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INTRODUCTION

Most rocks and ores require blasting to break them into smaller particles before an excavation takes place in mining operations.  Explosive materials are tools that benefit mankind, when used properly.  However, improper use can be disastrous.  Blasting is one of the most important parts in any mine because without the appropriate performance, the results are not only the failure of the blasting operation but the mine operation could be jeopardized. The major factors that influence blasting results are the properties of explosives being used, the initiation systems, the distribution of the explosive in the blast, rock structure, the overall geometry, and other factors.

This paper presents the basic knowledge of explosive and blasting operations which could be useful for most surface mining operations, construction and some other works that need explosives and blasting.

TYPES OF COMMERCIAL EXPLOSIVE PRODUCTS

There are many types of explosives used in commercial blasting, but we can classify them generally as dynamite, ANFO and slurry.

       The selection of the proper explosive is based on three criteria:

                - Its ability to function property in the proposed  environment

                - The performance characteristic of explosive

                - Cost

                Explosives are different in the following ways:

                - Strength

                - The ability to resist water and water pressure

                - Input energy needed to start the reaction

                - Minimum diameter in which detonation will occur

                - Flammability

                - Generation of toxic fumes

                - Detonation pressure

                - Ability to remain in original configuration

                - Ability to function under different temperature condition

                - Reaction velocity

                - Bulk density

                In the explosives selection process, we evaluate each of characteristics to determine which are importance to us and which are not.  For instance, if we had a project with dry blast holes, we do not need a product that has a high water resistance; or if we are using large diameter blastholes, it is not important whether the explosive functions reliably in a very small diameter blasthole.  We should purchase the explosive product that will function properly for the blast, at the lower cost.

EXPLOSIVE PROPERTIES

                Strength:   The term strength refers to the energy content of an explosive which is the measure of the force it can develop and its ability to do work.  There are no standard strength measurement methods used by explosives manufacturers.  Many different strength measurement methods have been used, such as the ballistic motor test, seismic execution, strain pulse measurement, calculation of detonation pressure, calculation of borehole pressure, determination of the heat release, and the bubble energy test.  However, none of these methods can be satisfactorily used for blasting design purposes. Theoretical or controlled test data does not necessarily represent the useful work energy delivered in the borehole.  The same kind of explosive may have different efficiencies  when used in different borehole diameters.  Strength ratings are misleading and do not accurately compare rock fragmentation effectiveness with explosive type.  One can say that strength ratings are only a tool used to identify the end results and associate them with a specific product.

                Sensitivity:   The sensitivity of  an explosive product is defined by the amount of in put energy necessary to cause the product to detonate reliably, sometimes called minimum booster rating, or minimum priming requirements.  Some explosives require little energy to detonate reliably such as dynamite, and some of the cap sensitive slurries can be detonated by a standard number 8 blasting cap.  A blasting cap alone will not reliably initiate bulk loaded ANFO and some slurries; a primer or booster would have to be used in conjunction with the blasting cap to get reliable detonation of these products.  The following table summarizes the sensitivity of several types of explosives:

 

 

Sensitivity

 

TYPE

HAZARD SENSITIVITY

PERFORMANCE SENSITIVITY

 

 

 

Granular Dynamite

Moderate to High

Excellent

Gelatin Dynamite

Moderate

Excellent

Bulk Slurry

Low

Good to Very Good

Cartridged Slurry

Low

Good to Very Good

Poured ANFO

Low

Poor to Good

Packaged ANFO

Low

Good to Very Good

                Explosive products are tested for sensitivity by various methods to determine their response to standard usage conditions:

Drop Impact Test.  Various weights are dropped from different heights onto the sample to determine relative sensitivity to falling objects or other forms of impact.  The standard test involves a 5-kg weight at various heights up to 100 cm.

Projectile Testing.  Various sizes of samples are subjected to impacts from either a bullet or a brass projectile to evaluate their sensitivity to this type of impact.

Friction Pendulum Test.  The sample is submitted to the action of steel sliding across steel to determine whether friction phenomena will initiate a reaction.

Sliding Rod.  This tests the ability of an explosive to withstand a glancing blow from a steel object without the generation of any noise, light, smoke, or any evidence of burning.  Various base plates are used to differentiate sensitivity thresholds, such as brass, steel, and nylon.  Tests are performed with or without grit.

Static Electricity Test.  The explosive sample is subjected to electrical discharge and observed to determine whether any reaction has occurred.  Blasting agents must withstand a 25,000-volt discharge.

Burning Test.  The tests are done to determine the ability of a bulk quantity of the material to withstand heat.  Various sizes of sample, ranging from 5 to 500 lb, are placing a in wood or a diesel fuel fire, in confined or unconfined states, for times ranging from 30 minutes to 6 hours.

Differential Thermal Analysis.  This test allows an explosive to be evaluated with regard to its ability over a range of heat loads and temperature.  The  heat is applied to a sample in a manner that will raise the sampleís temperature at a constant rate.  Any deviation from this rate (up or down ) indicates the presence of some sort of internal action in the sample.

        Density:  The density of an explosive is normally expressed in terms of weight per unit volume.  The density of  an explosive determines the weight of explosive that can be loaded into a specific borehole diameter, known as the loading density.  The specific gravity of an explosive is the ratio of an explosiveís weight to the weight of an equal volume of water.  It is commonly used as a tool to approximate the explosiveís strength and as a design parameter to relate explosives of different manufactures and different generic families.  In general terms, the higher the explosiveís density, the more energetic the product.  The following table lists various explosives and their approximate density ranges:

Approximate Density of Various Explosives (g/cc)

 

TYPE

DENSITY

 

 

Granular Dynamite

0.8 - 1.4

Gelatin Dynamite

1.0 - 1.6

Bulk Slurry

1.1 - 1.6

Cartridged Slurry

1.1 - 1.3

Poured ANFO

0.8 - 0.85

Packaged ANFO

1.1 - 1.2

                Velocity:  Explosive velocity, or detonation velocity, is the speed at which the detonation wave moves through the column of explosive.  For commercially used products, the velocity ranges from approximately 5,000 to 25,000 ft. per second.  Detonation velocity can be used as a tool to determine the efficiency of explosive reaction in field use.  The more velocity, the better the efficiency of the explosive.  Explosives with a lower velocity of detonation tend to release gas pressure over a longer period of time, compared to an explosive with a higher velocity, which may release gas pressure over a shorter period of time.  High-velocity explosives would be more favorable in hard rock.  In softer formations, a lower velocity explosive would give better results.  Typical explosive velocities are tabulated below:

Detonation Velocity (fps)

 

TYPE

 

DIAMETER

 

 

1 1/4-in.

3-in.

9-in.

 

 

 

 

Granular Dynamite

7 - 19,000

 

 

Gelatin Dynamite

12 - 25,000

 

 

Bulk Slurry

 

14 - 19,000

12 - 19,000

Cartridged Slurry

13 - 19,000

14 - 19,000

 

Poured ANFO

6 - 7,000

10 - 11,000

14 - 15,000

Packaged ANFO

 

10 - 12,000

14 - 15,000

Air-emplaced ANFO

7 - 10,000

12 - 13,000

14 - 15,000

Heavy ANFO

 

 

11 - 19,000

                Detonation Pressure:   The detonation pressure is the instantaneous pressure derived from a shock wave traveling through the explosive compound.  It is the pressure in the reaction zone behind the detonation front.  A high detonation pressure is one of the key characteristics that an explosive must have if it is used as a primer.

                Water Resistance:  The water resistance of an explosive may broadly be defined as a productís ability to withstand water penetration or the ability to detonate after the productís expose to water.  More specifically, water resistance is generally expressed as the number of hours a product may be submerged in static water and still be detonated reliably.  The standard water resistance test is used primarily for classification of dynamite explosives.  The test is performed by punching 16 1/4-in. holes in 1 1/4-in. x 8-in. cartridges of product, immersing these samples in water for various amounts of time, and then testing the detonability of a No. 6 strength detonator.  The product is then classified with regard to its ability to withstand water degradation, according to the following scheme:

Water Resistance

CLASS

HOURS

 

 

1

Indefinitely

2

32 - 71

3

16 - 31

4

8 - 15

5

4 - 7

6

1 - 3

7

Less than 1

Commercial explosives differ widely in their ability to resist the effects of water penetration.

                Cohesiveness:  The ability of an explosive to maintain its original shape is called cohesiveness of the explosive.  Sometimes we need the explosive to maintain its original shape, such as when blasting in cracked or broken rock.  But other times, such as when blasting in bulk loading, explosives should flow freely and not bridge the borehole nor form gaps in the explosive column.

EXPLOSIVE  PRODUCTS

                The products used as the main borehole charge can be broken in to three generic categories: dynamite, slurry, and ANFO blasting agents.  The term high explosive refers to any products used in blasting that react at a speed faster than the speed of sound in the explosive material. The reaction must also be accompanied by a shock wave for it to be considered a high explosive.  On the other hand, one commonly hears some of these high explosive called by other names, such as blasting agents.  The blasting agents are a classification for the storage and transportation of some explosives since they are less sensitive to initiation and, therefore, can be stored and transported under different regulations than would normally be used for more sensitive high explosives.

                The subclass of high explosives, called blasting agents, are materials or mixture which consists of a fuel and oxidizer.  The finished products, as mixed and packaged for shipping, cannot be detonated by a number 8 blasting cap in a specific test.  Normally, blasting agents do not contain ingredients which, themselves, are high explosives.  Some slurries containing TNT, smokeless powder or other high explosive ingredients may be classified as a blasting agent if they are insensitive to initiation by a number 8 blasting cap.

Dynamite

                Nitroglycerin was the first high explosive used in commercial blasting.  It has a specific gravity of 1.6 and detonation velocity of approximately 25,000 ft. per second.  Nitroglycerin is extremely sensitive to shock, friction, and heat, which makes its use in liquid form extremely hazardous.  In Sweden, in 1867, Nobel found that if the liquid nitroglycerin was absorbed into an inert material, the resulting product would be safe to handle and would be much less sensitive to shock, friction, and heat.  This product was called dynamite.  Most dynamites are nitroglycerin based products.  Some manufactures of dynamite have products in which they substituted non-headache producing high explosives such as nitrostarch or other compounds for the nitroglycerin.  Dynamites are the most sensitive of all explosives used today.  Because of their sensitivity, dynamites are somewhat more susceptible to accidental initiation.

                There are two major sub-classifications within the dynamite family:  granular dynamite and gelatin dynamite.  Granular dynamite is a compound which uses nitroglycerin as the explosive base, whereas gelatin dynamite uses a mixture of nitroglycerin and nitrocellulose which produces a rubbery waterproof compound.

                Granular Dynamite:  Under the granular dynamites classification, there are three sub-classifications which are straight dynamite, high-density extra dynamite and low-density extra dynamite.

Straight dynamite:  This product consists of nitroglycerin, sodium nitrate, carbonaceous fuels, sulfur and antacids.  The term straight means that the dynamite contains no ammonium nitrate. Straight dynamite is the most sensitive commercial high explosive in use today.  It should not be used for mining or construction applications since its sensitivity to shock could result in sympathetic detonation in wet blastholes rather than being initiated by the caps within the hole. On the other hand, straight dynamite is an extremely valuable product for blasting ditches since the sympathetic detonation is an attribute in ditching, because it eliminates the need for a detonator in each and every hole.  In ditching applications, normally one detonator is used in the first hole and holes are fired by sympathetic detonation.

High-density extra dynamite:  This dynamite is the most widely used in mining, quarries and construction applications.  It is similar to straight dynamite except that some of the nitroglycerin is replaced with ammonium nitrate.  The ammonia or extra dynamite is less sensitive to shock and friction than straight dynamite.

Low-density extra dynamite:  This product is similar to high-density extra dynamite except that more nitroglycerin is replaced with ammonium nitrate.  Since the cartridge contains a large proportion of ammonium nitrate, its bulk or volume strength is relatively low.  This type of dynamite is useful in weak rock or where a deliberate effort is made to limit the energy density, the energy per linear length of borehole.

