Blasting Operations

Blasting Operations Prepared by Dr. Ayman El-Midany 4th year Mining

WEEK ONE

Introduction General types of Explosives 

Commercial explosives

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Military explosives

Ingredient Ethylene glycol dinitrate Nitrocellulose (guncotton) Nitroglycerin Nitrostarch Trinitrotoluene (TNT) Metallic powder Black powder Pentaerythritol tetranitrate (PETN) Lead azide Mercury fulminate Ammonium nitrate Liquid oxygen Sodium nitrate Potassium nitrate Ground coal - Charcoal Paraffin Sulfur Fuel oil Wood pulp Lampblack Kieselguhr Chalk -Calcium carbonate Zinc oxide Sodium chloride

Chemical formula C2H4(NO3)2 C6H7(NO3)2O2 C3H5(NO3)3 C7H5N3O6 Al NaNO3 + C+ S C3H8N4O12 Pb(N3)2 Hg(ONC)2 NH4NO3 O2 NaNO3 KNO3 C CnH2n+2 S (CH3)2(CH2) (C6H16O3)n C SiO2 CaCO3 ZnO NaCI

Function Explosive base – lowers freezing point Explosive base – gelatinizing agent Explosive base Explosive base Explosive base Fuel sensitizer : used in high density slurries Explosive base Explosive base Explosive used in blasting caps Explosive used in blasting caps Explosive base : oxygen carrier Oxygen carrier Oxygen carrier – lowers freezing point Oxygen carrier Combustible, or fuel Combustible, or fuel Combustible, or fuel Combustible, or fuel Combustible, absorbent Combustible Absorbent – prevents caking Antacid Antacid Flame depressant (permissible explosives)

Chemical explosives 

is a compound or mixture which is capable of undergoing extremely rapid decomposition.

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

Chemical explosives When the explosive is detonated, gas is released,  temperature of the gas increases,  pressure also increases (Charles’ law).  move and break the rock. 

How to compare explosives  Detonation pressure Strength  Energy  Detonating velocity  Sensitivity  Fume class  Sensitiveness  Water resistance  Flammability  Density  Physical characteristics  Storage  Freezing

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How to compare explosives 

Strength : % of active material

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Velocity of Detonating (VOD): is the velocity at which the detonation wave moves through the explosive (ft/s or m/s)

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Fume class : the amount of toxic fumes which determine its safety to be used in particular situation such as underground operations.

How to compare explosives Detonation pressure : is the pressure behind the detonation front.  Energy  Sensitivity : the minimum energy/pressure needed for detonation.  Sensitiveness: measure of explosion wave spreading from one stick to another.  Flammability : easiness to ignite by flame or heat 

How to compare explosives

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Water resistance : is the ability to resist contamination or a reduction in strength when exposed to water. Sometimes determined by the length of time it can be submerged in water and still perform as designed.

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Density : is the explosive wt per given volume. A cartridge of 90 sticks per 50-lb case is denser than a cartridge of 110 sticks per 50-lb case. Aid in blast design.

How to compare explosives 

Physical characteristics: commercial explosives can take three basic forms: granular, gelatin, and slurry. The form depends on the formula, and the choice of form depends on the usage required. The package for the same explosive product may also vary according to usage. For example, a slurry can be pumped into a borehole with no container, or it can be packaged in polyethylene bags to permit handling in smaller amounts.

How to compare explosives Storage: how explosive can be stored without affecting its safety, reliability, and performance. Early nitroglycerin (NG) dynamites were extremely poor for storing due to separation of NG from the other components and creates an extremely hazardous condition.  Freezing : important for safety and performance especially in cold climate. Anitfreezing additives may be used. 

Drills and drilling The drilling system consists of the drill: the drill steel, or rod; and the bit. The bit penetrates the rock by the force it imposes on the rock. Bits are designed for percussion, rotary drilling, or both.  Hand held drills  External –percussion drills  Down-the-hole drills  Rotary drills 

Rock Shear strength Rock Sandstone

Condition

Shear strength, lb/in2

Soft Medium Hard gray Fine-grained brown Medium-grained friable gray

1500 3050 4720 3600 2840

Rock Shear strength Rock Limestone

Siltstone Dolomite quartzite

Condition Hard flossiliferrous Hard gray Medium crystalline

Shear strength, lb/in2 4160 6520 7600 3000 12700 10600

Drill Selection Size of project : drill type and size  Hole diameter : drill type and bit size  Depth of cut : long or short  Rock hardness: percussion (4-6.5) or rotary (23.5) on Mohs’ scale  Capital : machine price  Cost : cost per foot of borehole – need specialized operator 

WEEK TWO

Firing systems 

Blasting Cap

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Detonating systems

Blasting Caps 

Blasting Cap : are small cylindrical tubes that detonate cap-sensitive explosives. They are usually made of copper or aluminum and contains an explosive.

