Explosives, Theory of Breakage and Blasting OperationsBlasting Operations

Author: Partha Das Sharma, B.Tech(Hons.) in Mining Engineering,E.mail: sharmapd1@gmail.com, Website: http://miningandblasting.wordpress.com/

Introduction

General types of Explosives

• Commercial explosives

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• Commercial explosives

• Military explosives

Explosive Ingredients and their FunctionIngredient Chemical formula Function

Ethylene glycol dinitrateNitrocellulose (guncotton)NitroglycerinNitrostarchTrinitrotoluene (TNT)Metallic powderBlack powderPentaerythritol tetranitrate (PETN)Lead azideMercury fulminate

C2H4(NO3)2C6H7(NO3)2O2C3H5(NO3)3

C7H5N3O6AlNaNO3 + C+ SC3H8N4O12

Pb(N3)2 Hg(ONC)

Explosive base – lowers freezing pointExplosive base – gelatinizing agentExplosive baseExplosive base Explosive base Fuel sensitizer : used in high density slurriesExplosive base Explosive base

Explosive used in blasting capsExplosive used in blasting caps

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Mercury fulminateAmmonium nitrateLiquid oxygenSodium nitratePotassium nitrateGround coal - CharcoalParaffinSulfurFuel oilWood pulpLampblackKieselguhrChalk -Calcium carbonateZinc oxideSodium chloride

3 2 Hg(ONC)2NH4NO3O2NaNO3KNO3CCnH2n+2S(CH3)2(CH2)(C6H16O3)nCSiO2CaCO3ZnONaCI

Explosive used in blasting capsExplosive base : oxygen carrierOxygen carrier Oxygen carrier – lowers freezing point Oxygen carrierCombustible, or fuel Combustible, or fuelCombustible, or fuelCombustible, or fuelCombustible, absorbentCombustibleAbsorbent – prevents cakingAntacidAntacidFlame depressant (permissible explosives)

Chemical explosives

• is a compound or mixture which is capable ofundergoing extremely rapid decomposition.

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• An explosion can be broken down into fourphases• Release of gas• Intense heat• Extreme pressure, and• The explosion

Chemical explosives

When the explosive is detonated,

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• gas is released,• temperature of the gas increases,• pressure also increases (Charles’ law).• move and break the rock.

How to compare explosives

• Strength• Detonating velocity• Fume class• Water resistance

• Detonation pressure• Energy

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• Water resistance• Density• Physicalcharacteristics

• Storage• Freezing

• Energy• Sensitivity• Sensitiveness• Flammability

How to compare explosives

• Strength : % of active material

• Velocity of Detonating (VOD): is the velocity at

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• Velocity of Detonating (VOD): is the velocity atwhich the detonation wave moves through theexplosive (ft/s or m/s)

• Fume class : the amount of toxic fumes whichdetermine its safety to be used in particularsituation such as underground operations.

How to compare explosives

• Detonation pressure : is the pressure behindthe detonation front.

• Energy

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• Energy• Sensitivity : the minimum energy/pressureneeded for detonation.

• Sensitiveness: measure of explosion wavespreading from one stick to another.

• Flammability : easiness to ignite by flame orheat

How to compare explosives

• Water resistance : is the ability to resistcontamination or a reduction in strength whenexposed to water. Sometimes determined by the

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exposed to water. Sometimes determined by thelength of time it can be submerged in water andstill perform as designed.

• Density : is the explosive wt per given volume.Aid in blast design.

How to compare explosives

• Physical characteristics: commercialexplosives can take three basic forms:

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explosives can take three basic forms:granular, gelatin, slurry and emulsion. Thechoice of form depends on the usagerequired.

How to compare explosives

• Storage: how explosive can be stored withoutaffecting its safety, reliability, and performance.Early nitroglycerin (NG) dynamites were

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Early nitroglycerin (NG) dynamites wereextremely poor for storing due to separation of NGfrom the other components and creates anextremely hazardous condition.

• Freezing : important for safety and performanceespecially in cold climate. Anitfreezing additivesmay be used.

Drills and Drilling

• The drilling system consists of the drill: the drillsteel, or rod; and the bit. The bit penetrates therock by the force it imposes on the rock. Bits are

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rock by the force it imposes on the rock. Bits aredesigned for percussion, rotary drilling, or both.