                Gelatin Dynamite:  The gelatin dynamite used in commercial application can be classified into the following subclasses:  straight gelatin, ammonia gelatin and semigelatin dynamite.

Straight gelatin dynamite:  This dynamite traditionally contains nitroglycerin and nitrocellulose with sodium nitrate and carbonaceous fuel and sometimes sulfur added.  A straight gelatin dynamite is the most powerful nitroglycerin-based dynamite, and, because of its composition, would also be the most water resistant dynamite.

Ammonia gelatin dynamite:  This product is sometimes called special or extra gelatin.  It is a mixture of straight gelatin with additional ammonium nitrate added to replaced some of the nitroglycerin.  Ammonium gelatins are usually used as primers for blasting agents, or as a bottom load in small diameter blastholes.  Ammonium gelatins do not have the water resistance of a straight gelatin.

Semigelatin dynamite:  This product is similar in some respects to ammonium gelatins except that more of the nitroglycerin/nitrocellulose mixture is replaced by ammonium nitrate. Semigelatin dynamites are less water resistant than the ammonium gelatin dynamites and more economical.  They have more water resistance than granular dynamites because of their gelatinous nature and are more often used under wet condition.  Sometimes, they are used as primers for blasting agents.

Slurry Explosives

                A slurry explosive is made of ammonium, calcium or sodium nitrate; a fuel sensitizer, which can either be a hydrocarbon or hydrocarbon with aluminum or in some cases an explosive sensitizer such as TNT or nitrocellulose; along with varying amounts of water. Slurries can be broken down into two categories:  water gels and emulsions.  Water gels require sensitizer and a cross-linking agent while emulsions are composed of saturated solutions of ammonium nitrate and fuel in the form of an emulsion. In general, emulsions have a higher detonation velocity than do water gels.  The composition of emulsions and water gels are similar but they are somewhat different in characteristic.

                Slurries, in general, contain large amounts of ammonium nitrate and are made water resistant through the use of gums, waxes, cross linking in water gels or emulsifying agents in emulsion.  There are many varieties of slurries being used in blasting.  Some slurries may be classified as high explosives, while others are classified as blasting agents if they cannot be initiated by a number 8 blasting cap.  This difference in classification is important for magazine storage.

                An added advantage to slurries over dynamites is that they can be delivered as separate ingredient for on-site mixing.  The separate ingredients brought to the job site in large tank trucks are generally nonexplosive until mixed at the blasthole.

                Slurries can be broken down into two general classifications:  cartridge and bulk.

Cartridge slurries:  These products come in both small and large diameter cartridges.  Small cartridges, in general less than 2 inches in diameter, are made cap-sensitive so that they can be substituted for dynamite.  The large diameter cartridged slurries are not cap-sensitive and must be primed with a cap-sensitive explosive.  In general, large diameter slurries are the least sensitive.

Bulk slurries:  These slurries contain neither aluminum nor sensitizer and have the lowest cost. They are often the least dense and the least powerful.  Some low-cost slurries, however, can have less energy than ANFO.  In wet conditions where mechanical dewatering is not practiced , low-cost slurries offer competition to packaged ANFO.

ANFO

                Ammonium Nitrate and Fuel Oil is the most common of all explosives used today, because of its economics and efficiency.  ANFO is sometimes called a dry blasting agent.  The term ďdry blasting agentĒ describes any material in which no water is used in the formula.  The early dry blasting agents employed fuels of solid forms of carbon or coal dust combined with ammonium nitrate in various forms.  The early agents proved less successful since the solid fuels segregated during transportation and provided lower blasting results.  It was found that diesel oil mixed with porous ammonium nitrate prills gave the best overall blasting result, and therefore the term ANFO has become synonymous with dry blasting agents. An oxygen balanced mixture of ANFO is the lowest cost explosive energy available today.  Ground aluminum foil added to dry blasting agent increases the energy output and the cost.

                ANFO provides excellent performance as an explosive at a reasonable cost. It excels in gas production during detonation and provides a moderately high velocity of detonation, but it is very dependent upon charge diameter.  A major disadvantage of ANFO is the that it is water soluble and cannot be used in wet holes because its become desensitized.

                The performance or release of the available energy of ANFO is affected by:

                - Particle size and distribution

                - Fuel oil content

                - Degree of confinement

                - Column diameter

                - Priming

                - Moisture

                - Detonating cord downline

                The velocity of detonation is a good indicator of product performance.  The higher the velocity, the greater the release of available energy.

                The oxygen-balanced ANFO mixture is 5.7 % oil and 94.3 % ammonium nitrate blasting prills. Any deviation from the oxygen-balanced mixture will result in a loss of blasting energy.  The effect of fuel oil concentration on the explosive energy of ANFO is illustrated in the table below.

                This curve illustrates the explosive energy of ANFO mixtures with fuel oil content over the range of 1-10 %.  Note that the loss of blasting energy is much greater for mixtures that contain less than 5.7 % oil (Low oil) than for mixture that have more (High oil).  The effect of incorrect fuel oil content on blasting results are summarized in the table below.

Table Illustrating the Loss of Energy in ANFO with Incorrect Fuel Oil Content

CONDITION

FUEL OIL

CONTENT (%)

ENERGY LOSS

EFFECT ON BLASTING

 

 

 

 

Oxygen Balanced

5.7

0

Best Blast Results

 

 

 

 

Low Oil

5.0

5.3

Excess Oxygen. Considerable Loss of Energy, May Produce Orange Nitrous Oxide Fumes

 

4.0

12.1

As Above

 

3.0

20.0

As Above

 

 

 

 

High Oil

7.0

1.5

Insufficient Oxygen.  Minor Loss of Energy, Dark Fumes

 

8.0

2.9

As Above

 

9.0

4.9

As Above

INITIATION SYSTEMS

                An initiation system is a combination of explosive devices and component accessories specifically designed to convey a signal and initiate an explosive charge from a safe distance when properly configured and activated.  The signal function may be electric or non electric.  Electric initiation systems utilize an electrical power source with associated circuit wiring to convey electrical energy to the detonators.   Nonelectric initiation systems utilize various types of chemical reactions ranging from deflagration to detonation as a means of conveying the impulse to nonelectric detonators, or as in case of detonating cord, it is the initiator.

                Initiating systems contain devices meant to detonate, and therefore they should be treated with extreme care and caution.  They should never be physically abused, altered, taken apart, exposed to high temperature, friction, or impact.  Improper treatment of initiation systems may cause death or serious injury from premature detonation of the initiating device or explosives.

                As with other explosive materials,  all initiating devices should be locked up when not in use, closely accounted for, and kept away from children and unauthorized person.

                Many conditions exist at an operation that influence the selection of the proper initiation systems:

-   Type of explosive used:  Detonation cord used for initiating high explosives may cause disruption of less sensitive explosive.  Blasting agents should not be initiated by a No. 6 blasting cap.

-            Borehole temperature:  Different manufacturers produce products with different ranges of limiting temperatures; mostly the initiation systems should not be used in temperature exceeding 150o F or 60o C.

-            Geology:  Initiation systems should be fully activated before rock movement occurs to prevent cutoffs and subsequent misfires.

-            Hydrostatic pressure:  Different manufacturers produce products with different ranges of hydrostatic pressure resistance. 

-            Environmental constraints:  The type of system as well as the delay sequence must be correctly chosen so as to limit  ground vibrations and air blast.

-            Extraneous electricity:  Extraneous electricity is defined as any electrical energy, other than the actual firing current or a test current from a blasting galvanometer, that may be present in the

    blasting area.  Electric detonators are designed to be fired by a pulse of electrical energy, so they may be accidentally fired by extraneous electricity such as a stray current, static electricity, radio frequency energy, electrical storms and high-voltage power lines.  Extraneous electricity can be introduced into an electric blasting circuit by either direct contact (stray current or static current) or by electromagnetic radiation (EMR) ( inductive coupling, capacitive coupling, electromagnetic microwave, or radio waves).  As a result, consideration must be given to the potential hazard from extraneous electricity when using electric detonators.

Stray currents:  Stray current is defined as a current flow outside an insulated conductor system. Generally, it is generated by a defective insulation on electrical power systems or by electrically operated equipment.  Other sources of stray current include electric railway lines, electrified fences, or any other electric systems that use a ground return path ( either intentionally or accidentally).

                Electric current always flows from a high voltage to a lower one through whatever conductive paths are available to it.  Alternate conductive paths include the earth, damp timber, metal pipelines, track rails, metal fences, a conductive rock strata that lies on top or between two non-conductive strata, etc.  A potential hazard exists if an electric detonator becomes a part of these alternate conductive paths.

                In order to minimize the probability of stray current occurring, the following safety procedures should be implemented:

-   If any electrical power distribution systems or electrically operated equipment are located near a blast area, periodic checks should made of the wires and equipment insulation to ensure that it is in good condition.  Use ground fault detection devices that will open a circuit breaker if a fault occurs in the systems.

-   If  applicable, bond and cross-bond all metal tracks, pipes, frameworks, housings, etc., to each other and to a good earth ground.

-            Remove all potential sources of stray current, such power lines, electrical  equipment, batteries, etc., from the blast area prior to the start of any explosives loading.

-   Do not remove shunts from electric detonator legwires except for continuity testing, after which they should be reshunted, until just prior to tying them into the blasting circuit.  Keep all free ends from blasting circuit wiring shunted together except when firing the shot.

-   Use a well-insulated firing line that is not damaged and is not strung over any stray current sources.  Ensure that all splices are insulated from ground and other potential stray current sources.

-   Lock off and de-energize any stray current sources near the blast area when explosive materials are present.

 

Static electricity:  Static electricity is electrical energy that is stored at rest on some objects or persons similar to the way that an electric charge is stored on a capacitor.

                Static electricity is generated  by the contact and separation of two similar materials, the process is most efficient when the material are rubbed together and if one or both of the materials are poor conductors of electricity.  Some practical examples of the contact and separation that could generate static current are:  pneumatically loading of ANFO, moving a conveyor, snow and dust particles carried by high wind, the rubbing of layers of different synthetic fabric clothing, and during the formation of lighting storms.

                After the static electricity is generated, it must be stored or accumulated on some objects. The efficiency of accumulation of static electricity in objects increases when the moisture around them decreases.  This event can be observed in the dry season when the air is lacking in moisture.

                Static electricity can represent a potential hazard to an electric detonator when it discharges into an electric blast circuit and delivers sufficient energy to initiate the detonator.

                The generation of static electricity is inherent in nature and we have no control over it, however, we can prevent the accumulation of static charges to minimize the susceptibility of electric detonators to accidental detonation by static discharge.   This can be made possible by:  (1)  Safe operation procedure--cross-bond and ground to earth all persons or objects on which static charges might be stored; and (2)  Safe electric detonators--back up the safe operation procedures by using a detonator that reduces the probability of accidental detonation by a static discharge.

Electrical storms:  Lightning strikes associated with these storms are static discharges of gigantic proportion.  The extremely large amounts of electrical energy released by the powerful electric and magnetic fields associated with lightning represent a hazard to any explosive materials

                In view of the intense electrical power associated with lightning, all explosive loading operations should be discontinued, and all personnel in and around the blast area should retreat to a safe position until the storm has passed over.

High-voltage power lines:  When the blasting area is in the vicinity of high-voltage power lines,  there are four factors that should be considered when evaluating the effect that a power line may have on electric blasting circuit:

-   Stray currents:  The current flows through the neutral wires is low for a balanced three-phase system, but some unbalanced current may exist.  Since the neutral wires are usually grounded at every tower or pole, some stray current may leak out into the earth along the right-of-way.

-            Lightning strikes on the power line:  One of the purposes that the overhead ground wire serves is to intercept lightning strikes and dissipate the resulting surge current to earth.  However it must be recognized that the surge current from a  lightning strike could conceivably be carried  a great distance.

