthexplosivesinporousrockitmightreacharadiusofupto8D,butisnormallybetween2and4D(DuvallandAtchison,1957).
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VariationofpeakcompressivestresswithDistancefromblastholewall(Hagan,1974)
ThisfigureshownthatthevariationsincompressiveStressesgeneratedbytwofully-coupledcharges.Thecrushingoftherockisproducedatapressureof4GPaSothecurveoftheexplosive(A)whichproducesaTensionof7GPaontheblastholewallhasavarysharpDecreaseinpeakstressduetothelargeincreaseinSurfaceareduringthepulverizationoftherock
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2.Radialfracturing
• Duringpropagationofthestrainwave,herocksurrondingtheblastholeissubjectedtoanintenseradialcompressionwhichinducestensilecomponentsinthetangentialplanesofthewavefront.Whenthetangentialstrainsexceedthedynamictensilestrengthoftherock,theformationofadenseareaofradialcracksaroundthecrushedzonethatsurroundstheblastholeisinitiated(seethefigurebelow).• Thenumberandlengthoftheseradialcracksincreasewith:
1. Theintensityofthestrainwaveonthelastholewallorontheexteriorlimitofthecrushedzone,and
2. Thedecreaseindyanmictensilestrengthoftherockandtehattenuationofthestrainenergy.
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RadialCrack
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Radialfracturing
• Whentherockhasnaturalfractures,theextensionofthecracksiscloselyrelatedtothese.Iftheexplosivecoloumnsareintersectedlengthwisebyapre-existingcrack,thesewillopenwiththeeffectofthestrainwaveandthedevelopmentofradialcracksinotherdirectionswillbelimited.Thenaturalfracturesthatareparalleltoblastholes,butatsomedistancefromthem,willinterruptthepropogationoftheradialcracks.
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Radial fracturing and breakage through reflection of thestrain wave.
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3.ReflectionBreakageorSpalling
• Whenthestrainwavereachesafreesurfacetwowavesaregenerated,atensilewaveandashearwave.Thisoccurwhentheradialcrackshavenotpropogatedfartherthanonethirdthedistancebetweenthechargeandfreeface.Althoughtherelativemagnitudoftheenergiesassociatedwiththetwowavesdependsupontheincidentangleofthecompressivestrainwavemthefracturingisusuallycausedbytherefalectedtensilewave.Ifthetensilewaveisstrongenoughtoexceedthedynamicstrangthoftherock,thephenomenonknownassapllingwillcomeabout,backtowardtheinterionoftherock.Thetensilestrengthsoftherockreachvaluesthatarebetween5and15%ofthecompressivestrengths.
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4.GasExtensionFractures
• Afterthestrainwavepasses,thepressureofthegases causeaquasi-staticstressfieldaroundtheblasthole. Duringoraftertheformationofradialcracksbythe tangentialtensilecomponentofthewave,thegasesstart toexpandandpenetrateintothefractures.Theradialcracksareprolongedundertheinfluenceofthestress concentrationsattheirtips.Thenumberandlengthofthe openedanddevelopedcracksstronglydependuponthe pressureofthegases,andaprematureescapeofthesedue toinsufficientstemmingorbythepresenceofaplaneof weaknessinthefreefacecouldleadtoalowerperformanceoftheexplosiveenergy.
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5.Fracturingbyrelease-of-load
• Beforethestrainwavereachesthefreeface,thetotal energytransferredtotherockbyinitialcornpressionvariesbetween60and70%oftheblastenergy(Cooket al.1966).• Afterthecompressivewavehaspassed,astate ofquasi-staticequilibriumisproduced,followedbya subsequentfallofpressureintheblasthole asthegases escapethroughthestemming,throughtheradialcracks andwithrockdisplacement.• ThestoredStressEnergyis rapidlyreleased,generatinganinitiationoftensileand shearfracturesintherockmass.Thisaffectsalargevolumeofrock,notonlyinfrontoftheblastholes but behindthelineoftheblastcutaswell,havingregistered damagesinuptodozensofmetersaway,seeFigbelow.
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tSeparation oflayersofcompresiblemediumby
release-of-load
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t6.Fracturingalongboundariesofmoduluscontrastofshear fracturing• Insedimentaryrockformationswhenthebeddingplanes, jointsetc.,havedifferentelasticitymodulusorgeomechanicparameters,breakageisproducedintheseparationplaneswhenthestrainwavepassesthroughbecauseofthestraindifferentialinthesepoints..
Shear Fracturing (Hagan
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7.Breakageby flexion
• Duringandafterthemechanismsofradialfracturingand spalling,thepressureappliedbytheexplosiongasesupon thematerialinfrontoftheexplosivecolumnmakestherockactlikeabeamembeddedinthebottomofthe blasthole inthestemmingarea,producingthedeformationandfracturingoftheSamebuythephenomenaof flexion. Mechanismofbreakagebyflexion,after
Ouchterlony (1995)
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8.Fracturebyin-flightcollisions
• Therockfragmentscreatedbythepreviousmechanismsandaccelerated bythegasesareprojectedtowardsthe freeface,collidingwitheach otherandtherebyproducingadditionalfragmentationwhichhasbeendemonstratedbyultra-speedphotographs(Hino,1959;Petkof,1969).
