Blasting Operations

Introduction to Explosives

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

An explosion can be broken down into four phasesRelease of gasIntense heatExtreme pressure, andThe explosionChemical 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 explosivesStrengthDetonating velocity Fume classWater resistanceDensityPhysical characteristicsStorageFreezingDetonation pressureEnergySensitivity SensitivenessFlammabilityHow to compare explosivesStrength : % of active material

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

Fume class : the amount of toxic fumes which determine its safety to be used in particular situation such as underground operations.How to compare explosivesDetonation pressure : is the pressure behind the detonation front.EnergySensitivity : 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 heatHow to compare explosivesWater 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.

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 explosivesPhysical 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 explosivesStorage: 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 drillingThe 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 drillsExternal percussion drillsDown-the-hole drillsRotary drills

Rock Shear strengthRockConditionShear strength, lb/in2SandstoneSoftMediumHard grayFine-grained brownMedium-grained friable gray15003050472036002840Rock Shear strengthRockConditionShear strength, lb/in2LimestoneHard flossiliferrousHard grayMedium crystalline416065207600SiltstoneDolomite quartzite30001270010600Drill SelectionSize of project : drill type and sizeHole diameter : drill type and bit sizeDepth of cut : long or shortRock hardness: percussion (4-6.5) or rotary (2-3.5) on Mohs scaleCapital : machine price Cost : cost per foot of borehole need specialized operator

Firing systemsBlasting CapDetonating systems

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

There are three types of blasting caps: Common caps Millisecond delays (MS delays) Standard delaysCommon Blasting CapsDetonated by a fuseNow they are the least commonCopper 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 CapsWhen 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.

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

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 CapsBase chargeIgnition chargeFuseElectric Blasting CapsMore controllable methodContains 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

Delay Blasting CapsAre caps that are detonated by electricity in various time-delay intervals.Two types : standard and millisecond (MS).Advantages of (MS) are:Reduce ground vibrationImprove fragmentationProduce less flyrockReduce costsReduce overbreakDelay BlastingFlyrock, 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.Free face1st rowFlyrockDelay 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 BlastingFree faceBlasting CircuitsThree types of circuits Single-series

Straight parallel

Parallel-seriesTheory of BreakagePurpose 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 costTheory of BreakageInvolves two basic processes:Radial crackingFlexural 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 BreakageBoreholeFree faceCompression wavesRadial crackingTheory of BreakageThe distance from the borehole to the free face is the burden.The denser the rock the faster the wavesProper 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 RuptureThe 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 RuptureAfter 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.StemmingIs 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 gravelConfine and delay the escape of expansive gases and increases the explosives efficiency (reduces the explosive used).Reduce the flyrocks, increase ground vibration, and air blastRifling : in case of impropoer stemming, blowing of the stemming material.Angle of BreakageIs the measured angle at which a homogeneous material can be expected to break from the explosive charge to the free face.135 90Free faceBlast DesignIs the safe and economic way to do blasting

Factors affecting blasting designGeological factors (out of blasters control)Controllable factorsBorehole dia.BurdenSpacingStemmingDesign of the delay firing system.Hole DiameterDepends onThe availability of the equipmentThe depth of the cutThe 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 firingShorten the depth of the cutDecrease the hole diaUse decking techniqueBurden & spacing determinationBurden is the distance from the blast hole to the nearest perpendicular free face.Free faceSpacingBurdenBurden & spacing determinationAndersen FormulaB= (dL)0.5 B : burden, ftd : borehole dia, inL : borehole Length, ftLangefors FormulaV= (db/33) [Ps/cf(E/V)]0.5

V : burden, mdb : dia of drill bit, mmP : degree of packing = 1-1.6 kg/dm3s : wt strength of explosives (1.3 for gelatin)c : rock constant, generally 0.45f : 1 degree of fraction, for straight hole = 1E/V = ratio of spacing to burdenSpacing determinationSpacing is the distance between blast holes fired in the same rowIt is necessary to complete burden calculations before determining the spacing.

S= (BL)0.5

B : burden, ftL : borehole Length, ftControlled BlastingTo 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 PresplittingControlled Blasting Line drillingProvides 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 loadedRequires more drilling more than the other controlled blasting methods.Is not very effective in non-homogeneous formationsControlled Blasting Line drillingUnloaded line drill holesFree faceCushion BlastingRequires a single row of holes ( 2 to 3.5 in) in dia.Permits a reduction in the No. of holes required by line-drillingUnlike 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.Pre-splittingCreates a plane of shear in solid rows along the desired excavation before the production blast.All holes are loaded like cushion blastingReduces overbreakReduces the vibration