-            Capacitive and inductive coupling:  This refers to electrical energy that may be introduced into a blasting circuit by electric and magnetic fields that are associated with a power line.  If lightning strikes the phase wires or if there is a fault in the system, a current surge may be produced that could result in stronger inductive coupling.

-   Wire thrown over the power line:  The firing line or other blasting circuit wires could be thrown up into contact with the high-voltage phase wires by the force of the blast. This will result in a short circuit from the phase wire, through the blast circuit wires to earth. Several blasters have been killed or injured by this short circuit.

INITIATING DEVICES

                Good results from any blasting operation can be achieved only when the initiating devices used to detonate the explosive charge are carefully chosen and properly utilized.  Initiating devices include detonators (electric blasting caps, nonelectric blasting caps for use with safety fuse, and nonelectric delay blasting caps for use with detonating cord) detonating cord, detonating cord delay units, MS connectors, and devices for assembling and lighting ďcap and fuseĒ units.

Electric Initiating Devices

                The most widely used detonator is the electric blasting cap.  With the proper electrical energy and blasting circuitry, large numbers of electric blasting caps can be initiated from a single current source at a safe location from the blast area.

                The electric blasting cap has a cylindrical metal shell containing several powder charges.  Electric energy is delivered into the cap by two plastic-insulated, metal wires called ďleg wiresĒ which enter the cap through a rubber or plastic plug.  The plug, securely crimped in the open end of the cap shell, forms a water-resistance closure and firmly positions the leg-wire ends inside the cap shell.  The end of the leg wires are joined together inside the caps by a short length of high resistant wire called the ďbridge wireĒ which is embedded in the capís ignition mixture.  When sufficient electrical current passes through the system, the bridge wire becomes hot enough to ignite the ignition.  In instantaneous electric caps, the ignition mixture causes the primer charge to detonate, subsequently detonating a high-explosive base charge.  In a delay electric blasting cap, the ignition mixture initiates the delay powder train which burns a predetermined time before igniting the capís primer charge.  The burning rate of the delay powder and the length of its column determine the time interval between application of the adequate electrical energy and the detonation of the cap.  Cap strength depends on the amount of base charge in the cap. Higher strength caps have a greater quantity of base load.  Figure 1 page 44  shows the details of electric blasting caps.

WARNING:  Electric blasting caps of different manufactures should never be mixed in a blasting circuit since their ignition system may not be electrically compatible and dangerous misfires may occur.

Nonelectric Initiating Devices

                Nonelectric blasting caps sometimes called ordinary blasting caps or fuse caps, provide a nonelectric method of initiating explosive charges when properly used in conjunction with safety fuse.  The safety fuse conveys a flame at a relatively uniform rate to the blasting caps, where it ignite the ignition charge.

                Nonelectric blasting caps consist of aluminum shells loaded with three charges:  a base charge of high-velocity explosive in the bottom of the shell, a primer charge in the middle  and a charge of ignition powder on the top.  The ignition powder ensures that the flame is picked up from the safety fuse, and the primer charge converts the burning into detonation and initiates the high-velocity explosive base charge. Figure 2 page 44 shows the details of nonelectric blasting caps.

Safety Fuse

                Safety fuse is the medium through which the flame is conveyed at a relatively uniform rate for ignition of the blasting cap.  The core of safety fuse is a black powder train, tightly wrapped by covering of tape, textiles, and waterproofing material such as asphalt and plastics.  The functions of the covering are:

-   To protect the powder train from water, oil, or other substance which might effect its burning rate or desensitize it.

-   To protect the core from abrasion or other abuse while maintaining flexibility.

-   To minimize the chance of setting fire to the charge of explosive by sparks coming through the side of the fuse before the fire has reached the caps.

-   To prevent intercommunication of firing between adjacent links of fuse.

                There are several manufactures and brands of safety fuse.  Before using safety fuse, the user should know the burning rate of the particular fuse he is using.  In general, the burning rate of safety fuse is 120 seconds per yard but manufactures make no warranty or representation regarding the burning speed of their products because of the many circumstances and conditions the fuse is subjected to after leaving the factory.  These  include the different altitude, weather, storage condition, character of tamping, and mishandling.  All of these effect the burning rate of the fuse.

                Burning rate of safety fuse also depends on the external pressure of it, the greater the pressure or confinement, the faster a fuse will burn, and the burning rate under these conditions may be appreciably faster than the published unconfined burning rate.  On the other hand, a reduction of external pressure slows the burning rate of the fuse and, therefore, the elevation above sea level must be considered.

Safety Fuse Lighting Devices

Hot-wire fuse lighter:  This device is similar in appearance to a fireworks sparkler. It consists of a wire covered with an ignition composition that burns slowly and at a fairly steady rate with an intense heat.  The hot wire fuse lighter is lighted by a match and can be used to ignite fuse merely by holding the burning portion of the lighter against the freshly cut end of the fuse.  Figure 3  page 45 shows the hot-wire fuse lighter.

Igniter cord system:  Safety fuse can be ignited efficiently with igniter cord and igniter cord connectors. This system enables the blaster to fire multi-hole blasts in sequence with a single lighting and without cutting different lengths safety fuses.  The safety fuse must be cut in equal lengths, and the designed timing will then be from the igniter cord rather than from the length of the fuse.

                Igniter cord is a small cord which burns progressively along its length with a short, hot, external flame at the zone of burning.

                Igniter cord connectors are metal shell, aluminum or brass, device for connecting the igniter cord to the safety fuse.  These connectors are crimped to the precut lengths of safety fuse on the free end not crimped to the blasting cap.  This operation is done in a separate location, such as a central capping station, at the same time the blasting caps are crimped.  The connectors are designed to protect the end of safety fuse from moisture and can be attached to the igniter cord at the face with a minimum of effort.

Thermalite Connector and Igniter Cord:  The thermalite connector is a small metal capsule with an internal ignition compound that burns with an intense heat.  The use of this product as a system for lighting a single charge is strongly recommended.  The connector should be crimped to the fuse, and a short section of igniter cord should be inserted under the ďlipĒ of the connector and closed by thumb pressure. The igniter cord is then lighted.  This ignites the thermalite connector, and the ignition compound in the connector lights the fuse.  Figure 4  page 45  shows the igniter cord with thermalite connector.

Detonating Cord

                Detonating cord is a round, flexible cord containing a center core of high explosive.  It is relatively insensitive and requires a proper detonator, such as a No. 6 strength cap, for initiation. Detonating is used to initiate other explosives, its ability to detonate the other depends in part on the density of the high explosive core (usually pentaerythritol tetranitrate:  PETN) or the grains of PETN per linear foot of cord.  The most widely used cords have approximately 25 to 60 grains-per foot (5.3 to 12.87 grams per meter), although core load with as little as four grains per foot and up to 400 grains per foot is available.  Make sure that in all cases detonating cord is of sufficient strength to initiate the primer or explosive charge.

Millisecond Delay  Blasting

                Millisecond (MS) delay blasting  permits the explosive engineers to divide the shot into smaller charges, which are detonated  in a predetermined millisecond sequence at specific time intervals. Millisecond delay initiation of the explosive charge is a technique used in most surface and some

underground rock blasting operations.  It serves to enhance fragmentation and direct rock movement for increasing productivity.  MS delay blasting is also used to manage adverse geologic conditions found on the site and to optimize the blast design and control of vibration and air blast.  It can reduce ground vibration by dividing the explosive energy into smaller charges using a timing sequence and a delay interval, which provides for lateral relief for charges in the second and later rows in surface blasting operations.

                The major advantage of MS delay blasting are:

-            Improved fragmentation

-            Reduction of ground vibrations and air blast

-            Reduction of overbreak and flyrock

-            Improved productivity and lower cost

                There are five major systems of millisecond  delay blasting. All systems employ a detonator with a pyrotechnic millisecond delay element.  They differ in how the signal is transmitted to the detonator and in the manner in which they are sequenced to obtain additional MS delay firing times.  The five major MS delay blasting systems are:

-          Standard electric MS delay blasting.  In this system, all MS delay electric detonators in a shot receive an impulse of electrical energy at the same time and rely upon their internal delay time to provide for the requirements of the specific MS delay design.

-          Sequential electric MS delay blasting.  This is a system whereby electric MS delay detonators are wired into multiple circuits that receive separate pulses of electrical energy at predetermined MS delay time intervals from a sequential blasting machine.

-          Standard nonelectric MS delay blasting.  In this system, blastmaster in-the-hole nonelectric MS delay detonators are initiated by a signal tube, which is initiated by detonating cord on the surface.

-          Sequential nonelectric MS delay blasting.  This system is similar to system number 3 but MS detonators are sequentially initiated by either blastmaster surface nonelectric MS delay trunkline connectors or by detonating cord  trunklines with MS delay connector.

-          Combination electric and nonelectric MS delay blasting.  In this system, blastmaster in-the-hole nonelectric MS delay detonators are sequentially initiated by surface electric MS delay detonators.  The surface electric MS delay detonator may be sequenced by a sequential timer.

Blasthole Initiation Sequencing

                Millisecond delay blasting can be used both in a single row round and in a multiple row round. When each charge is given sufficient time to break its quota of burden from the rock mass before the next charge detonates, the ground vibration, air blast and flyrock are minimized, and the fragmentation is increased.  If a free face is not available, an inner blasthole may crater upward, resulting in poor fragmentation, little forward displacement, and an increase in the possibility of flyrock and overbreak, while increasing ground vibration and air blast.

                The delay interval necessary for optimum fragmentation varies with the type of rock and burden distances.  It appears that delay intervals of between 10 and 60 milliseconds between adjacent blastholes in a row provides the best result.

Some Examples of Delay Pattern

 

Square Pattern:  Generally, a square pattern is used with a V or echelon pattern.  A square drill pattern with a millisecond delay firing order is illustrated in Figure 5 page 46 .  This type of shot will produce a muckpile that is stacked up and well-fragmented for a shovel or dragline.

                If more than one free face exists, a square drill pattern is used to shoot the corner in an echelon pattern results in the same true burden and spacing relationship as a rectangular pattern in a V-MS pattern.

Rectangular Pattern:  Rectangular drill patterns are normally drilled on a staggered configuration  for better explosive distribution.  For this drill pattern, a row-by-row pattern is preferred as illustrated in Figure 6 page 46.  This pattern will give good movement and consequently a lower muckpile for easy digging.  It is also used to move overburden in casting techniques where optimum displacement is desired.

                When trying to displace as much of the muckpile as possible, it is important to get as many as rows of holes moving out at the same time as possible.

Sequential Timer MS Delay Pattern

                The sequential timer makes possible delay patterns with only one or two holes firing on the same delay time.  It also makes possible delaying within each individual hole to reduce vibrations to an acceptable level.  The interaction between delays, each firing on controlled sequence, has greatly improved fragmentation and materially reduced fly rock in most formations.

                Sequential timer programs are custom designed to individual operations.  The following pattern is offered as an example of a program that is currently being used in many areas:

V-Pattern:  The V-pattern  has been used very successfully with timer settings of 33, 42, and 58 milliseconds on blasts of up to 240 holes.  The timer setting choice is usually determined by the formation, the burden, spacing, and the depth of the hole.  A change in timer setting physically affects the angle of movement and can materially affect the level of vibration from the blast.  Figure 7 page 47 shows the V- pattern.