• Hand held drills• External –percussion drills• Down-the-hole drills• Rotary drills

Theory of Breakage

Purpose of blasting• One solid piece → smaller pieces (fragmentation) → to be moved or excavated (movement).

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→ to be moved or excavated (movement).• Underground blasting, for example, requires greater fragmentation than surface blasting because of the size of the equipment that can be used and the difficulty of access.

• Get the desired results with a minimum cost

Theory of Breakage

Involves two basic processes:• Radial cracking• Flexural rupture

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• Flexural rupture

• Rock is stronger in compression than in tension. Therefore, the easiest way to break rock is to subject it to a tensile stress greater than its ultimate strength in tension.

• Rocks are heterogeneous (contain different types of rocks). They differ in their density.

Theory of Breakage

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Borehole

Free face

Compression waves

Radial cracking

Theory of Breakage

• The distance from the borehole to the free face is the burden.

• The denser the rock the faster the waves

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• The denser the rock the faster the waves• Proper fragmentation when enough to travel to the face and back overcoming the tensile strength of the rock.

• Along the face the outermost edge is stretched in tension which causes cracks.

Flexural Rupture

• The second process in breaking rock by bending the rock to the point where the outside edge, the

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side in tension, breaks.• Caused by the rapid expansion of gases in borehole.

• Analogous to the bending and breaking of a beam.• Movement or displacement are required in addition to cracking.

Flexural Rupture

• After detonation the redial cracks expands and the gas starts to the movement by

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and the gas starts to the movement by putting a CS against the borehole wall causing its bending.

• The deeper the hole, the greater the burden and borehole spacing.

Blast Design

• Is the safe and economic way to do blasting

•• Factors affecting blasting designFactors affecting blasting design

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•• Factors affecting blasting designFactors affecting blasting design• Geological factors (out of blaster’s control)• Controllable factors

• Borehole dia.• Burden• Spacing• Stemming• Design of the delay firing system.

Burden & spacing

Burden is the distance from the blast hole to the nearest perpendicular free face.

Spacing

20Free face

Burden

Burden & spacing determination

Andersen Formula B= (dL)0.5 • B : burden, ft d : borehole dia, in• L : borehole Length, ft

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• Langefors’ Formula V= (db/33) [Ps/cf(E/V)]0.5

• V : burden, m db : dia of drill bit, mm• P : degree of packing = 1-1.6 kg/dm3• s : wt strength of explosives (1.3 for gelatin)• c : rock constant, generally 0.45• f : 1 degree of fraction, for straight hole = 1• E/V = ratio of spacing to burden

Spacing determination

Spacing is the distance between blast holes fired in the same row

• It is necessary to complete burden calculations before determining the spacing.

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before determining the spacing.

S= (BL)0.5

• B : burden, ft• L : borehole Length, ft

Controlled Blasting

To control overbreak and to aid the stability of the remaining rock formation.

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• There are following methods:• Line drilling (unloaded), • Cushion blasting• Smooth-wall blasting• Presplitting

Controlled Blasting – Line drilling

• Provides a plane of weakness to which the rock can break.• Helps to reflect shock waves,• Reduces the shattering effect of the rock outside the perimeter.

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perimeter.• Do not exceed 3 in in dia and are spaced one to four diameters apart (due to cost).

• Are not loaded• Requires more drilling more than the other controlled blasting methods.

• Is not very effective in non-homogeneous formations.

Controlled Blasting – Line drilling

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Unloaded line drill holes

Free face

Cushion Blasting

• Requires a single row of holes ( 2 to 3.5 in) in dia.• Permits a reduction in the No. of holes required by line-drilling• Unlike line-drilling holes, the cushion holes are loaded with

light charges.• Holes are fully stemmed between charges, allowing no air gap,

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• Holes are fully stemmed between charges, allowing no air gap, and are fired after the production shot has been excavated.

• The stemming acts as a cushion to protect the finished wall from the shock waves. The larger the borehole, the greater the cushion.

• Not suitable for underground - tough stemming requirements.• Drawbacks: (1) requires removal of excavated material before

firing (costly due to production delay – no excavation for entire area at once). (2) Sometimes the production shot can break back to the cushion holes, creating redrilling problems and causing loading changes.

Smooth-wall Blasting

• Similar to cushion blasting

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Pre-splitting

• Creates a plane of shear in solid rows along the desired excavation before the production

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the desired excavation before the production blast.

• All holes are loaded like cushion blasting• Reduces overbreak• Reduces the vibration