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1、CONTROLLEDBLASTINGTECHNIQUTESControlledblastingisusedtoreduceover-breakandminimizefracturingoftherockattheboundaryofanexcavation.Thefourbasiccontrolledblastingtechniquesare:linedrilling,presetting,cushionblastingandsmoothblasting.Linedrilling,theearliestcontrolledblastingtechniqu
2、e,involvesdrillingarowofcloselyspacedholesalongthefinalexcavationline,providingaplaneofweaknessto-whichtobreak.Linedrillholes,2or4diametersapartandcontainnoexplosive・Thesadjacenttothelinedrill-holesnormallyareloadedlighterandareoncloserspacingthantheotherblastholes.Themaximumdept
3、hforlinedrillingisabout30ft.Linedrillinginvolvesnoblastinginthefinalrowofholes,andthusminimizesdamagetothefinalwall.Presetting,sometimescalledpres-hearing‘involvesasinglerowofboreholes,usually2to4in.indiameter,drilledalongthefinalexcavationataspacingof6to12boreholediameters.Dynam
4、itecartridgesIto1.5in.insizeon1to2ft.centersusuallyarestring-loadedondetonatingcord,althoughspecialsmall-diametercartridgeswithspecialcoupleareavailablefortotalcolumnloading.Inunconsolidatedformations,closerspacing'swithlighterpowderloadsarerequired・Thebottom2to3ft.ofboreholeusua
5、llyisloadedsomewhatheavierthantheremainder.Stemmingbetweenandaroundtheindividualchargesisoptional・Thetop2to3ft.ofboreholeisnotloaded,butisstemmed.Thedepththatcanbupreliterateislimitedbyholealignment,with50ft.beingaboutmaximum・Thepreliterateholesarefiredbeforebeforetheadjacentprim
6、aryholestoprovideafractureplanetowhichtheprimaryblastcanbreak.Inpresettingitisdifficulttodeterminetheresultsuntiltheadjacentprimaryblastisshot.Forthisreason,presettingtoofarinadvanceisnotrecommended・Presettingseldomisdoneunderground.Cushionblastinginvolvesdrillingarowof2-to6-in・d
7、iameterboreholesalongthefinalexcavationline,loadingwithalightwell-distributedchargecompletelystemmedandfiringafterthemainexcavationisremovedratherthanbefore,asinpresetting.Theburdenontheholesisslightlylargerthanthespacing・Wedgesmaybeusedtoabutthechargestotheexcavationsideofthebor
8、eholeandminimizedamagetothefinalwall・Plo
