k.v.ramana ,deilling and blasting

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Opencast mining

Drilling and blasting keyfeatures



Introduction

Drilling is used in mining for: placement of explosives geological exploration installation of rock bolts for support installation of rock anchors for hanging of

equipment

Tuesday, December 8,2015

Supervisors developmentprogramme

2



Rotary percussive drilling achieves the breakage of rock by a combination of the

following actions:Percussion

piston operated by either pneumatic or hydraulic power strikes a drill rod

which transmits the shock waves produced by the impact to the drill bit !

Rotation

fter each strike of the piston" the rod and bit are turned so that on the next

strike the tungsten carbide inserts of the bit impact on a different piece of

rock!

#eed or thrust load

$n order to maintain the drill bit in contact with the rock a thrust force isapplied to the drill rod!

#lushing

%ater or air is in&ected through the drill rod and bit to the bottom of the

hole in order to flush out the drill cuttings!



3



Drilling machine

'he prime mover" converting electric" hydraulicor pneumatic energy into mechanical energy bymeans of a piston and a rotary mechanism!

Feed mechanism pneumatic" hydraulic piston or mechanical

mechanism that applies thrust to the drill rodand bit!



4



Drill rod'ransmits the percussive and rotary forces down the drill hole from the drill to

the bit! lso termed a steel " stem or pipe !

$n the case of relatively short holes (e!g! up to ) m*" only one steel is used at any

one time! #or the drilling of longer holes (up to +,, m for production blasting*"additional rods are attached" generally by means of screw threads at the endsof the rods" as the hole is deepened! 'he length of rod depends on the travel ofthe feed mechanism! series of connected drill rods is termed a drill string !

Bit

pplies the drill energy directly to the rock to achieve penetration!

Circulation fluid

-leans the hole" suppresses dust and cools the bit!



5



Percussive drill bits break rock by repetitive impaction and indentation! 'here are two ma&or types of percussive drill bit: the bra.ed bit and

the button bit!

a)Brazed Bit

/ra.ed bits are made up from either one or four rectangularprisms of cemented tungsten carbide! 'hese inserts are bra.ed into

the end of the steel drill bit! 'hey can be arranged as either cross bit

(inserts mounted at 0,o intervals around the bit face* or 12bit (angles

between the inserts 3, o and +,, o*! n advantage of the bra.ed bit is

that" when worn" it can be re2sharpened by grinding the tungsten

carbide inserts!



6



Drilled holes are included in the bit body" usually onecentre hole and four others located around the outsideof the bit! 'hese holes provide the passages for waterduring drilling that cools the bit inserts" removes rockchips and suppresses dust! 'he drill bits are always oflarger diameter than the rod so that there is anannulus" or gap" between the rod and the walls of the

drill hole! $t is through this gap that the drill waterflushes rock cuttings to the surface! 'his system isalso applied to button bits as described below!





'his type of bit features cylindrical tungsten

carbide inserts that are press2fitted into holes inthe end of the bit body !'he ends that protrudefrom the bit are domed! 'he button bit design isgenerally limited to tools greater than ), mm

diameter because of the space available formounting the buttons! 'he advantage of button overbra.ed bits in larger holes is that there is a moreeven distribution of cutting elements over the baseof the hole" resulting in higher drilling rates!4enerally" button bits cannot be re2sharpened!



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"



'op hammer rigs are used underground" in quarries andin surface mines using small diameter holes (such asgold mines when bench heights are kept relatively lowto improve grade control*! 'op hammer drills perform

best with small diameter holes and relatively shortdepths" as their penetration rate decreases withdepth and drill deviation increases with depth!

5ow ever recent drilling technology is permitting drill

machines with top hammers to drill up to +), mm diameters with penetrations up to 6, meters 7hour



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$n a compressed air rock drill" the supply line comes

in to a manifold! 'he percussive action of the drill isproduced by the action of a valve which directs liveair to either side of the piston! 'he piston strikesthe base of the drill steel" held in a chuck at the

front of the machine! water hose is also coupledto the drill! %ater is fed through the machine body"down a central hole in the drill steel to the bit!