BLASTHOLE DESIGN

                Any blasthole design must encompass the fundamental concepts of an ideal blast design, which are then modified, when necessary, to account for geologic conditions.  The engineer must select the proper variable to match the specific field conditions during the design of the blast.  There are two types of variables in blasting design, the controlled variables and uncontrolled ones.  Uncontrolled ones are the variables over which we have little control such the geology, rock characteristics, and regulations or specifications (such as the distance to the nearest structures).  The variables over which we do have control are called controlled. Some examples of these are:

-   Hole  diameter

-   Hole depth

-            Subdrilling depth

-            Stemming distance

-            Stemming material

-            Burden and spacing

-            Number of holes in the blast

-            Direction of rock movement

-            Timing

-   Types of explosive and initiation system

Burden

                Burden is defined as the shortest distance to relief at the time the hole detonates, either a ledge or the internal face created by a row of holes that have been previously shot on an earlier delay.  The proper burden is one of the most importance decisions made in any blast design.  If the burden is too small, rock is thrown a consideration distance from the face, air blast levels are high and the fragmentation may be excessively fine.  If the burdens are too large, severe backbreak occurs behind the last row of blastholes and may cause the blast hole to geyser causing flyrock.  Excessive burdens cause over-confinement of blast holes, which result in high levels of ground vibration per weight of explosive being used.  Rock breakage can be extremely coarse and often causes bottom or toe problem.  Of all the variables, the burden has the least allowable tolerance.  If the operator selects the proper burden distance, other variables are more flexible and will not produce the drastic differences in the results as would the same proportion of error in the burden dimension.

                To approximate burden, the  following empirical formula is helpful:

                                     B  =  ((SGe/SGr) + 1.5) * De

                       where:

                                     B      =   burden in ft.   

          SGe  =  specific gravity of explosive                 

                                     SGr   =  specific gravity of rock

                                     De     =  diameter of explosive in inches.

Example:

An operator has designed a blast in limestone using 3-inch diameter blastholes. The blastholes were loaded with ANFO which has a specific gravity of  0.8, limestone has a specific gravity of 2.6.

Solution:

B = ((0.8/2.6) + 1.5 ) * 3

   =      

                If  we have a successful burden for one size of drill hole we can determine a burden for another size of drillhole by using the following ratio:

                                   B2  =  B1(De2 /De1)

                       where:

                                   B1      =     Burden successfully used on previous blast

                                  De1     =     diameter of charge of  B1

                                   B2      =     new burden

                                   De2    =     new explosive diameter

Example:

The operator has used three inch diameter blastholes with six feet of burden in a sandstone blast.  ANFO has been used as the explosive.  The operator decided to increase the blasthole size to five inches while still using ANFO as the explosive.  The new burden should be:

                                 B1  = 6 ft, De1 = 3 in., De2 = 5 in., B2 = ?

                                 B2   = 6 *(5 /3) = 10 ft.

Spacing

                The distance between adjacent blastholes, measured perpendicular to the burden, is defined as the spacing.  Spacing calculations are a function of burden.

                                     S   =  1.0-1.8 (B)

                        where:

                                     S  =   spacing (feet)

                                     B  =   burden (feet)

                If spacings are significantly less than the burden, it tends to cause early stemming ejection and premature splitting between blastholes due to the rapid release of gases to the atmosphere, and result in noise and air blast.  When the spacings are too large, the fragmentation of rock may be poor, and may cause an uneven floor.  Consequently, burden and spacing decisions are made by careful analysis of geology, explosives, condition at the site, and experience.

Stemming Distance

                In most cases, a stemming distance of 0.7 * (B) is adequate to keep material from ejecting prematurely from the hole.  But if the blast is poorly designed , the stemming distance equal to 0.7B may not be adequate to keep the stemming from blowing out.

                The stemming distance is the function of  burden, charge diameter, strength of explosive and specific gravity of rock. The relationship for stemming is:

                                 T      =  0.45 De(St/Sgr)^0.33

                      where:

                                 T       =  stemming in ft.

                                 De     =  diameter of explosive in inches.

                                 St      =   relative bulk strength of explosive

                                 Sgr    = specific gravity of rock

                Generally, stemming distance is between 0.7-1.3 of burden distance.  A delicate balance exists between not enough or too much stemming.  Flyrock and excessive air blast can be caused when there is not enough stemming.  Too much could lead to excessive vibration and poor fragmentation.

Optimum Size of Stemming Material

                If we want to minimize the stemming depth in order to break cap rock we should use the proper size of stemming material.  Very fine drilling dust will not hold in the blasthole and is easily ejected; very coarse material tends to bridge the hole when loading and may be ejected; round gravel or sand from river will not function as well as crush stone.  The optimum size of stemming material would be the material that has an average diameter of approximately 0.05 times the diameter of the blasthole.  Material must be granular to function properly, never use a coarse, large, or sharp rock that could damage the initiating system.  The most common material used for stemming is drilling cuttings, since they are economic and conveniently located at the collar of the blasthole.  If the drill holes are wet to the collar, the use of crushed stone can result in better fragmentation and control.  If drilling dust were used instead of crushed stone or drilling chips, it may be necessary to increase the stemming distance to equal the burden distance.

Subdrilling

                Subdrilling is a term that defines the depth to which a blasthole will be drilled below the proposed to ensure that breakage will occur to the grade line.  Blastholes may not break to full depth, especially when blasting takes place in dense rock.  On the other hand, if there is a soft seam or bedding plane located at grade line, no subdrilling should be used.  Sometimes, a blasthole may be backfilled to confine the gasses and keep them away from a soft seam.  Too much subdrilling is waste and may cause excessive ground vibration because of the increased confinement.  Insufficient subdrilling can cause a high bottom, ultimately causing excessive equipment wear and the need for secondary blasting.  The rule of thumb for subdrilling is to subdrill holes to the depth equal to 20 % to 50 % of the burden (J = 0.2-0.5)B.  In most instance, subdrilling is approximately  0.3(B).

                                     J  =  0.3B

                          where:

                                     J  = subdrilling in ft

                                     B = burden in ft.

     Example:

A 3-inch diameter blasthole was used in limestone.  The burden was design to be 7.5 ft.  The amount of additional drilling, or subdrilling, which would be needed below grade to ensure breakage to grade is determine by:

                                      J  =  0.3B

                                          =  0.3* 7.5

                                          =  2.25  ft 

Angle  Drilling

                In many countries, angle drilling is used on production hole as well as controlled blasting hole.  In the United States, angle drilling normally is only used for controlled blasting applications such as presplitting or trim blasting.  To better understand the rationale for angle versus vertical drilling, you would have to compare the advantages and disadvantages of both methods and look at them form a site specific application.

                Advantages of angle drilling

-   less backbreak

-   less problem at grade

-   more throw, especially on low benches

-   better fragmentation on low benches

-   loose rock better held on face by gravity

                Disadvantage of angle drilling

-   harder to collar hole

-            difficult to maintain accurate angle

-   more problem with geologic discontinuities

-   easier to hang steel in hole

Selection of Blasthole Size and Bench  Height

                For the selection of  blasthole size, we would consider drilling economics, the effect of fragmentation, air blast, flyrock, and ground vibration.  The larger diameter blastholes tend to be more economic but would cause more problems with air blast, flyrock, ground vibration and fragmentation.  To gain the best result of blasting with the best fragmentation and the least result of unwanted effects, the operator would consider the stiffness ratio, which is the bench height divided by the burden distance, and must be between 3-4.

Example:

A small cement plant needs good fragmentation of limestone because it has small loading equipment.  The operator has a track drill capable of drilling a 4 inch diameter hole.  What should be the bench height of the blast?

Solution:

For the 4-inch borehole, using ANFO with a specific gravity of 1.8, and with a specific gravity of limestone if 2.6:

burden  =  ((2*0.8/2.6) +1.5 )*4    =    8.5

For a good fragmentation  B  = L/3,  where B  = burden (ft), L = minimum bench height (ft)

for B =  8.5 ft, then minimum bench height  25.5 ft  ( around 25 ft)

                A simple method used to approximate a blasthole length where the stiffness ratio is greater than two (which produces a fair result for both fragmentation and the undesirable results) would be used as follows:

                                   L    =  5 *  De 

                        where:

                                    L     =  minimum bench height in ft.

                                    De   =   diameter of explosive in inches.

                Since the minimum length of a blasthole, in feet, is approximated by multiplying the charge diameter in inches by five, this formula is known as ď Rule of FiveĒ.

                However, this rule of thumb covers only the recommended minimum length of blasthole:

                                    L  =  2*B

                         where:

                                     L  = minimum bench height in ft.

                                     D =  burden distance in ft.

Decking

                The decking technique is a technique where the explosive column is divided into two ore more charges.  This is accomplished by loading an inert materials, such as drill cuttings or crushed stone, between the explosive charge.  The purposes of decking is to give confinement of explosive gases where a soft seam or void is encountered, or to assure a better energy distribution.  Sometimes this technique is used to reduced the explosive weight per delay  when blasting in an area of vibration constraint.  This decision is often made upon analysis of scaled distance calculation or analysis of seismic records.

                A guide for calculation minimal deck thickness is:

                                  Td  =  6*D

                         where:

                                    Td  =  decking thickness (inch.)

                                     D   =  borehole diameter (inch.)    

Explosive Factor (or Sometimes Called Powder Factor)

                Explosive factor is the mathematical relationship between the weight of explosives and a given quantity of rock.  The explosive weight is normally expressed in pounds (or kg.) and the rock quantity is normally expressed in cubic yards (or cubic meters) or tons.

                To calculate cubic meters per borehole, the following formula is used:

                                     V  =  B * S * H

                           where:

                                      B  =  burden dimension (m.)

                                      S   =  spacing dimension (m.)

                                      H  =  bench height (m.)

                                      V  =  rock volume (cubic meters.)

 

                To convert rock volume to cubic yards, multiply V by  30.48.

                To calculate tons of rock per borehole, multiply V by the density of rock (tons/cubic meter).

Example:

Limestone has an explosive factor of 0.3 kg/cubic meter to break the rock into an average size of 6 inches.  In blasting a 4-inch diameter borehole, the burden is 2.5 m, the spacing is 4.0 m and the bench height is 8.0 m.  How much explosive (ANFO) per borehole should to be used?

Solution:

Rock volume per borehole, V, is calculated by:

                                V   = B* S* H

                                 = 2.5*4* 8  = 80 cubic meters

Explosive per borehole = 0.3 *80 = 24 kg.

In surface blasting, the normal explosive factor is between 0.1-1.0 kg/cubic meter.

Explosive Consumption

                The amount of explosive required to break the rock into an average size is called explosive consumption.  There are many factors in the value of explosive consumption such as the types of rock (rock strength) to be blasted, geometry and geology of the rock, kind of explosive to be used, size of borehole, delay systems, initiation systems, product size, and etc.  For example, ANFO consumption is greater when used in small boreholes (close to critical diameter) than when used in the bigger boreholes, blasting with more free face consumes less than blasting with less free face, loose rock needs less explosive than dense rock and so on.  In any blasting, the operators need to try and adjust the amount of explosive to achieve the optimum product size.  They also must consider crushing and grinding cost compared with blasting cost.  In some operations, particularly when blasting in hard rock, drilling cost must also be considered.                                                             

SPECIALIZED  BLASTING TECHNIQUES

                There are many specialized design techniques which are used in different blasting situations. Some techniques are used to produce cosmetically appealing final walls with little or no concern for stability within the rock mass.  Other techniques are used to provide stability by forming a fracture plane before any production blasting is conducted.  The second technique may or may not be as cosmetically appealing, but from a stability standpoint, it performs its function.  Controlled blasting for overbreak control can be broke down in to three types, presplitting, trim or cushion blasting and line drilling.

                Presplitting:  This method is not a new blasting technique.  It became a recognized blasting technique for wall control when it was used in the mid-1950s on the Niagara power project.  The pyramids of ancient Egypt were built by craftsman that used a nonexplosive method of presplitting.  The technique was employed by pounding wooden wedges into holes drilled into the rock.  The wooden wedges were soaked with water and the forces generated by the expanding wedges caused fractures to occur between holes.  The block could then be removed.  In northern climates, man found that he could use the forces generated by freezing water to cause rock to fracture.  Holes were drilled into the rock mass and filled with water.  Cracks developed between holes as water froze during the winter.  In the spring, the block could be removed.  The wooden wedges and the freezing water exerted static pressure on the rock mass similar to what occurs from explosive gas pressure.