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BubbleEnergy
• Theperformanceofexplosivesupontherockis,therefore,agroupofelementalactionswhichperformsimultaneouslyinafewmiliseconds,associatedwiththeeffects ofthestrainwavewhichtransportstheStressEnergy,andwiththeeffectsoftheexplosiongasesorBubbleEnergy.• Theestimatescarried outbyHagan(1977)havedemonstrated thatonlya15%ofthetotalenergygeneratedin theblastingisusedasaworkingtoolinthemechanismsofrockfragmentationanddisplacement.
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BubbleEnergy
• RascheffandGoemans(1977)haveestablishedamodelthattheoreticallydistributestheenergy,asrepresented innextFigurefromtestsmadeuponcubicblocksof rockplacedunderwaterinswimmingpools.Theseinvestigatorsassurethatapproximately53%oftheexplosiveenergyisassociatedwiththestrainwave.Thisvaluedependsupontheconditionsoftheexperimentandvery differentresultscanbefoundthatgofrom5to50%ofthe totalenergy,dependinguponthevarioustypesofrock thataretobefragmentedtheexplosivesused.• Therefore,inhardrocktheStrainEnergyofabreakingexplosiveismoreimportantinfragmentationthanthe BubbleEnergy,andthecontraryistrueforsoft,porousor fissuredrocksandinlowdensityexplosives.
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From the tests carred out by Rascheff and Geomans, Table summerizesthe energy distribution of the strain wave.
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ExplosiveRockInteraction
• Lowends usedasimplifiedmodelofratioexplosive rock interactiontodescribethepartitionofexplosiveenergyin theprocessofrockblasting.Theenergyispartitioned intodifferentzoneshatarerelatedtotheratiopressure volumeexpansionofthegasesduringthedifferentphasesofblasting.Anillustrationofthispartitionof energyisgiveninFigure.
P3
P4
P53
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1
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volumepressure
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RockBlastingTechniqu
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tTheenergiesassociatedwiththedifferentzonesgiven inthefigureare,asfollows
• Whentheexplosivedetonatesintheblasthole,thehigh pressuregasesattheinitialorexplosionstateP3sendashockwaveintotherock.Thestrainsfromthisshock neartheblasthole aregreaterthanthedynamiccompressive andshearstrengthoftherock.Theycausevarying amountsofrockcompressionandcrushinginthesurroundingareaoftheblastholedependinguponthe strengthandstiffnessoftherock.Withrockcompressionandcrushingthevolumeoftheblastholeincreasesand thepressuredecreasesuntilthestraininihe rockbalances thepressure.
P3
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volume
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t• ThisisshownP4onthepressure/volumecurveofFigureandiscalledblastholeequilibriumstate.Duringtheexpansion,theworkbeingdonebythe explosiveiscalledbrissanceenergyandconsistsofthe strainenergystoredintherock(Zone2)andthekinetic energyoftheshockwave(Zone1).Thekineticshock energyisessentiallylostasusefulworkduringtheblastingprocessandappearsascrushedrocksurroundingtheblastholeandasseismicwavespropagatedintotheground.
P3
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pressure
• ThestrainsintherockcomingfromtheresidualblastholepressureP4causefracture.Theexplosionproductgasesenteratleastthecracksexistingbetweenthehole andthefreeface,resultinginfragmentationandpossibly contributingtotheheave.Whenthegasesreachthefree facethroughtheburden,theprocessendsmoreorless abruptly.Thepressureofthegasesatescapeisshownat P5inFigure.Duringescape,theburdeniscompressivepressedbythegasinthecrackswithastrainenergy storedintherock(Zone4).Thisenergyhaslittle influenceonfragmentationandheave.
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t• TheenergyfromZones2and3isthemostusefulinrockblastingandiscalledFragmentationEnergy.• Atthetimeofescape,someoftheenergyinthegases (Zone5)movestheburdenandrepresentsheaveenergy. Therestofthisenergyislostasheatandnoiseinthe escapinggases.
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CraterBlasting
• Theconceptanddevelopmentofcraterblastingattributed toC.W.Livingston(1956),openedanewschoolof thoughtforbetterunderstandingofthephenomenonof blastings andthecharacterization oftheexplosives.• Bauer(1961),Grant(1964)andLang(1976)amongothers,widenedthefieldofapplication ofthistheory,convertingitintoabasictoolforthe studyofsurfaceaswellas undergroundblastings.• Acraterblastisthatwhichiscarriedoutwithconcentratedsphencalorcubicchargesandwithgoodapproximation usingrelativelyshortcylinderchargesthatare detonatedinsidetherockmasstobefragmented.
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t• InFigure,theinfluenceoftheenergytransmittedby theexplosivetotherock,dependinguponthedepthofthe chargeandthevolumeofmaterialaffectedbytheblast.Whenthechargehasaveryshallowburial(a)mostoftheenergyistransmittedtotheatmosphereinformofairblast,uptoanexcessivedepth(c)wherealltheenergyis appliedupontherock,fragmentingitandproducinga highintensityvibration.Betweenthetwosituations, therewillbeonethatproducesalargercrater.
Effectsofincreasingdepthofburialoncratershapes
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tAftereachtest,thevolumeofthecraterwillbemeasured,andafterwards,withallinformationinhand,thevolume-depthcurvewillbeestablished.
ThemoundissubdividedintotheZoneofcompletefragmentationandthatofextremeortensilefragmentation. Inblastings withinvertedfaces,thscratersizesare influencedbytheeffectofgravityandthestructural charactenstics oftherock,formingelongated,elliptic shapedcavitieswhichcorrespondtotheextremeruptureorstressedzones.