Controlled Blasting_082055.pdf

Blasting

1. Presented by:
Dr Clement Kweku Arthur

2. Learning Objective
Havingworkedthroughthislecture,thestudentwillbe
ableto:
Describethevariouscontrolledblastingtechniques

3. Introduction
Thissecondtechniquemayormaynotbecosmetically
appealing,butfromstabilitystandpoint,performsits
function.
Overbreakcontrolmethodscanbebrokendowninto
threetypes:
Presplitting
Trim(Cushion)blasting
Linedrilling

4. Introduction
Blastingtechniqueshavebeendevelopedtocontrol
overbreakatexcavationlimits.
Theoperatormustdecidetheultimatepurposeofthe
controltechniquebeforeselectingofthetechniquecanbe
made.
Sometechniquesareusedtoproducecosmetically
appealingfinalwallswithlittleornoconcernforstability
withintherockmass.
Othertechniquesareusedtoprovidestabilitybyforminga
fractureplanebeforeanyproductionblastisconducted.

5. Presplitting
“Pre-splitting”isacontrolled(special)blasting
techniquethatinvolvessimultaneouslyfiringalineof
blastholesthataredrilledclosetogetherandhaveavery
largeburden.
Theblastholesareusuallydrilledalongthedesign
excavationboundaryor“finallimit”inaconstruction
project,mine,orquarry.
Simultaneousfiringofthechargesproducesatensile
stressintherock,creatingaplanarcrackbetween
blastholes.

6. Presplitting
Presplittingalongtheexcavationboundaryisdesigned
toprovideanopenplaneofweaknessthatwillprotect
surroundingrockfromtheeffectsofproduction
blastholesthatarefiredafterthepresplit.

7. Presplitting
Presplitblastholesareusuallychargedwitharelatively
smallamountofexplosives,comparedtoproduction
blastholesinthesamerockmass.
Explosivesenergyisusedefficientlytoproducestresses
thatexceedtherock’stensilestrength,butarebelowits
compressivestrength(ie.Energyisnotwastedby
crushingtherock).
Firingthepre-splitchargeswithanexcessiveburden
ensuresthatexplosiongasesareforcedtoopenupacrack
insteadofdisplacingtherockmass.

8. Presplitting
Firingexplosivesthatareconfinedbyanexcessiveburden
alwaysproducesrelativelylargegroundvibrations.
ThePPVsfrompre-splitblastingcanbeupto5timesthe
levelsproducedbyfreefaceblastswiththesameexplosives
chargeweightperdelay.
Resultsproducedbypre-splittingcanvarygreatly,
dependingonrockmassproperties.

9. Presplitting
Thetechniqueisdesignedtoproduceauniformcrack
withoutcausinganysignificantdamagetorockoneither
sideofthe“split”.
However,itispossibleforapre-splitblasttoproduce
extensiveradialcracksand“openup”geological
discontinuities(eg.jointsandbeddingplanes)if
blastholesaretooclosetogetherorovercharged.

10. Presplitting

11. Presplitting
Pre-splittingcanproduceeffectiveresultsinblasthole
diametersfrom38to311mm,although75mmisthemost
commonsizeforconstructionprojects.
Blastholescanbechargedwithdecoupledexplosives,or
“deckcharged”withsmallamountsofbulkexplosivesto
producetherequiredenergydistribution.

12. Presplitting
Decoupledchargesmayalsobepreparedbyattaching
smalldiameterpackagedexplosivessuchas‘POWERGEL’
“MAGNUM”tohighenergydetonatingcorddownlines.
Pre-splittingwithdecoupledchargesappearstobemost
effectivewitha“decouplingratio”of2to2.5(ie.the
blastholediameterisabout2to2.5timesthecharge
diameter).

13. Presplitting

14. Presplitting
Ifairblastmustbeminimised,pre-splitblastholesshould
beeffectivelystemmed(eg.withdrillcuttingssupported
byaplasticbag/“plug”)andalldetonatingcordtrunklines
coveredbysuitablematerial(eg.200mmofsand)to
mufflenoise.
Apre-splitlinewouldideallybedrilledandblastedasa
separateoperationbeforetheadjacentproduction
blastholesarefired.