Rotation may be produced by either a rifle bar andratchet system that is integral to the pistonmechanism or by an independent motor mountedoutside the hammer mechanism!



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$n the case of an integral rotation system" the hexagonal drillsteel fits into a similar hole in the chuck! 'he chuck engages the chuck bushing! $n the bushing" a 8chuck nut9 engages the straight splines onthe stem of the piston so that rotation of the piston causesrotation of the drill steel! 'he rifle nut in the head of the

piston engages spiral flutes on a rifle bar! ratchet mechanism allows the bar to rotate during the power stroke but not on the return stroke! -onsequently" the piston must rotate on the return stroke

and through the various connections the drill steel alsorotates!'his system is applied to hand2held rock drills as it limits the

si.e and weight of the machine!





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#ompressed air drilling mac$ine



$ndependent rotation is applied to drilling machines

mounted on large mechanised rigs! -ylindricalgearing is used to transmit rotation to the drillsteel! dvantages of this system are:

#or the same diameter piston the piston receives

more energy because" when the rifle bar iseliminated" the piston area on which thecompressed air acts is increased!

'he independent systems allow rotation andpercussion can be ad&usted to suit the rockconditions!



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'he development of hydraulic rock drilling inthe later +0,s and early +0;,s represented an

important technological advance! 5ydraulic

systems are inherently more efficient and alsosafer than compressed air power%



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Surface mines and quarries Down2the2hole (D'5*" or $n the hole ($'5*" drills differ from

conventional drills in that the piston itself is located 8down thehole9 at the end of the drill string and imparts percussive forcedirectly to the bit!

$t has the advantage that no energy is dissipated along thelength of the drill string and the penetration rate remainsvirtually constant" regardless of the hole depth!

#or the drilling of large diameter holes of +)2<<3 mm diameter"a D'5 rig is easier to design than the equivalent top hammer drill!=arge diameter holes improve the efficiency of blasting and D'5systems are in competition with rotary drilling for diametersabove +), mm in hard rock surface mines and large quarries!



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Di d t





1

Hydraulic top hammer More efficient than pneumatic.

Less drill rod fatigue, hence

lower consumables costs.

Penetration rates 50-100%

higher than pneumatic.

Less noise.

Greater flexibility - rotation,

pressure and percussion rate

variability.

Easier to automate

High capital cost of rigs.

Maintenance more complex.

Down-the-hole hammer Compared to pneumatic top

hammer:

Constant penetration rate ashole deepens.

Less bit wear due to flushing air.

Longer rod life.

Hole deviation greatly reduced.

Reduced air consumption.

Less noise.

Low penetration rates.

Hammer size limited by hole

diameter.Risk of losing hammer down

hole.

Advantages

Dis advantages



'he drill piston is powered by compressed air deliveredthrough the core of the rods at supply pressures rangingtypically from (3)2<), psi*! simple pneumatic orhydraulic motor mounted on the surface rig produces

rotation and flushing is achieved by the exhaust air fromthe hammer!

'he piston si.e in a D'5 drill is limited by the diameterof the hole but productivity can be improved byincreasing the air supply pressure! 'he introduction ofhigh pressure compressors means that D'5 designs withoperating pressures of +!; to <!> ?Pa are now available



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Sr no Hardness value mineral

01 1 Talc

02 2 &ypsum

03 3 calcite

04 4 'luorite

05 5 (patite

06 6 )rt$oclase

0 *uart+

08 8 Topa+

0" " Sap$ire

10 10 Diamond



1"



Hardness value Mineral

3%0 ime stone

3%5 Dolomite

4%0 &ranite

6%0 Trap roc-

6%5 c$ert

%0 *uart+



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1%.oc- mass properties

2%/plosive properties%

3%last geometry, angles o t$e blast$oles toards t$e ree ace%

4nitiation



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Detonation Theory: n e!plosi"e# or $lasting agent# is a

compound or a mixture of compounds" which" when initiated byheat" impact" friction" or shock" is capable of undergoing a

rapid decomposition" releasing tremendous amounts of heat

and gas!