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There are three types of blasting caps: • Common caps • Millisecond delays (MS delays) • Standard delays

Common Blasting Caps Detonated by a fuse  Now they are the least common  Copper or aluminum Cylinder  38 mm long X 6 mm dia, closed at one end.  Contains two types of charges : igniting charge and the base charge.  A safety fuse is inserted into the open end of the cap to ensure that the flame reaches the igniting charge completely. To prevent water and contaminants from entering the cap and inhibiting detonation. 

Common Blasting Caps 

When the fuse is ignited, the powder core burns, acting as a vehicle through which the fire is transmitted to the igniting charge end of the cap.

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The burning fuse spits a flame resembling a jet flame called as “ignition spit”.

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When the flame travels to the cap, it ignites the ignition charge, which detonates the base charge, which in turn detonates the explosive charge that is being primed with the cap.

Common Blasting Caps Base charge

Fuse

Ignition charge

Electric Blasting Caps More controllable method  Contains charges like the common cap, but instead of safety fuse the cap contains two wires that meet at a bridge wire.  when electric current is applied, the bridge wire burns, igniting the charge in the cap.  Enables the blaster 

• to choose the suitable time of detonation • to shoot more holes than the safety fuse method

Delay Blasting Caps Are caps that are detonated by electricity in various time-delay intervals.  Two types : standard and millisecond (MS).  Advantages of (MS) are: 

•Reduce ground vibration •Improve fragmentation •Produce less flyrock •Reduce costs •Reduce overbreak

Delay Blasting Flyrock

1st row

Free face

Flyrock, excessive movement of blasted rock in the air, caused by not using delay blasting. Second row cannot move toward face and therefore must either fly or remain in place.

Delay Blasting •

Delay blasting can help reduce flyrock by permitting the rock to move in the direction desired rather than moving haphazardly through the air.

• Blasting without delays requires more drilling and explosive to break the rock because the rock tends to resist breakage and lack of a sufficient number of free faces. • Delay blasting reduces overbreak.

Delay Blasting Free face

Other Blasting Caps Vented caps : with vents to delay blasting to reduce the combustion rate in the blasting cap.

Composition Electric caps : contains a mixture of chemical compounds.

Detonating Systems The main components of an electric detonating system • Blasting cap • Blasting wire • Power source or blasting machine Detonating cord • Transmit detonations from blasting cap to explosives • Less sensitive than blasting caps • Is a high explosive and burns at speed > 4 miles/second

Blasting Circuits Three types of circuits • Single-series • Straight parallel

• Parallel-series

Blasting Circuits

Problems

Theory of Breakage Purpose of blasting • One solid piece → smaller pieces (fragmentation) → 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 • 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 Free face

Borehole

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 • 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 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 putting a CS against the borehole wall causing its bending. • The deeper the hole, the greater the burden and borehole spacing. • M = wl2/8 where w is the load (burden), l is the borehole length.

Stemming • Is non-explosive material that is placed in the borehole between the top of the explosive column and the collar of the hole. • Sand, drill fines, or gravel • Confine and delay the escape of expansive gases and increases the explosive’s efficiency (reduces the explosive used). • Reduce the flyrocks, increase ground vibration, and air blast • Rifling : in case of impropoer stemming, blowing of the stemming material.

Angle of Breakage • Is the measured angle at which a homogeneous material can be expected to break from the explosive charge to the free face.

Free face

135° 90°

WEEK THREE

Blast Design • Is the safe and economic way to do blasting Factors affecting blasting design • Geological factors (out of blaster’s control) • Controllable factors •Borehole dia. •Burden •Spacing •Stemming •Design of the delay firing system.

Hole Diameter Depends on •The availability of the equipment •The depth of the cut •The distance of the nearest structure. • Max dia. Depends on the hole depth L (ft) = 2D (in) There are four methods to decrease the explosives amount: •Use delay firing •Shorten the depth of the cut •Decrease the hole dia •Use decking technique

Burden & spacing determination Burden is the distance from the blast hole to the nearest perpendicular free face. Spacing

Burden

Free face

Burden & spacing determination Andersen Formula B : burden, ft

B= (dL)0.5 d : borehole dia, in

L : borehole Length, ft

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.

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. There are 4 methods • Line drilling (unloaded), Fig.8-2 • 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. • 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

Free face

Unloaded line drill holes

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, 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

Pre-splitting • Creates a plane of shear in solid rows along the desired excavation before the production blast. • All holes are loaded like cushion blasting • Reduces overbreak • Reduces the vibration