15. Presplitting
However,duetoschedulingissues,itisusuallymore
convenienttofirepre-splitchargesasthefirstpartofthe
adjacentproductionblast(ie.asthisavoidstheneedto
bringdrillingandchargingequipmentintothearea
twice).
Whenapre-splitisfiredaspartofaproductionblast,pre-
splitchargesshouldfireabout100to200msbeforethe
nearestproductionblastholes(ie.toenablethepre-split
fracturetodevelopproperlybeforemorepowerfulcharges
detonatenearby).

16. Trim Blasting
Trimblastinginvolvesfiringalineofblastholesthatare
drilledclosetogetheralongthedesignexcavation
boundary,aftertheadjacentproductionblasthasbeen
excavated.
Theseblastholesareusuallydrilledwitharelatively
smallburdenandspacing,aftertheproductionblasthas
beenfired(ie.Topreventthemfrombeingdamagedby
anygroundmovementor“spalling”).

17. Trim Blasting
Trimmerblastholesarechargedwithlowenergy
explosivesordecoupledchargesandfired
simultaneouslytoremovethesmallburden.
“Trimmer”blastholesareusuallychargedandfiredina
similarwaytopre-splitting,tocracktherockalonga
planebetweenblastholesandproduceasmoothrock
facewithminimumdamageandover-break.

18. Trim Blasting
Atrimblastchargeusuallyconsistsofsmalldiameter
packagedexplosives(eg.‘POWERGEL’“MAGNUM”)
attachedtoahighenergydetonatingcorddownline,or
continuouslengthsofaspecialproductsuchas
“Powersplit”.
Theburdenshouldgenerallybeabout20blasthole
diameters,withthespacing30%lessthantheburdento
assistcrackingalongaplanebetweenblastholes.

19. Trim Blasting
Trimblastingtendstogivebetterresultsthanpre-
splittinginweakor“blocky”rock,andisusuallyless
expensivethanpre-splittingbecauseblastholespacings
arelarger.
Arockfacecanbebadlydamagedbyproduction
blastholesthatarefirednearbybeforea“trimblast”is
drilled.
Thus“buffer”blastholesthatarechargedwithless
powerfulexplosivesmaybeusedtoprotectrockfaceson
finaldesignlimits:

20. Trim Blasting

21. Line Drilling
Linedrillingisaspecialtechniquethatmaybeusedto
minimiseblastdamagetothesurroundingrockmassin
criticalsituations(eg.excavationsforrockcrushersor
bins).
Thismethodinvolvesdrillingarowofparallelholes
veryclosetogetheralongthedesignexcavationlimit,
andleavingthemuncharged.

22. Line Drilling
Theseopenholesareintendedtoforma“planeof
weakness”thatwillcauseproductionblastholesto
breakneatlyalongthedesignexcavationboundary.
Theholediameterforlinedrillingisusually38to75
mm,withaspacingbetweenholesofabout2to4times
thediameter.

23. Line Drilling
Thedistancebetweenthelastrowofchargedblastholes
andtheline-drilledholesisusuallyabout50to75%of
thenormalburdendistance.
Ifanychargedblastholesaretooclosetothedesign
boundary,theymaycausedamagetotherockmass
beyondthelineofunchargedholes.
Theverycloseholespacingbetweenline-drilledholes
requiresexceptionaldrillingaccuracytoavoidholes
intersectingeachother.

24. Line Drilling
Thisfactorwillgenerallyrestrictthemaximumlength
oflinedrilledholestorelativelyshort“lifts”.
Line-drillingisonlyoccasionallyusedandthenin
specialapplicationswhereanyothertechniquewould
causetoomuchoverbreak(ie.becauseofthetimeand
expenseofdrillingmanycloselyspacedholes).

25. QUESTION TIME
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