'he decomposition is a self2propagating" exothermic reaction

called an explosion! 'he stable end products are gases that

are compressed" under elevated temperature" to very high

pressures!$t is the sudden rise in temperature and pressure from

ambient conditions that results in a shock wave" or a

detonation" travelling through the un reacted explosive!



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'he velocity of detonations lies in the approximaterange of (+),, to 0,,, m7s*" well above the speed

of sound in the explosive material!

Deflagration is the chemical burning of explosiveingredients at a rate well below the sonic velocity!

$t is associated with heat only and carries no

shock! Deflagration occurs when less than idealhole2loading conditions or explosive formulation

are involved



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ll commercial explosives are mixtures of carbon" hydrogen"

oxygen" and nitrogen! 'he maximum energy release upon detonation

occurs when the explosive mix is formulated for oxygen balance! n

oxygen2balanced mixture is one in which there is no excess or

deficiency in oxygen" such that the gaseous products formed are

chiefly 5<O (water vapour*" -O< (carbon dioxide*" and @<(nitrogen*!

$n actual blasting practice" small amounts of noxious gases such as

@O (nitric oxide*" -O (carbon monoxide*" @5> (ammonia*" -5>

(methane*" and solid carbon" are formed resulting in non ideal detonations and somewhat less than ideal

pressures and energies! -ommercial explosive formulation attempts

to achieve an oxygen2balanced mixture!



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DensityA 'he density of an explosive is defined as the weight

per unit volume or the specific gravity! -ommercial explosivesrange in density from ,!) to +!;! Bxplosives with a density less

than + will float in water!

'herefore" in water2filled holes" an explosive with a density greaterthan + is required!

#or certain granular explosives such as dynamite" density correlates tothe energy released in a given borehole volume! 5owever" for waterbased explosives" this is not the case" and often the reverse is true!Density is most useful in determining the loading density

=D or the weight of explosives one can load per unit length

of borehole



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'he ability of an explosive to withstand exposure to water for long

periods of time without loss of strength or ability to detonate

defines the water resistance!

numerical rating is used based on the results of tests performed

on the explosive! 5owever" explosive manufacturers individually

rate products based on a relative basis as good" fair" or poor

rating! 'he presence of moisture in amounts greater than )C

dissolves chemical components in dry blasting agents and alters the

composition of gases produced" contributing to the formation of

noxious fumes and lower energy output! 4elled granular productshave good water resistance" and certain water2based mixtures

have an excellent rating!



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Bxtreme low temperatures affect the stability as

well as the performance of explosives!

'he sensitivity and detonation velocity are

hampered for certain water2based explosives at

low temperatures while dynamites can becomedangerously unstable below free.ing temperatures!

Bxplosives manufacturers recommend the

appropriate range of temperature for

storage and use!



2



elocityA'he detonation velocity is the speed at which the detonation front moves through a column ofexplosives!

#or high explosives such as dynamite" the strength of anexplosive increases with detonation rate!

#or dry blasting agents and water2based explosives" fieldloading conditions greatly affect detonation velocity! Euchconditions include borehole diameter" density" confinementwithin the borehole" the presence of water" and other

factors!'he speed of detonation is important when blasting in hard"

competent rock where a brisance effect

is desired for good fragmentation%



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'he specific gas volume is the amount of gas created

by one kg of explosive under normal condition (odegrees and ;, mm hg* expressed in liters7kg!

'he explosion heat is the amount of energy released

upon detonation! $t is usually expressed in k&7kg! 'he effect of the

gas pressure wave depends on the amount of heat and

gas volume created by the explosion !$n heatexpansion 6,2 >, C of heat is converted into

mechanical work!



2"



'he combined effect of specific gas volume andexplosion heat is that the heat released actuallyexpands the gases produced !