                The purpose of presplitting is to form a fracture plane across which the radial cracks from the production blasting cannot travel.  This methods may cause a fracture plane which may be cosmetically appealing and allow the use of steeper slope with less maintenance.  Presplitting uses lightly loaded, closely spaced drill holes, and is fired before the production blast.

                The following ďrules of thumbĒ can be applied to presplit blast design:

The explosive load per foot of hole which will not damage the wall but will produce sufficient pressure to cause the splitting action to occur, is given by the equation

                    dec   =  Dh*2 / 28

         where:

                    dec   =  explosive load per foot, lb/ft

                    Dh    =  diameter of empty holes, in.

If  this explosive load is used, the spacing between holes in a presplit blast can be determined by:

                     S   =  10 Dh

          where:

                     S   =  spacing, in.

                    Dh  =  diameter of the empty hole, in.

The constant ď10Ē in the above formula is conservative.  It is used to make sure that the presplit distance is not excessive and that the presplit will occur.  Field experience indicates that often this value can be increased to 12 and sometimes 14.

                Presplit holes would normally be drilled first, ahead of the production holes.  The choice can be made between loading and firing the presplit or completing the drilling and loading of the main blast.  In the latter cases, the presplit line would be fired instantaneously 100 to 150 millisecond before the main blast.  The presplit line is formed ahead of the main blast and allows the stress and gas from the blast to be driven back from the buffer row through the radial cracks to terminate at the presplit line. The presplit holes may be vertical or inclined at 10 - 15 degrees from the vertical.  The angle is somewhat indicated by rock structure although a slight angle is preferred regardless of  long-term stability as well as for best initial results with large production holes.

                The figure illustrates the upper bench where two benches will finally run together to from a final face between berms.  Presplit requirements become clear when presplit holes needed for the bench are considered.  The drill must be capable of drilling close to the previously produced bench face at an angle of up to 15 degrees beneath itself so the face can be continued to depth.  The presplit holes in this case are limited in size to 102 to 127 mm. ( 4 to 5 in.)

                The back row of the main production blast, termed the buffer row, must be carefully designed with respect to standoff distance from the presplit row and spacing as well as the explosives loaded. Subsequently, main blastholes after the buffer row are designed at regular spacing, burden, and loading for the type of material being blasted.

                No subdrilling is normally used on the presplit and buffer row.  This is to prevent damage to the bench below or to the wall at that point.  Figure 8 page 47 shows a presplit blast coupled to a 250 mm (9 7/8 in.) diameter hole production blast.

                Trim (or Cushion) Blasting:  Trim blasts are designed to produce a final wall similar to a presplit blast, but they are fired after the production holes.  Trim blasting is designed to give wall control using large diameter holes for both the production and the final row of holes.  The idea is to eliminate costly small diameter blasthole work, along with the associated hole loading difficulties.  The explosive load per foot of hole is determined by the same formula as in a presplit blast.  The spacing is normally larger than used in presplitting because there is relief toward which the holes can break.

                The spacing could be determined by:

                                 S    =  12-16 Dh

                      where:

                                 S     =  spacing in.

                                Dh   =  diameter of the empty hole, in.

                The constant value (12-16) used depends on the rock hardness, from hard to moderately soft, and they are for adequately decoupled charges.

                A buffer row is designed as the last row of the main blast, with increased stemming to prevent cratering back at the surface through the trim row.  Normally at least two other regular rows of holes would be used in front of the buffer row to complete the trim blast.  These two rows are designed at normal spacing, burden, and loading.  All blastholes in the trim blast would be vertical and would be of production size.

                Figure 9 page 48 shows wall trim blast using 250 mm.(9 7/8 in.) diameter holes in hard brittle rock. 

GROUND VIBRATION AND AIR BLAST

                One of the most troublesome and controversial issues facing mining and other industries related to blasting is that of ground vibration and air blast produced form blasts.  With the general trend toward larger blasts, increased population, and spread of urban area, vibration problems and complaints have also increased.  Complaints range from cracking in structures to affecting sexual performance to outright demolition of a residential structure.  Although some of these claims are exaggerated, others are quite legitimate.

Ground Vibration

                Many damage criteria for ground vibration have been established during the last few decades.  Damage criteria are based on amplitude calculation, displacement, velocity, and acceleration.  The amplitude calculation was conservative, the formula was found to be inadequate in view of the more complex blasting designs.  Furthermore, acceleration was considered as the criterion for structural damage, and was frequency dependent.  An acceleration of less than .01 g was considered safe, 0.1-1.0 g was caution, and greater than 1.0 g was damage.

                The criterion of blast damage developed based on particle velocity is the most generally accepted among all criteria.  It was generally agreed that the particle velocity of ground motion in the vicinity of the structure was the best criterion, hence, attempts were made to convert past damage criteria in term of particle velocity.

                Selected particle velocity damage criteria are listed as follows:

United Stated Bureau of Mines (USBM), 1971

PARTICLE VELOCITY (in./sec.)

DEGREE OF DAMAGE

 

 

< 2

Safe

2 - 4

Plaster Cracking

4 - 7

Minor Damage

> 7

Major Damage to Structure

Canmet, Bauer, and Calder (1977)--Established Damage For Equipment and Structures

TYPE OF STRUCTURE

TYPE OF DAMAGE

PARTICLE VELOCITY AT WHICH DAMAGE STARTS (in./sec.)

 

 

 

Rigidly Mounted Mercury

    Switches

Trip Out

0.50

House

Plaster Cracking

2.0

Concrete Blocks in a New House

Crack in Block

8.0

Cased Drill Holes

Horizontal Offset

15.0

Mechanical Equipment (Pumps,

    Compressors)

Shaft Misalignment

40.0

                The results of various researchers indicated that a peak particle velocity of less than 2 in/sec. would result in a low probability of structural damage to residential dwellings.  Above 2 in/sec., the probability of damage would increase and is independent of the particle velocity component in the longitudinal, transverse, or vertical direction.  This damage criterion was also assumed to be independent of the frequency range from 1 cps to approximately 500 cps.

  Peak Particle Velocity And Scaled Distance

                Peak particle velocity is the highest displacement per unit time in reference to the speed or excitation of the particles in the ground resulting from vibratory motion.  The scaled distance is a scaling factor that incorporates the charge weight influence on the source functions as a generator of vibration.  The scaled distance is derived as a combination of distance and charge weight influencing the generation of seismic or air blast energy.

                Experiments have shown that the peak level of ground motion at any given point is inversely proportional to the square of the distance from the shot point.  The empirical scaling formula relating peak particle velocity to scaled distance has been developed from results actually obtained in the field

 using vibration monitoring equipment.  Scale distance combines the effects of total charge weight per delay on the initial ground shock level with increasing distance from a blast.

                Peak particle velocity, scaled distance and the site factor can be expressed in the following equation:

                                 V    =  K (  d / W ^0.5)^ -m

                       where:

                                  V    =  maximum peak particle

                                  d    =  slope distance between the shot and the nearest dwelling (ft)

                                  W  =  total weight of explosive per minimum of 8-msec. delay

                                  d / W ^0.5   = scaled distance for a cylindrical charge (ft/lb^0.5)

                                  K, m  = site factors

                The site factors are determined from a logarithmic plot of peak particle velocity versus scaled distance.  The straight line best representing the data has a negative slope, m ,and a Y-axis intercept, K, at a scaled distance of 1.  In almost all cases, the scatter from data obtained is so great that in order to establish a statistically good approximation of the site factor, an extremely largely large number of tests would have to be conducted.  As this is not practical in most instances, limit lines can be established that encompass all data obtained from many different sites.

                Generally, the influence of rock characteristic reduces, or attenuates,  the rate of ground vibration with the increasing of distance.  It is impossible to make a reliable theoretical prediction of the rate of ground motion attenuation to be expected at a locality.  The site factors, K and m of the previous equation, which are determined by a ground motion monitoring program, are empirical allowances for the effect of local rock characteristics on ground motion.

                When a large number of blasts were monitored for peak particle velocity in many areas of the United States, and the data were combined, safe scaled distances were established for field use.  The equations developed are:

                             D/(W)^0.5 ³ 50 ft/(lb)^0.5             (1)

                   and    D/(W)^0.5 ³ 20 ft/(lb)^0.5              (2)

                These equations represent the minimum scaled distance recommendations for safe blasting.  Equation (1) is recommended on sites where no instrument readings are made because the equation introduces a safety factors to allow for the possibility of high seismic energy generation and propagation effects.  Equation (2) is recommended only for sites that are actually instrumented and if peak particle velocities of less than 2.0 in./sec. were obtained.  If readings were scarce or questionable and a mine operator wished to use a smaller scaled distance to allow for larger explosive consumption per delay, then a consultant or permission from the regulatory agency in the area was required.

                Typical values for safe blasting distances and scaled distances of 50 and 20 are tabulated in the following table.

 

       SAFE  BLASTING                  MAXIMUM  CHARGE WEIGHT

   DISTANCE TO NEAREST                        PER DELAY

           STRUCTURE

                                                       D /(W)^0.5  =  50    D / (W)^0.5 = 20

                                                               (lb)                                  (lb)

                    ft

                  100                                         4                                     25

                  500                                      100                                  625

                1,000                                     400                                 2500

                 2,000                                   1600                              10000              

                It should be noted that this damage criterion of using a scaled distance of 50 implied that the probability of producing a peak particle velocity greater than 2.0 in/sec was very small.

                To simplify calculations, D /(W)^0.5  = 50  can be rewritten as:      

                                 D/(W)^0.5  =    Ds

                    and:                               D  =    Ds * (W)^0.5

                                  W   =    (D /Ds)^2

                                   D    =  distance from blast to nearest structure  (ft)

                                   Ds   =  scaled distance   (ft / (lb)^0.5)  = 50

                                   W    = weight of explosive per delay (lb)

                For example, if the closest structure to the blast is located 1500 ft away, and a scaled distance of 50 ft is used, the weight per delay is calculated as:

                                   W  = ( D /D)^2   =  (15000 / 50)^2   =  900   lb.

                This mean that the operator should blast with the maximum weight per delay of not more than 900 lb (such as 3 holes with 300 lb per hole on the same delay).  However, this criterion alone is inefficient because it did not take into account the predominant frequency of blast wave.

                In 1974 the USBM began to reanalyze the blast damage problem and in 1980, Siskind et al. published the results of a comprehensive study of ground vibration produced by blasting on 76 homes from 219 production blasts.  Time and frequency properties of mining blasts as reported in RI 8507 are as follows:

(1)   The amplitude, frequencies, and duration of ground vibrations change as they propagate, because of:

        - intersection of various geologic media and structural interface.

        - spreading out the wave train through dispersion, or

       - absorption that is greater for the higher frequencies.

(2)        Close to the blast, the vibration character is affected by factors of blast design, mine geometry, charge weight per delay, delay interval, direction of initiation, burden, and spacing.

(3)   At large distances from the blast, the factors of blast design become less critical and the transmitting medium of rock and soil overburden dominates the wave characteristics.

(4)        Particle velocity amplitudes are approximately maintained as the seismic energy travels from one material into another(e.g. rock to soil.)

(5)        Vibration frequency, displacement, and acceleration amplitudes depend strongly on the propagating media.

(6)        Thick soil overburden and large distances create long-duration, low-frequency wave trains. This increases the response and damage potential of nearby structure.

(7)   Coal mines shots are characterized by a trailing large-amplitude, low-frequency wave because of a larger overburden layers.

(8)   The combined effect of large shots, thick overburdens, good confinement, and long range propagation make coal mine blast vibrations potentially more serious than quarry and construction blasts because of their lower frequencies.

(9)        Natural frequencies of midwalls are somewhat higher than for the structure corners

(10)        Damping for midwalls was generally lower than that for structure corners.  Damping controls the decay of oscillation, so that when a structure is critically damped, it will return to its equilibrium position without oscillating.

