RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

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

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

MechanicsmRockBreakageLesson4

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

BasicConcept

• Duringthedetonationofanexplosivechargeinsiderock,theconditionpresentedarecharacterizedbytwophasesofaction:

• 1st.phase.Astrongthimpactisproducedbytheshockwavelinkedtothestrainenergy,duringashortperiodoftime.

• 2nd.phase.Thegasesproducedbehidthedetonatinfrontcomeintoaction,athightemperatureandpressude,carryingtheThermodynamicorBubbleEnergy.

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

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.

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

1.Crushingofrock

• Inthefirstinstantsofdetonations,thepressureinfrontofthestrainwave,whichexpandsincylindricalformreachesvaluesthatwellexceedthedynamiccompressivestrengthoftherock,provokingthedestructionofitsintercrystallineandintergranullarstructure.• Thethicknessofthesocalledcrushedzoneincreaseswithdetonationpressureoftheexplosiveandwththecouplingbetweenthechargeandtheblastholewall.Highstrengthexplosivesinporousrockitmightreacharadiusofupto8D,butisnormallybetween2and4D(DuvallandAtchison,1957).

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

VariationofpeakcompressivestresswithDistancefromblastholewall(Hagan,1974)

ThisfigureshownthatthevariationsincompressiveStressesgeneratedbytwofully-coupledcharges.Thecrushingoftherockisproducedatapressureof4GPaSothecurveoftheexplosive(A)whichproducesaTensionof7GPaontheblastholewallhasavarysharpDecreaseinpeakstressduetothelargeincreaseinSurfaceareduringthepulverizationoftherock

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

2.Radialfracturing

• Duringpropagationofthestrainwave,herocksurrondingtheblastholeissubjectedtoanintenseradialcompressionwhichinducestensilecomponentsinthetangentialplanesofthewavefront.Whenthetangentialstrainsexceedthedynamictensilestrengthoftherock,theformationofadenseareaofradialcracksaroundthecrushedzonethatsurroundstheblastholeisinitiated(seethefigurebelow).• Thenumberandlengthoftheseradialcracksincreasewith:

1. Theintensityofthestrainwaveonthelastholewallorontheexteriorlimitofthecrushedzone,and

2. Thedecreaseindyanmictensilestrengthoftherockandtehattenuationofthestrainenergy.

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

RadialCrack

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

Radialfracturing

• Whentherockhasnaturalfractures,theextensionofthecracksiscloselyrelatedtothese.Iftheexplosivecoloumnsareintersectedlengthwisebyapre-existingcrack,thesewillopenwiththeeffectofthestrainwaveandthedevelopmentofradialcracksinotherdirectionswillbelimited.Thenaturalfracturesthatareparalleltoblastholes,butatsomedistancefromthem,willinterruptthepropogationoftheradialcracks.

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

Radial fracturing and breakage through reflection of thestrain wave.

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

3.ReflectionBreakageorSpalling

• Whenthestrainwavereachesafreesurfacetwowavesaregenerated,atensilewaveandashearwave.Thisoccurwhentheradialcrackshavenotpropogatedfartherthanonethirdthedistancebetweenthechargeandfreeface.Althoughtherelativemagnitudoftheenergiesassociatedwiththetwowavesdependsupontheincidentangleofthecompressivestrainwavemthefracturingisusuallycausedbytherefalectedtensilewave.Ifthetensilewaveisstrongenoughtoexceedthedynamicstrangthoftherock,thephenomenonknownassapllingwillcomeabout,backtowardtheinterionoftherock.Thetensilestrengthsoftherockreachvaluesthatarebetween5and15%ofthecompressivestrengths.

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

4.GasExtensionFractures

• Afterthestrainwavepasses,thepressureofthegases causeaquasi-staticstressfieldaroundtheblasthole. Duringoraftertheformationofradialcracksbythe tangentialtensilecomponentofthewave,thegasesstart toexpandandpenetrateintothefractures.Theradialcracksareprolongedundertheinfluenceofthestress concentrationsattheirtips.Thenumberandlengthofthe openedanddevelopedcracksstronglydependuponthe pressureofthegases,andaprematureescapeofthesedue toinsufficientstemmingorbythepresenceofaplaneof weaknessinthefreefacecouldleadtoalowerperformanceoftheexplosiveenergy.

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

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.

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

tSeparation oflayersofcompresiblemediumby

release-of-load

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t6.Fracturingalongboundariesofmoduluscontrastofshear fracturing• Insedimentaryrockformationswhenthebeddingplanes, jointsetc.,havedifferentelasticitymodulusorgeomechanicparameters,breakageisproducedintheseparationplaneswhenthestrainwavepassesthroughbecauseofthestraindifferentialinthesepoints..

Shear Fracturing (Hagan

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

7.Breakageby flexion

• Duringandafterthemechanismsofradialfracturingand spalling,thepressureappliedbytheexplosiongasesupon thematerialinfrontoftheexplosivecolumnmakestherockactlikeabeamembeddedinthebottomofthe blasthole inthestemmingarea,producingthedeformationandfracturingoftheSamebuythephenomenaof flexion. Mechanismofbreakagebyflexion,after

Ouchterlony (1995)

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

8.Fracturebyin-flightcollisions

• Therockfragmentscreatedbythepreviousmechanismsandaccelerated bythegasesareprojectedtowardsthe freeface,collidingwitheach otherandtherebyproducingadditionalfragmentationwhichhasbeendemonstratedbyultra-speedphotographs(Hino,1959;Petkof,1969).

