Use of explosives

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LOADING AND BLASTING

Load/Blast Explosive Products

Explosive Reaction'

An explosive is a chemical compound or mixture that undergoes a rapid decomposition when initiated by energy in the form of heat, impact, friction, or shock. This decomposition produces mostly gases and a large amount of heat. These very hot gases produce extremely high pressures, and it is these pressures in the range of one million pounds per square inch that cause the rock to fragment.

The principal reacting ingredients in an explosive are fuels and oxidizers. Common fuels used in commercial explosives include fuel oil, carbon, aluminum, TNT, smokeless powder, monomethylamine nitrate, and monoethanol amine nitrate. Fuels also often act as a sensitizer for the mixture. The most common oxidizer is ammonium nitrate, although sodium nitrate and calcium nitrate are also used.

There are also many other ingredients used including water, gums, thickeners and cross-linking agents, stabilizers, emulsifiers, and microballoons. Most ingredients of explosives are composed of the elements oxygen, nitrogen, and carbon. For explosive mixtures, energy release is optimized at zero oxygen balance. In theory the gaseous products of detonation for an oxygen balanced mixture are water, carbon dioxide, and nitrogen gas, although there are always trace amounts of carbon monoxide, nitrous oxide, NO, and CH4 produced. Gases resulting from an improper oxygen balanced mixture are not only inefficient in terms of energy release but they are also poisonous.

When a column of explosive is initiated, the detonation spreads throughout the column. The primary reaction occurs between a shock front at the leading edge and a rear boundary, known as the Chapman- Jouguet or C-J plane. Part of the reaction may occur behind the C-J plane particularly if some of the explosive ingredients are coarse. The length of this reaction zone, which depends on the explosive ingredients, particle size, density, and confinement, determines the minimum diameter at which the explosive can be used.

The initial pressure, called the detonation pressure, is created by the supersonic shock front moving out from the detonation zone. The detonation pressure gives the explosive its shattering action in the vicinity of the borehole. The detonation pressure is followed by a sustained pressure called explosion pressure or borehole pressure. Borehole pressure is created by the rapid expansion of the hot gases within the borehole. The detonation and borehole pressures, are significant factors in determining how a given explosive will perform.

Rock Breakage Fundamentals

When an explosive is initiated, the mixture of solids and liquids that comprise the explosive are converted within a few nanoseconds (10 to the -9 seconds) to an extremely high density gas. As the conversion takes place along the borehole, a tremendous shock energy is applied to the wall of the borehole. A shock wave radiates outward from the borehole in all directions. The rock in contact with the explosive is pulverized and the borehole expands slightly. The radiating shock wave causes radial cracking in all directions up to several diameters away from the hole. These radial cracks are caused by tensile stresses perpendicular to the direction of the shock wave, commonly referred to as hoop stresses.

Beyond several borehole diameters the shock wave is reduced in intensity and becomes merely a vibration, slightly deforming the rock elastically. As this shock wave moves in all directions, the portion moving away from the free face is eventually absorbed by the rock over distance. The portion moving towards the free face is partially reflected back into the rock as a tensile wave. This produces tiny cracks in the rock between the free face and the borehole as the shock wave now travels back from the free face to the borehole, as rock fails more easily in tension than compression. The movement of the shock wave

travelling to the free face and being reflected back is known as preconditioning. The success of the remainder of the blasting process will depend on how effective the initial preconditioning was.

Rock movement and fragmentation begin to occur as the high pressure gasses start to escape into the radial and concentric cracks in the preconditioned rock. The gasses force apart the cracks causing further grinding and cracking of the rock. The amount of explosive gas produced by the detonation will help determine the looseness and heave of the muckpile.

Properties of Explosives

There are many different properties of explosives that may be used to determine how they will perform in field conditions. These properties include detonation velocity, density, water resistance, fume class, detonation pressure, borehole pressure, sensitivity, weight strength, bulk strength, storage life, gas volume and cost.