(11)        Maximum amplification for a one-story and two-story structure occurred when ground motions between 5 and 12 Hz were recorded.

(12)        Corner motion amplification factors for all of the homes studied were as high as 4, and for midwalls the factors were as high as 8

(13)        Normally, ground motion measurements above 45 Hz produce little or no amplification in corner structure and/or midwalls.

The Main Conclusions Drawn From the Latest USBM, RI 8507 Are:

(1)        Particle velocity is still the best single ground descriptor

(2)        Particle velocity is the most practical descriptor for regulating the damage potential for a class of structures with well-defined response characteristics (e.g. single-family residences)

(3)        Where the operator want to be relieved of responsibility of instrumenting all shots, he could design for a conservative square root scale distance of 70 ft /(lb)^0.5. The typical vibration level at this scaled distance would be 0.08-0.15 in /sec.

(4)        Damage potentials for low-frequency blast (<40 Hz) are considered higher than those for high-frequency(>40 Hz), with the latter often produced by close-in construction and excavation blasts.

(5)        Home construction is also a factor in the minimum expected damage levels. Gypsum board (drywall) interior wall are more damage resistant than older, plaster-on-wood-lath construction.

(6)        Particle safe criteria for blasts that generate low-frequency ground vibration are 0.75 in /sec for modern gypsumboard houses and 0.50 in /sec. for plaster-on-lath interiors. For frequency above 40 Hz, a safe particle velocity maximum of 2.0 in /sec is recommended for all houses.

(7)   The change of damage from blast generating peak particle velocities below 0.5 in /sec is not only small ( 5 % for worst cases) but decreases more rapidly than the mean prediction for the entire range of vibration levels (almost asymptotically below about 0.5 in /sec)

(8)   All home eventually crack because of a variety of environment stresses, including humidity and temperature changes, settlement from consolidation, and variation in ground moisture, wind, and even water absorption from tree roots. Consequently, there may be no absolute minimum vibration damage threshold when the vibration (from any cause, for instance slamming a door) could in some cases precipitate a crack about to occur.

                An alternative recommended blasting level criteria has been developed using both measured structure amplification and damage summaries.  In 1983, the United Stated Office of Surface Mining (USOSM) published its final regulations concerning the use of explosives for the control of ground vibration and air blast.  These regulations apply only to surface coal mining operations.  However, many organization in the aggregate, crushed stone, and other non-coal surface mining operations have opted to comply with these new regulations as operating guidelines.

                The OSM regulations were derived in part from the USBM database and proposed recommendation from RI 8507.  They did not incorporate all of the USBM recommendation, but were designed to offer more flexibility in meeting performance standards and to prevent property damage.  The operators are now given a choice of employing any one of three methods to satisfy the OSM regulations.  These three methods are:

-   Method 1:  Limiting Particle Velocity Criterion

-   Method 2:  Scaled Distance Equation Criterion

-   Method 3:  Blast Level Chart Criterion

                Method 1 requires that each blast be monitored by a seismograph capable of the monitoring peak particle velocity.  Providing that the maximum particle velocity stays below the levels specified in the table below, the operators are considered in compliance.

Table Showing Maximum Permitted Particle Velocity at Various Distances From Blast Site

DISTANCE FROM BLAST SITE (ft.)

MAXIMUM ALLOWABLE PEAK

PARTICLE VELOCITY (in./sec.)

 

 

0 - 300

1.25

301 - 5000

1.00

5001 and Beyond

0.75

                Method 2 requires the operators to design shots in accordance with the table given below, which specifies a scale distance design factor for use at various distance between a dwelling and blast site.  The scale distance factors vary with distance to reflect the varying limits on peak particle velocities.  No seismic recording is required for this method, providing that the scale distance factors specified by OSM  in the table are observed.

Table Showing Scaled Distance Factors for Various Distances From Blast Site

DISTANCE FROM BLAST SITE (ft.)

SCALED DISTANCE FACTORS TO BE

USED WITHOUT SEISMIC MONITORING

 

 

0 - 300

50

300 - 5000

55

5001 and Beyond

65

                Example:  If a residential structure is 1000 ft. from a blast, the scaled distance factor of 55 is selected, and the safe weight per delay then is:

                                     W  = (D /Ds)^2

                                         = (1000 /55)^2

                                         =  330  lb.

                Method 3 allows an operators to use peak particle velocity limits that vary with frequency.  This method requires frequency analysis of the blast-generated ground vibration wave as well as particle velocity measurements.

                The wave form must be analyzed to determine the predominant frequency and peak particle velocity corresponding to this frequency.  In most cases, electrical instrumentation and digital analysis by a competent seismologist will be necessary to analyze the intensity of each frequency.  This method may represent the best means of evaluating potential damage to residential structures as well as human annoyance from blasting.  Figure 10 page 48 shows the relation between particle velocity and frequency as a blast-damage indicator (USOSM Regulation using method 3).

AIR BLAST

                Like ground vibration, air blast is another undesirable by-product produced from blasting.  Air blast damage and annoyance are directly related to factors such as blast design, weather and terrain conditions, and human response.

                At a particular air temperature, the wavelength of the sound wave is calculated by the relationship:

                             Wave length  =  Velocity (ft/sec.)/Frequency (Hz)

                The velocity of sound in air at 32 degrees F is 1086 ft/sec.  This velocity  increases by about 1 % for each 10 F increase in air temperature.  Audible air blast is called noise while air blast at frequencies below 20 Hz is inaudible to the human ear, and is called concussion.  Concussion is measured and reported as ďair overpressureĒ,  i.e., air pressure over and above atmospheric pressure.  Overpressure is expressed in pounds per square inch (psi) or in decibels (dB).

                Decibels are an exponential expression for sound intensity that approximates the response of the human ear.  The relationship between psi and dB is:

                                  dB     =    20  log(P/Po)

                        and:    psi     = 2.9*10-9 *  anti log (dB/20)

                    where:    dB = overpressure in dB

                                   log = common logarithm

                                   anti log  = 10*(x)

                                   P  = overpressure in psi

                                   Po =  reference pressure = 2.9 *10-9

The Figure Below Illustrates Overpressure Equivalence For Both Types of Units

dB

psi

 

 

 

 

 

 

180

3.0

Ü

Structural Damage

170

0.95

Ü

Most Windows Break

160

0.30

 

 

150

0.095

Ü

Some Windows Break

140

0.030

Ü

OSHA Maximum For Impulsive Sound

 

 

Ü

USBM TRP 78 Maximum

130

0.0095

Ü

USBM TRP 78 Safe Level

120

0.003

Ü

Threshold of Pain For Continuous Sound

 

 

Ü

Complaints Likely, OSHA Maximum For 15 Minutes

110

0.00095

 

 

100

0.0003

 

 

90

0.000095

Ü

OSHA Maximum For 8 Hours

80

0.00003

 

 

Causes Of Air Blast

                There are four main types of air blast overpressure defined as:

-     Air pressure pulse(APP); produced from direct rock displacement at the face or mounding at the blasthole collar.

-     Rock pressure pulse(RPP); produced from the vibrating ground.

-     Gas release pulse(GRP); gas escaping from the detonating explosive through rock fracture.

-          Stemming release pulse(SRP); Gas escaping from the blown-out stemming.

                Elements and conditions that can enhance the four causes of air blast are:

-          Detonating cord trunk lines and downlines

-     Lack of proper stemming material.

-          Inadequate stemming height.

-          Overdug or overloaded front row of holes in premature burden movement.

-     Drill patterns too small or large.

-          Delay sequence.

-          Atmospheric condition.

-          Secondary blasting.

-     Gas escape through fractures.

-     Mud seams providing an easy path for explosive gas into air at free surface.

Known Methods And Techniques to Reduce Blast Damage And Complaints

                In contrast to ground vibration, a greater number of the variables have a significant effect on air blast.  The table below shows the degree to which many variables contribute to the generation of air blast:

VARIABLES

SIGNIFICANT

MODERATELY

SIGNIFICANT

INSIGNIFICANT

 

 

 

 

Within Control of Mine Operators

 

 

 

     Charge Weight Per Delay    

X

 

 

     Length of Delay

X

 

 

     Burden and Spacing

X

 

 

     Stemming (Amount)

X

 

 

     Stemming (Type)

 

X

 

     Charge Length and Diameter

 

 

X

     Angle of Borehole

 

 

X

     Direction of Initiation

X

 

 

     Charge Weight Per Blast

 

 

X

     Charge Depth

X

 

 

     Bare Versus Covered Detonating Cord

X

 

 

     Initiating Systems--Electric Versus

          Non-electric Initiators

 

 

X

 

 

 

 

Not In Control of Mine Operators

 

 

 

     General Surface Terrain

 

X

 

     Type and Depth of Overburden

X

 

 

     Wind

X

 

 

     Atmospheric Conditions

X

 

 

                The following techniques and methods have been successful in reducing and resulting annoyance complaints:

1.          Depending on monitor capabilities, airblast should not exceed the following maximum levels as recommended by the USBM, RI 8485:

                              134 dB     @   0.1  Hz  high pass system

                              133 dB     @   2.0  Hz  high pass system

                              129 dB     @   6.0  Hz  high pass system

                              105 dB     @   C- slow weighting scale (events <= 2-sec. duration)

      This set of criteria is based on a minimal probability of the most superficial type of damage in residential-type structures.  Any one of the above will represent safe maximum air blast levels.  Although many of the currently used instruments are designed to be the 2 Hz corresponding to 133 dB.  These new recommended levels should provide 95-98 % non-damage probability and 90-95 % annoyance acceptability.

2.   In the absence of monitoring, the following cube-root scaled distances should be maintained following the USBM, RI 8485

                        Coal highwall                                        180  ft/lb

                        Coal parting                                          500  ft/lb

                        Quarries and mines                                250  ft/lb

                        Construction and excavation                  500   ft/lb

                        Unconfined blast                                   800   ft/lb

           Figure 11 page 49 shows the combined air blast measurements of all sites.

3.          Where methods 1 and 2 are too restrictive, monitoring of the blast site is recommended to determine safe blasting levels.

4.   To avoid air blast reinforcement from the simultaneous arrival of air blasts from different holes, the time for successive detonation should be:

 

                  T  >=  2 (S / V)

       where:

                  T  =  time between hole detonations (sec)

                  S  =  spacing between holes (ft)

                  V =  velocity of sound in air with respect to temperature  (ft /sec)

      This equation allows for a factor of safety.  However, where a surface initiation system is used to delay the firing of holes, T should be chosen short enough to avoid hole cutoffs.

5.   Use maximum confinement of explosive, at least a stemming height equal to the burden.  And use additional stemming on front row if excessive backbreak from the previous shot is present. It should be noted that a trade-off exists here in regards to decreasing airblast while increasing ground vibration.  Confinement can be obtained by choosing a large depth of burial or using coarse angular stemming material (such as crushed stone).

6.          Cover all surface detonating cord (one foot or more) where air blast is a sensitive problem.  At large distances in excess of 1000 ft, detonating cord becomes less of a problem because the high frequencies are attenuated.

7.          Reduce charge weight per delay by:

     - Lowering bench heights

     - Decreasing hole diameters

     - Decking

8.          Avoid or delay blasts where atmospheric and wind conditions couple to result in airblast focusing.  Postpone blasting when fog or cloudy conditions exist with no wind, during strong wind, early morning or late afternoon, and during the day when the surface temperature is falling.

9.   Use bottom hole initiation rather than top hole initiation.

10.          Avoid very short delay periods.  Be sure that the blast proceeds in the proper sequence.  Use a longer delay interval between rows than between holes in a rows.

11.          Consider beam condition when designing blast arrays.

     -Rate of the progression of holes firing along a free face should be less than the velocity of sound in air.

     -Minimize the number of opening holes having the same delay period.

     - Avoid the use of long charges in holes whose length is large compared with the burden  on the hole.