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

BubbleEnergy

• Theperformanceofexplosivesupontherockis,therefore,agroupofelementalactionswhichperformsimultaneouslyinafewmiliseconds,associatedwiththeeffects ofthestrainwavewhichtransportstheStressEnergy,andwiththeeffectsoftheexplosiongasesorBubbleEnergy.• Theestimatescarried outbyHagan(1977)havedemonstrated thatonlya15%ofthetotalenergygeneratedin theblastingisusedasaworkingtoolinthemechanismsofrockfragmentationanddisplacement.

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

BubbleEnergy

• RascheffandGoemans(1977)haveestablishedamodelthattheoreticallydistributestheenergy,asrepresented innextFigurefromtestsmadeuponcubicblocksof rockplacedunderwaterinswimmingpools.Theseinvestigatorsassurethatapproximately53%oftheexplosiveenergyisassociatedwiththestrainwave.Thisvaluedependsupontheconditionsoftheexperimentandvery differentresultscanbefoundthatgofrom5to50%ofthe totalenergy,dependinguponthevarioustypesofrock thataretobefragmentedtheexplosivesused.• Therefore,inhardrocktheStrainEnergyofabreakingexplosiveismoreimportantinfragmentationthanthe BubbleEnergy,andthecontraryistrueforsoft,porousor fissuredrocksandinlowdensityexplosives.

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

From the tests carred out by Rascheff and Geomans, Table summerizesthe energy distribution of the strain wave.

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

ExplosiveRockInteraction

• Lowends usedasimplifiedmodelofratioexplosive rock interactiontodescribethepartitionofexplosiveenergyin theprocessofrockblasting.Theenergyispartitioned intodifferentzoneshatarerelatedtotheratiopressure volumeexpansionofthegasesduringthedifferentphasesofblasting.Anillustrationofthispartitionof energyisgiveninFigure.

P3

P4

P53

2

1

4 5

volumepressure

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

tTheenergiesassociatedwiththedifferentzonesgiven inthefigureare,asfollows

• Whentheexplosivedetonatesintheblasthole,thehigh pressuregasesattheinitialorexplosionstateP3sendashockwaveintotherock.Thestrainsfromthisshock neartheblasthole aregreaterthanthedynamiccompressive andshearstrengthoftherock.Theycausevarying amountsofrockcompressionandcrushinginthesurroundingareaoftheblastholedependinguponthe strengthandstiffnessoftherock.Withrockcompressionandcrushingthevolumeoftheblastholeincreasesand thepressuredecreasesuntilthestraininihe rockbalances thepressure.

P3

P4

P53

2

1

4 5

volume

pressure

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t• ThisisshownP4onthepressure/volumecurveofFigureandiscalledblastholeequilibriumstate.Duringtheexpansion,theworkbeingdonebythe explosiveiscalledbrissanceenergyandconsistsofthe strainenergystoredintherock(Zone2)andthekinetic energyoftheshockwave(Zone1).Thekineticshock energyisessentiallylostasusefulworkduringtheblastingprocessandappearsascrushedrocksurroundingtheblastholeandasseismicwavespropagatedintotheground.

P3

P4

P53

2

1

4 5

volume

pressure

• ThestrainsintherockcomingfromtheresidualblastholepressureP4causefracture.Theexplosionproductgasesenteratleastthecracksexistingbetweenthehole andthefreeface,resultinginfragmentationandpossibly contributingtotheheave.Whenthegasesreachthefree facethroughtheburden,theprocessendsmoreorless abruptly.Thepressureofthegasesatescapeisshownat P5inFigure.Duringescape,theburdeniscompressivepressedbythegasinthecrackswithastrainenergy storedintherock(Zone4).Thisenergyhaslittle influenceonfragmentationandheave.

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t• TheenergyfromZones2and3isthemostusefulinrockblastingandiscalledFragmentationEnergy.• Atthetimeofescape,someoftheenergyinthegases (Zone5)movestheburdenandrepresentsheaveenergy. Therestofthisenergyislostasheatandnoiseinthe escapinggases.

P3

P4

P53

2

1

4 5

volume

pressure

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t

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.

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

t• InFigure,theinfluenceoftheenergytransmittedby theexplosivetotherock,dependinguponthedepthofthe chargeandthevolumeofmaterialaffectedbytheblast.Whenthechargehasaveryshallowburial(a)mostoftheenergyistransmittedtotheatmosphereinformofairblast,uptoanexcessivedepth(c)wherealltheenergyis appliedupontherock,fragmentingitandproducinga highintensityvibration.Betweenthetwosituations, therewillbeonethatproducesalargercrater.

Effectsofincreasingdepthofburialoncratershapes

RockBlastingTechniqu

e–MiningEngine

eringDe

partmen

tAftereachtest,thevolumeofthecraterwillbemeasured,andafterwards,withallinformationinhand,thevolume-depthcurvewillbeestablished.

ThemoundissubdividedintotheZoneofcompletefragmentationandthatofextremeortensilefragmentation. Inblastings withinvertedfaces,thscratersizesare influencedbytheeffectofgravityandthestructural charactenstics oftherock,formingelongated,elliptic shapedcavitieswhichcorrespondtotheextremeruptureorstressedzones.