(a) Detonation Velocity

The velocity of detonation (VOD) is the rate at which the chemical reaction driving the shock front travels through an explosive column. For most explosives the VOD will show a rapid increase in speed from the point of initiation until the VOD attains the steady state velocity or uniform VOD of the explosive. The steady state VOD will normally be attained within a few charge diameters of the point of initiation. The steady state velocity remains fairly constant for a given explosive, but varies from one explosive to

another depending on composition, particle size, and density. The steady state velocity is also affected by charge diameter and borehole confinement.

The VOD determines the rate of energy release. A high VOD explosive will have a large shattering effect which will be useful in strong homogeneous rock. A lower VOD explosive will have less shattering effect, but often produces a larger gas volume resulting in more heave and a looser muckpile.

'(b) Density

Density is normally expressed in terms of specific gravity, which is the ratio of the density of the explosive to that of water. The density of the explosive will determine the charge weight per foot of borehole that is attainable.

(c) Water Resistance

Water resistance is the ability of an explosive product to withstand exposure to water without losing sensitivity or undergoing a reduction in velocity of detonation from published specifications.

'(d) Fume Class

Fume class is a measure of the amount of toxic gases, primarily carbon monoxide and oxides of nitrogen, produced by the detonation of an explosive.

(e) Detonation Pressure

The detonation pressure of an explosive is primarily a function of the detonation velocity squared times

the density. Some blasting theories support the use of a high detonation pressure and the resulting strong shock wave as being essential to break dense competent rock.

(f) Borehole Pressure

Borehole pressure, sometimes called explosion pressure, is the pressure exerted on the borehole walls

by the expanding gases of detonation after the chemical reaction has been completed. Borehole pressure is generally considered to play the dominant role in breaking most rocks and in displacing all types of rocks encountered in blasting.

(g) Sensitivity

Sensitivity is defined as an explosive's susceptibility to initiation. Sensitivity to a number 8 strength cap defines the product as an explosive. Lack of sensitivity to a number 8 strength cap results in a classification as a blasting agent.

(h) Weight Strength

Weight Strength is more correctly known as relative weight strength. It is the measure of the energy available per weight of explosive as compared to an equal weight of ANFO. It is calculated by dividing the absolute weight strength of the explosive by the absolute weight strength of ANFO and multiplying by 100.

(i) Bulk Strength

Relative bulk strength is a measure of the energy available per volume of explosive as compared to an equal volume of bulk ANFO at a density of .81 (various explosive companies may use .85 as the standard which will change the relative rating of other products). It is calculated by dividing the absolute bulk strength of an explosive by the absolute bulk strength of bulk ANFO and multiplying by 100.

'(j) Storage

The important considerations with respect to storage are the shelf life the explosive has prior to use and the length of time the explosive can be left loaded in the borehole and still detonate.

(k) Cost

The cost of an explosive is an easily determined item. For comparative purposes the cost of a given explosive will be compared as a ratio to the cost of ANFO on a by weight and by volume basis.

(l) Gas Volume

Gas volume is a measure of the total volume of gas produced by the detonation.

Comparison of explosives

Blast Vibrations

When an explosive is detonated the shock waves are transmitted outwards in all directions. The vibrations directed towards the free face induce cracking that will help ensure fragmentation when the borehole gasses begin to move the rock. The vibration directed away from the free face becomes wasted energy and may produce damage underground or on surface.


There are many methods of predicting the vibration level for a given explosive charge. All the methods have used actual data as the basis for establishing an empirical relationship. There is no current method available to predict the vibration level with total certainty. The vibration level produced is a function of the following items.

'(a) Charge Weight

The larger the weight of explosive detonated, the higher the level of the blast vibrations that will be produced. The explosive is a source of energy. Increasing the weight of the explosive will increase the total energy available to break rock and the portion that escapes as waste energy.

(b) Actual Delay Interval

The firing times of delay detonators are average times, not actual times. As a multiple delay blast is initiated, each charge will produce vibrations. These vibrations will attenuate over time and distance. If two charges are detonated within 5 or 10 milliseconds of each other, the vibration level produced will likely be higher than the level for the charges individually. The amount of time that should be allowed between each charge is generally determined by experience and depends on the ground type, the explosive used, and the type of delay detonator used.