12. Do not mudcap in a populated area unless absolutely necessary.

                In addition to the above, blasting records should be maintained for both ground vibrations and airblasts to provide protection against local claims.  A public relations program, if implemented and executed sincerely, may avoid lengthy and costly litigation.                                                                         

BLASTING SAFETY

                Explosives are tools that, when used properly, benefit mankind.  However, improper use can be disastrous.  Prevention of explosives accidents depends on careful planning and faithful observance of proper blasting practices.  The users must remember that they are dealing with a powerful force and that various devices and methods have been developed to assist them in directing this force.  The slightest abuse or misdirection of explosives may cause serious injury or kill yourself or others.  It is impossible to include warnings or approved methods for every conceivable situation.  Explosive safety depends on a thorough knowledge of explosives and common sense.

                The following is a discussion of blasting safety from the moment the explosives have been delivered to the magazine to the moment the shot is fired.  Due to the complexity of the topic and the numerous variables found on-site, this discussion could not be all-inclusive.

                Storage:  All explosives, blasting agents, and initiatiators must be stored in magazines that have been constructed, approved, and licensed in accordance with local, state, and federal regulations. Magazines must be kept locked at all times, except when explosive materials are delivered into the magazine or removed for delivery to the blasting site.  Admittance to the explosive storage must be restricted; only authorized persons must have access to the keys of storage magazines.

                Maintaining accurate, current records of explosive materials is essential, both for regulatory requirements and to assure that stock are being used properly, the oldest stocks should be used firs

                The explosive materials that are taken from the storage magazine should be kept in their original containers.  However, small quantities of explosive material can be place in day boxes, powder chests, detonator boxes, or other special containers designed for this purpose.  Any explosive materials which are not used at the blast site must be returned to the storage magazine as soon as possible.  Magazine inventory records should reflect the quantity of explosive removed, the amount returned, and the net quantity used at the blast site.

                The magazine and the area within 25 ft. of the magazine must be kept clean of all empty cartons, packaging material, and other combustible materials.  Bags, boxes, liners, other packaging from the blasting operation should be disposed of in an approved manner and not returned to the magazine area.  Smoking, open flame, matches, or other flame-producing devices are prohibited inside of or within 50 ft. of storage magazines or explosive materials.  The loading and unloading of explosive materials should be done with care.

                Transportation:  Vehicles used for transporting explosive materials must be in good mechanical condition, with particular attention given to tires, brakes, the electrical system, steering, and chassis. Explosive material should be transported in a closed-body vehicle.  If an open-body vehicle is used, day boxes, powder chests, or detonator boxes must provide security and protection for the products being hauled.  The body of the vehicle hauling explosives should be lined with wood or other non-sparking material.  Explosive should  not be transported in the same truck with detonators.  If they are in the same truck, the detonators should be isolated from the explosives by placing them in a separate compartment, a day box, or detonator box designed for the purpose.

                Blasting Design Considerations:  The design of the blast is based on everything that follows the delivery of the explosives to the blast size.  As a standard procedure, the blasters should follow the practice of making a pre-blast log or loading charges when planning the blast.  The loading chart could be used to facilitate powder distribution in the blast area and assure the layout of the detonators to effect the designed delay pattern.  In addition, to showing the depth of the holes, explosives to be loaded, and the sketch of delay pattern, the chart could also note the amount of water in the holes and any information noted from the drilling logs, such as soft seams, caves, or fissures.

                The blast design should be the task for the persons who have the proper training and experience to determine the burden, spacing, diameter and the depth of holes, required stemming, amount and type of explosives, initiation system; and the delay pattern to control throw, minimize backbreak, flyrock, vibration, air blast; while producing the desired fragmentation and displacement.

                Loading Procedure:  All equipment not required for the loading operation should be removed from the blast pattern before loading of the blastholes begins.  The persons used on the blast site should be kept to a minimum and limited only to the blaster and the explosive crew who work under his direction.

                All tools and equipment required in the loading procedure should be kept of first quality and good repair.  Improper test instruments must not be permitted.

                Before any explosive material is loaded into blasthole, the depth, general condition, and the amount of water in the hole should be determined.  The primer or cartridge of explosive containing the detonator is generally loaded into the hole first.  The detonator should be well protected inside the cartridge, and slowly dropped into the hole.  The primer must be protected before any cartridges are loaded on it, specially when loading in a vertical or sub-vertical hole.  This is done by lowing several cartridges or pouring a few feet of free-flowing explosives materials on top of the primer.

                During loading, the explosive column should be checked periodically to make sure that the powder rise is consistent, the hole is not choking, and there are no openings where free-running explosive might be diverted to cause a concentration buildup.  To unknowingly load into these areas could cause blowouts or excessive flyrock.  The normal loading for any hole could be determined by the blast design loading chart, and periodic measuring will eliminate the probability of any gross overloading.

                Detonator Safety:   All types of detonators are sensitive to accidental initiation by heat, shock, and impact.  Although the modern detonator is designed to precise specifications and manufactured under extremely rigid quality control, it must be remembered that they contain sensitive primary explosives and pyrotechnics.  All blasting operations must be suspended during the approach and duration of an electrical storm with personnel moved to a safe location.  The blasters should be knowledgeable of all safety precautions regarding the detonator and associated initiating system components before loading.  All initiation systems must be used in accordance with the methods prescribed by the manufacturer.

                To provide initiation insurance and prevent unfired explosive due to rock movement and cutoffs, all blastholes over a certain depth should contain two detonators. Usually, one detonator is placed at the bottom of the hole, and the other detonator on the same or next later delay period,  should be placed in the upper portion of the hole.  The second detonator should be placed far enough down in the powder column so that the introduction of stemming will not cause it to be separated from the main charge.

                Electric Detonator Safety:  Electric detonators are designed to be fired by a pulse of electrical energy, so they are also susceptible to accidental firing by extraneous electricity, stray current, static electricity, radio frequency energy, electric storms, and high voltage power lines.  The electric detonators should be checked with proper test instrument before primer making.  Before stemming, the electric detonators should be checked again to make sure that they have not been damaged during loading.  Safety practice procedures dictate that the detonator be rechecked so that a replacement detonator may be introduced if needed.

                Nonelectric Detonator Safety:  Although nonelectric firing systems are generally less susceptible to accidental firing by stray current or radio frequency energy than electric firing systems, they also contain sensitive primary explosives and pyrotechnics.  As such, they should not be considered impervious to accidental initiating by lightning, static electricity, or strong electric fields.  The loading of explosives should be suspended, and personnel should be removed to a safe area, at the approach of, and for the duration of, an electrical storm regardless of the initiation system.

                Stemming and Hooking up The Blast:  After all the blastholes have been loaded, the electric detonators or the nonelectric system downlines should be checked before any stemming is introduced into the hole.  A blasterís galvanometer can be used to check the electric detonator, a special test meter can be used to check other nonelectric systems.  When a test instrument cannot be used to check nonelectric systems, these can be checked by visual inspection or by tugging lightly on the downlines to make sure that they have not been damaged or severed during the loading operation.  If there is any evidence that the primer or initiator has been damaged or weakened, a backup or ďinsuranceĒ primer should be introduced into the hole before the stemming begins.  If  the stemming is needed as soon as possible after the holes are loaded, the primer and initiating system in each blasthole should be checked before any stemming is place in the hole.

                While stemming the hole, caution must be taken to assure that the legwires or downlines are not damaged.  In wet holes, the subsidence of primer or excessive tension on legwires or downlines should be checked regularly.  The stemming material should be relatively clean, and, if used in a vertical hole, free flowing.  Avoid using large rocks since they may cut the legwires or downlines or bridge or choke in the hole.

                Blast Firing Considerations:  After completion of loading, a shot should be fired as soon as possible.  But if operation could not be suspended after loading is completed, a shot may be fired at noon time break, a shift change, or at quitting time.  Loading time should be scheduled so that the exposure time of a loaded blast is kept minimum.

                Before firing a blast, a standard procedure is required to clear the blasting area of all personnel and equipment, block roads, and post guards to access ways in to the blast area.  All personnel working in or near the blast area must be in a safe, protected  area when the blast is fired.  A blast warning signal device should be a siren, whistle, horn, or similar device which could be significantly heard to a distance of 1.0 km.  The warning signal must be known by all persons working at or around the blast site and should be posted conspicuously around the blast area.  Roads leading to the blast area should be appropriately marked with warning signs noting that blasting operations are being conducted in the immediate area.  If  possible, the approximate time for firing the blast should be posted.

                Flyrock, or the wild, uncontrolled throw of rock from the detonation of an explosive charge, probably causes more personal injury and property damage than any other aspect of the operation.   Flyrock can be caused by a number of factors:  overloading of the blast, lack of burden and confinement, insufficient stemming (too little or poor stemming), lack of relief, improper timing (delay pattern), structure of rock being blasted (such  as cave, crack, slips, mud seams, etc.) and loose rock on top of a blast.

                The blast pattern design criteria described above (burden, spacing, hole diameter, explosive charge, stemming and initiation system) are intended to minimize flyrock.  Changing any parameter of a blast should be done only after considering the possibility of flyrock generation.

                Firing The Blast:  The blaster (the one who actually fires the blast) is responsible for the custody of the blasting machine, blasting switch, or other type of device used to fire the blast.  After the blasting circuit has been completed and checked, the leading line is run from the blasting site to the blasterís shelter or to a protected area from which the blast is to be fired.  Before hooking the blast circuit into the leading line, the line should be checked with a blasterís galvanometer to assure continuity.  Once the circuit has been verified, the blaster connects the leads and waits for the firing signal.  Upon receiving the signal, the blaster fires the shot.

                Post  Blast:  After the blast has been fired , the blaster should stay under the shelter until he is sure to be safe from any flyrock.  Before leaving the shelter, the blaster should disconnect the lead and take the blasting machine or lock the firing switch to prevent any accidental re-firing.

                The blast site should be inspected before an ďAll ClearĒ is given for personnel  and machinery to return to the blast site area.  Make sure there is no evidence of misfires or unexploded explosive materials.  Ample postblast waiting time is necessary for reducing smoke, toxic fumes, dust, loose rock, and other hazardous conditions.  More postblast waiting time is required in underground operations in which roof inspection and proper ventilation are important factors. 

                Misfires:  A misfire is defined as the failure of an explosive charge to detonate at the proper time.  The best advice that can be given regarding the handling of misfires is to take every precaution to prevent their occurrence.  Anytime misfired holes, portions of misfired holes, or unexploded explosive materials remain after a blast is fired, a hazardous situation is created that will exist until the unfired explosive materials have been disposed of properly.  Explosive disposal requires sound judgment and a comprehensive understanding of explosives by the blaster.  Most misfires occur because of improper techniques by the blaster, or due to a change in the geologic structure from the expected norm.  It is important that any investigation into the cause of misfires is conducted fairly and with an open mind.  Any preconceived idea of the cause may mask the true cause, and prevent the establishment of procedures that will prevent future misfires.

                The disposal of misfire should be handled only by an experienced individual familiar with the explosive materials and initiating systems used in the blast.  Persons handling misfire should have firsthand knowledge of how the blast was loaded, or must have accurate records and data giving detailed information on the type, weight, and location of all the explosive materials and the initiation system components used.  The specific recommendation concerning misfire handling procedure cannot be made, as every misfire must be evaluated on an individual basis.  All information regarding the misfire must be analyzed completely and a plan of action must be outlined to safely handle it.  The operation must follow the specific federal, state and local regulations governing the handling of misfires.

                The safest and surest way of handling any types of misfire is to be able to reconnect all of the unfired charges and fire them successfully.  This procedure is not always possible or practical, as often misfired charges have been cut off by rock movement and are located in areas that are inaccessible to the blaster.  In some instances, rock has been torn away by the charge with very little burden or with burden that has been shattered and displaced.  The firing of these misfired charges can result in uncontrolled and excessive flyrock.  Attempts to confine misfired charges with mats, broken muck, and screenings require extreme caution and should be made only after the situation has been thoroughly evaluated.  In firing misfired charges under these circumstances, one must assume the worse consequences that can happen and take appropriate precautions.