'he greater the gas volume and the hotter thegases "the more effective the explosive is! 'he performance of an explosive is not only

determined by the total energy released by theexplosionF it also depends on the rate of energyrelease and how effectively the energy is utilisedfor breaking and moving the blasted material!



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$n short both the explosive properties and theproperties of the materials influence the explosiveseffectiveness!

'he charging density is the amount of explosive in a

certain hole volume !'he higher the charging density"the better the crushing ability of the explosivecharge !

'he strength per unit volume gives the extractingefficiency of an explosive at different chargingdensities compared to the extracting efficiency ofdynamites at the same charging densities!



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'he blast hole si.e is the first consideration of anyblast design! 'he blast hole diameter" along with thetype of explosive being used and the type of rockbeing blasted" determines the burden (distancefrom the blast hole to the nearest free face*!

ll other blast dimensions are a function of theburden! 'his discussion assumes that the blaster

has the freedom to select the borehole si.e! ?any operations limit borehole si.e based on

available drilling equipment!



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Hole diameter Charge per meter(kg/Mt)

254 0

230 50

203 45

18 30

152 20140 16

12 12

115 10

102 6

8" 4%3

6 3



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Practical blast hole diameters for surface





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Practical blast hole diameters for surfaceconstruction excavations range from 6 (;) mm* toapproximately +) inches (63 cm*! =arge blast hole

diameters generally yield low drilling and blastingcosts because large holes are cheaper to drill per unitvolume" and less sensitive" cheaper blasting agentscan be used in larger diameter holes! =arger diameter

blast holes also allow large burdens and spacing andcan give coarser fragmentation!

%



Epacing is defined as the distance between ad&acentblast holes" measured perpendicular to the burden!

%here the rows are blasted one after the other"

the spacing is measured between holes in a row!



35



Epacing that is too wide causes inadequatefracturing between holes" toe problems" and is

accompanied by humps on the face t$e spacing is

measured at an angle rom t$e original reeace%



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Epacing is calculated as a function of the burdenand also depends on the timing between holes! Epacing that is too close causes crushing and

cratering between holes" large blocks in the burden"and toe problems!



'he true spacing is twice the true burden" even thoughthe holes originally were drilled on a square pattern!

#ield experience has shown that the use of millisecond

(ms* delays between holes in a row results in betterfragmentation and also reduces ground vibrationsproduced by the blast! %hen ms delays are usedbetween holes in a row" the spacing2to2burden ratio

must be reduced to some$ere beteen 1%2 and1%8, it$ 1%5 being a good rst approimation



3



'he burden is defined as the distance from ablast hole to the nearest free face at theinstant of detonation!

$n multiple row blasts" the burden for a blast

hole is not necessarily measured in the directionof the original free face!

'he free faces developed by blast holes fired on

lower delay periods must be taken into account!



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$t is very important that the proper burden becalculated" accounting for the blast holediameter" relative density of the rock andexplosive" and" to some degree" the depth of the

blast hole!n insufficient burden will cause excessive air blast

and fly rock! 'oo large a burden will produce

inadequate fragmentation" toe problems" andexcessive ground vibrations!

%



3"



#or bulk2loaded charges (the charge is poureddown the hole*" the charge diameter is equal to

the blast hole diameter! #or tamped cartridges"

the charge diameter will be between thecartridge diameter and the blast hole

diameter" depending on the degree of tamping!

#or un tamped cartridges" the charge diameteris equal to the cartridge diameter!



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'he burden2to2charge2diameter ratio is seldom less than

<, or seldom more than >," even in extreme cases! #or

instance" when blasting with a low density blasting agent

such as @#O in a dense formation such as basalt" the

desired burden may be about <, times the charge

diameter! %hen blasting with denser slurries or

dynamites in low density formations such as sandstones"

the burden may approach >, times the charge diameter!