(c) Geometry

The geometry of one explosive charge relative to another will also play a major role in the vibration level produced. If the charges are in adjacent boreholes, the vibration levels produced will be higher than if the boreholes were on opposite sides of a stope, or on different horizons.

(d) Confinement

The degree of confinement, or the distance to a free face, will also greatly effect the vibration levels produced. If the confinement or distance is too great, then the energy that would normally be absorbed by fracturing the rock will be available as vibration energy. This is why if a blast or hole freezes, or doesn't break properly, there are very high blast vibrations produced.

(e) Stress Levels

The higher the stress levels in the rock, the less the vibrations will be attenuated over distance. The lower the stress levels, the higher the attenuation.

(f) Distance

The prediction of blast vibration levels within a 200 foot radius of a blast is much easier than a prediction of vibration levels at a distance of one mile. Blast vibrations will attenuate over distance and time. Close to a blast the vibration waveform is high in frequency, high in velocity and high in displacement. This

vibration will die out rapidly and can be reasonably predicted. As distance increases, the frequency of the vibration decreases, the displacement decreases, and the velocity decreases, but the rate of the decay keeps decreasing.

The greater the distance the vibration is monitored, from a multiple delayed blast, the more the delayed shots tend to look like one shot. With time the vibrations from the different shots tend to merge together and will travel for great distances. The total charge weight within a range of delay intervals will give a better idea of the vibration levels produced at large distances.

When trying to design blasts to minimize the vibration levels produced beyond the mine property, remember that the human body is sensitive to vibrations at levels about 100 times less than the generally accepted level of producing any damage to a house.

(g) Structure

The rock structure will also play a significant role in the blast vibration levels produced. A structure may attenuate or absorb the vibrations, or it may block, direct, or funnel the vibrations increasing their overall effect.

Air Vibrations

Air blasts, like ground vibrations, are another undesirable by-product of blasting, and another waste of explosive energy. There are four main types of air blast overpressures: air pressure pulse, produced from direct rock displacement at the face; rock pressure pulse, produced from the vibrating ground; gas

release pulse, resulting from gas escaping from cracks in the rock; stemming release pulse, caused by gas escaping from blown out stemming.

Blasting Fumes

In an ideal detonation, an oxygen-balanced explosive with the atomic constituents carbon, hydrogen, nitrogen and oxygen would form only the gaseous reaction products carbon dioxide, water vapour, and nitrogen gas. In real detonations, due to incomplete reaction of the explosive and subsequent reactions with the surrounding air, other reaction products will always be present. The primary toxic fumes produced are carbon monoxide, nitrous oxide (NO), and nitric oxide (NO2). The later two are commonly grouped as oxides of nitrogen (NOx).

The amount of nonideal detonation products formed depend on a number of factors: the explosive composition and its homogeneity, the presence of water and the water resistance of the explosive, the velocity of detonation, the charge diameter, the loaded density, the type of initiation, and the confinement of the explosive. Before and during the detonation, additional reactions can occur between the explosive and the surrounding rock; for example, when the rock contains sulphide or other reactive components.

Safety

The definition of an explosive is: an explosive is a chemical compound or mixture that undergoes a rapid decomposition when initiated by energy in the form of heat, impact, friction, or shock. All explosives are designed to detonate and should be handled at all times with this in mind. The safe handling of any explosive must take into account the factors that could initiate an explosive.

References


ROCK BLASTING AND EXPLOSIVES ENGINEERING, by Per-Anders Persson, Roger Holmberg, and

Jaimin Lee, CRC Press


EXPLOSIVES AND ROCK BLASTING, by Atlas Powder Company.


THE SCIENCE OF INDUSTRIAL EXPLOSIVES, by Melvin Cook, published by Ireco Chemicals.


BACK TO BASICS...SERIES, by Richard Dick, Dennis D'Andrea, Larry Fletcher, published in the Society of Explosive Engineers over several months in 1993.


CORRESPONDENCE AND TECHNICAL DATA, from ICI Explosives, ETI Explosives and Dyno

Explosives.

Up hole loading

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Research opportunities

References

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