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ir blast and fly rock often occur because of an insufficient





42

ir blast and fly rock often occur because of an insufficientcollar distance (stemming column* above the explosive charge!s the blast hole diameter increases" the collar distancerequired to prevent violence increases! 'he ratio of collar

distance to blast hole diameter required to prevent violencevaries from +>:+ to <3:+" depending on the relative densitiesand velocities of the explosive and rock" the physical conditionof the rock" the type of stemming used" and the point of

initiation! larger collar distance is required where the sonic velocityof the rock exceeds the detonation velocity of the explosiveor where the rock is heavily fractured or low density! top initiated charge requires a larger collar distance than abottom2initiated charge! s the collar distance increases" thepowder distribution becomes poorer" resulting in poorerfragmentation of the rock in the upper part of the bench!



4round vibrations are controlled by reducingthe weight of explosive fired per delay

interval!

'his is done more easily with small blastholes than with large blast holes!

$n many situations where large diameter

blast holes are used near populated areas"several delays" along with decking" must be

used within each hole to control vibrations!



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=arge holes with large blast patterns are best suited to anoperation with: (+* a large volume of material to be moved

(<* large loading" hauling" and crushing equipment

(6* no requirement for fine" uniform fragmentation!

(>* an easily broken toe

()* few ground vibration or air blast problems (few nearby

buildings*

(* a relatively homogeneous" easily fragmented rock without

many planes of weakness or voids! ?any blasting &obs

have constraints that require smaller blast holes!



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Bxcessive sub grade drilling is unnecessary" expensive"

and may cause excessive ground vibrations because ofthe high degree of confinement of the explosive in thebottom of the blast hole" particularly when the primeris placed in the bottom of the hole!

$n multiple2bench operations" excessive sub drilling maycause undue fracturing in the upper portion of thebench below" creating difficulties in collaring holes inthe lower bench!

$nsufficient sub drilling causes a high bottom" resultingin increased wear and tear on equipment and expensivesecondary blasting or hand excavation in structuralfoundations!



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-ollar distance is the distance from the top of the

explosive charge to the collar of the blast hole! 'his .oneusually is filled with an inert material called stemming togive some confinement to the explosive gases and to reduceair blast!

well2graded" crushed gravel works best as stemming" but itis common practice to use drill cuttings because ofavailability and economics!

-ollar distances that are too short result in excessiveviolence in the form of air blast and fly rock and may causeback break!

(breaking beyond the back wall*!



46



-ollar distances that are too long create large blocks in the

upper part of the muck pile! 'he selection of a collardistance is often a trade off between fragmentation and

the amount of air blast and fly rock that can be tolerated!

'his is true especially where the upper part of the bench

contains rock that is difficult to break or is of a differenttype!

'he difference between a violent blast and one that fails to

fragment the upper .one properly may be a matter of only

a few feet of stemming!


4



#ield experience has shown that a collar distance equalto ;, percent of the burden is a good first

approximation!

-areful observation of air blast" fly rock" and

fragmentation will enable further refinement of this

dimension!

%here adequate fragmentation in the collar .one cannot

be attained while still controlling air blast and flyrock" deck charges or satellite (mid2spaced* holes may

be Required



48

deck charge is an explosive charge near



deck charge is an explosive charge nearthe top of the blast hole" separated fromthe main charge by inert stemming!

$f large blocky materials are being createdin the collar .one and less stemming wouldcause excessive ir blast or fly rock" the

main charge should be reduced



4"



-ollar or direct priming (placing the

primer at or near the collar of the blasthole with the blasting cap pointing

toward the bottom of the hole* of blast

holes normally causes more violence thancenter or toe priming and requires the

use of a longer collar distance! Tuesday, December 8,2015


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Detonating use consists o a narro tube lledit$ $ig$ eplosive%

$en an eplosion is initiated at one end bymeans o

a detonator, t$e eplosive ave travelsalong t$e use it$ a $ig$ velocity andcauses t$e detonation o ot$er $ig$eplosives $ic$ lie in its pat$% Detonatinguse is used or procuring t$e almostsimultaneous eplosion o a number oc$arges%



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k.v.ramana ,deilling and blasting

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