                If misfired charges could not be reconnected and re-fired safely, the next approach is to try to neutralize it as much as possible.  In small diameter holes, it is often possible to wash out the explosive column with a stream of water.  It is rather impractical to try to wash out large diameter cartridges from drill holes, it is often possible to wash out the stemming to permit the removal of these cartridges with non-sparking retrieving tools.  When large diameter holes have been loaded with bulk blasting agents, sometimes water can be used to was out the charge.  Water will neutralize ANFO and similar types of materials that have little or no water resistance.

                If ,due to rock movement, the detonator leads or detonating cord lines have been severed, it may be possible to wash out the stemming or to place another primer in the hole to fire the charge from the top column.                 When it is impossible to reinitiate, retrieve, or neutralize misfired explosives, it may be necessary to consider drilling, loading, and firing holes adjacent to the misfire.  If performed successfully, this procedure will dislodge or displace the misfired explosives for eventual disposal.

Some Unsafe Practices That Happen In Thailand:

                There have been many unsafe practices about the uses of explosive materials and blasting techniques in Thailand.  These practices happen from time to time.  Examples are:

                Storage of an Explosive:  Most of the mines in Thailand store explosive materials in residences, particularly the small miners.  Although, by the law, all explosives materials must be stored in approved magazines.  But they build the magazines only to obtain the required approvals.  The reasons for these practices could be for convenience, or to prevent theft.

                Transportation of an Explosive:  Sometimes, explosive materials are transported in unproved or inappropriate vehicles, especially when small amounts of explosive materials are transported.  They use normal trucks, pickup trucks, buses, or even the trains.

                Unsafe Caps and Fuses:  The most popular, cheap and unsafe caps and fuses are used in the small mines or sometimes even in the ďnon-standardĒ mines.  These caps and fuses are made in military bases, they are illegal.  The caps and fuses are made together in one unit.  Caps are made of explosive materials  which are ignited by unsafe fuses, made of  black powder within plastic covers.  These plastic covers are not waterproof, fireproof and the burning rates are not uniform.  The reason for these practices could be the lower cost compared to commercial caps and safety fuses.

SUGGESTIONS AND WARNINGS TO AVOID THE MORE COMMON CAUSES OF ACCIDENTS

ďDO  AND  DONíTĒ  INSTRUCTIONS  AND  WARNINGS

        

General Warning:

                All explosive material are dangerous and must be carefully transported, handle, stored, and used.

DO follow federal, state and local laws and regulations.

DO lock up explosive materials when you are done with your duties.

DO control explosive materials, which have been removed from a magazine, to prevent possession by children or other unauthorized persons.

DONíT expose explosive materials to excessive heat from flame-producing devices, friction, or electrical impulses.

DONíT allow any sources of ignition within 100 feet of the blast area (except for lighting safety fuse) or within 50 feet of a magazine or vehicle containing explosive materials.

DONíT fight fires in explosive materials. Remove all personnel to a safe location immediately and guard the area against intruders.

DONíT shoot with a weapon into material, magazines, or vehicles loaded with explosive materials.

DONíT use explosive materials that are deteriorated or damaged.

DO contact your supervisor or the manufacturer when you have any questions on the use of explosive materials.

DO call the police when the explosive materials are lost or stolen.

When Transporting Explosive Material:

DONíT park vehicles containing explosive material in areas which are congested or where people congregate.

DO load and unload explosive materials carefully.

DO transport explosive materials in accordance with Federal, State and local laws and regulations.

DO keep matches, lighters, open-flame, and other sources of ignition at least 50 feet away from parked vehicles carrying explosive materials.

DONíT leave a vehicle containing explosive materials unattended.

When Storing Explosive Material:

DO post ď EXPLOSIVES-KEEP OFFĒ signs conspicuously near magazines.  These signs should be located so that a bullet passing through them at right angles cannot strike a magazine.

DO locate magazines in the most isolated places available.  They should be separate from each other, and inhabited building, highway, and passenger railways by distances not less than those in the U.S. ďTable of DistancesĒ.

DO store explosive material only in a magazine which is clean, dry, well-ventilated, reasonably cool, properly located, substantially constructed, securely locked, weather-resistant, fire-resistant, and theft-resistant and, when required by the nature of material, bullet-and missile-resistant.

DONíT store explosive materials in a wet or damp place, with flammable or other hazardous material, or near sources of excessive heat.

DONíT store detonators in the same package or magazine with other explosive materials.

DO store only explosive materials and blasting accessories in magazines.

DONíT allow combustible material to accumulate within 25 feet of magazines.

DO rotate stock so the oldest material in the magazine is first out.

DONíT exceed recommended storage time and temperature for explosives.

When Using Explosive Materials:

DONíT use any explosive material unless you are completely familiar with safe procedures for their use, or are under the direction of competent, experienced persons.

DONíT allow metallic cutters to come into contact with any metallic fasteners when opening packages of explosive material.

DO close partially used packages of explosive.

DONíT carry explosive materials on your person.

DONíT insert anything except safety fuse in a blasting cap.

DONíT use any explosive materials that has been water-soaked even if they appear to be dried out.

DONíT handle explosives during an electrical storm.  All persons should retire to a place of safety.

DONíT attempt to investigate the contents of a detonator or try to pull the wires, fuse, or detonating cord out of any detonator or delay device.

DONíT use any explosive materials missing either a safety standard or a brand name.

DO keep explosive materials away from children, unauthorized persons, and livestock.

When Preparing the Primer:

DONíT force a detonator into an explosive material.  Insert the detonator in a hole made with a punch suitable for that purpose.

DONíT use a cast primer or booster if the hole is too small for the detonator.  Never try to enlarge the hole.

DONíT make up primers in a magazine or near other large quantities of explosive materials.

DONíT make more primers than necessary for immediate need.

DO make up primers in accordance with established methods.  Make sure that the detonator is completely encased in explosive and also make sure that in loading no tension will be placed on the wires, safety fuse, or detonating cord leading out of any detonator or delay device.

When Drilling and Loading:

Do carefully examine the surface or the face before drilling to determine the possible presence of unfired explosive materials.  Never drill into explosive material or into any hole that has contained explosive material.

Do check each borehole carefully to assure that it is in safe condition for loading.

Do avoid placing any unnecessary part of the body over or in front of the borehole when loading, tamping, and stemming.

DONíT force explosive materials into a borehole.

DONíT slit, drop, deform, tamp, or abuse the primer and DONíT drop another cartridge directly on the primer.

DONíT load a borehole that contains any hot or burning material. Temperatures in excess of 150o F (66o C) are dangerous.

DONíT drill a borehole near a hole loaded  with explosive materials.

DONíT stack more explosive materials than are needed near a working area during loading.

DO recognize the possibility of static electrical hazards from pneumatic loading and take adequate precautionary measures.

When Tamping:

DONíT hit the primer.  DONíT  tamp explosive materials with a metallic device except for jointed poles with nonferrous metal connectors.  Avoid violent tamping.

DONíT kink or damage safety fuse, detonating cord, or wires of detonators when tamping.

When Blasting Electrically:

DO test all electric blasting cap circuits for continuity, when required, using only a blasting circuit test instrument designed for that purpose.

DO be sure that all wire ends are clean before connecting.

DONíT attempt to fire electric blasting caps with more, or less, current than recommended by the manufacturer.

DO keep the electric cap wires or lead wires disconnected from the power source and short-circuited until ready to fire.

DONíT use electric blasting caps made by different manufacturers in the same circuit, or caps of different styles or functions even if made by the same manufacturer, unless such a use is approved by the manufacturer.

DONíT load any borehole near electric power lines, unless the firing line, including the electric blasting cap wires, is anchored or so short that it cannot reach the power lines. 

DONíT have electric wires or cables near electric blasting caps or other explosive materials except at the time for the purpose of firing the blast.

DO keep the firing circuit completely insulated from the ground or other conductors.

DONíT use or uncoil the wires of electric blasting caps during electrical or dust storms or near other source of large charges of static electricity.

DONíT uncoil the wires or use electric blasting caps in the vicinity of radio-frequency transmitters. Consult the manufacturers.

When Blasting with Detonating Cord:

DO select detonating cord that has the characteristics consistent with correct blasting methods and type of explosive materials being used.

DO handle detonating cord with the same respect given other explosive materials.

DO avoid damaging detonating cord prior to firing.

DO cut the line of detonating cord from the spool before loading the remainder of the charge.

DO make tight connections in accordance with established methods.  Cord-to-cord connections should be made only where the detonating cord is dry.

DO avoid loops, sharp kinks or angles that direct the cord back toward the oncoming line of detonation.

DO connect detonators to detonating cord by methods recommended by the manufacturer.  The detonators should always be pointed toward the desired direction of detonation.

DONíT attach detonators to detonating cord until everything is in readiness for the blast

When Blasting with Non-electric Blasting Caps:

General:

DO follow the manufacturerís instructions and warnings.  Emphasize proper hook-up procedures and safety.

DO discontinue operations in surface blast areas during electrical storms.

DONíT hold nonelectric leads during firing; personal injury or death may result.

DONíT use the shock tubing leads or detonating cord leads for any purpose other than that intended by the manufacturer.

Miniaturized Detonating Cord System:

DO use explosives that are insensitive to initiation by the miniaturized detonating cord lead.

DONíT join two lengths of miniaturized detonating cord.  It will not propagate through such a connection.

DONíT smoke or allow open flame within 25 feet of blasting machines designed for gas initiated nonelectric blasting caps.

DO stay away from the blast area after connections are made ready for firing, unless the entire system has been properly purged and disconnected from the primary source of ignition.

Shock Tube System:

DONíT trim heat seals from the shock tube ends.  Moisture entry will cause failure.

DONíT join lengths of shock tube.  It will not propagate through such connections.

When Blasting With Safety Fuse:

DONíT use lengths of safety fuse of less than 1meter.  Know the burning speed of  safety fuse by conducting a test burn, and make sure you have enough time to reach safety after lighting.

DO handle safety fuse carefully to avoid damaging the covering.  In cold weather, warm it before using to avoid cracking the waterproof covering.

DONíT cut safety fuse until you are ready to insert it into a blasting cap. Cut off an inch or two to ensure a dry end.  Cut safety fuse squarely across with a clean sharp blade.  Seat the safety against the cap charge and avoid twisting after it is in place.

DO crimp blasting caps only with a crimper designed for the purpose.

DONíT light safety fuse until sufficient stemming has been placed over the explosive material to prevent excessive heat from coming into contact with the explosive material.

DONíT hold other explosive materials in your hands when lighting safety fuse.

DONíT drop a primer with lighted safety fuse down the hole.

DONíT use safety fuse in agricultural blasting.

DONíT use matches, cigarette lighter, cigarettes, pipes, cigars, carbide lamps or other unsafe methods to ignite safety fuse.

DO use only ignitercord with ďthermaliteĒ connectors for multiple-fuse ignition.

DO use only hot-wire lighters or thermalite connector for single-fuse ignition.

DO use the ď buddy systemĒ when lighting safety fuse--one lights the fuse, the other times and monitors.

In Underground Work:

DONíT store excessive supplies of explosive materials in an underground mine.

Before and After Firing:

DONíT fire a blast without a positive signal from the one in charge.

DONíT fire from a position in front of the blast.

DO make certain that all persons, equipment and surplus explosive materials are in a safe place, that all access routes into the blast area have been posted with guards, and adequate warning has been sounded.

DONíT attempt to investigate a misfire too soon.

DO comply with existing Federal, State and local laws and regulations for safe fume levels , misfire waiting time, etc., before returning to the blast area.

DONíT drill, bore, or pick out a charge of explosive material that has misfired.  Misfires should be handled only by or under the direction of a competent and experienced person, and then only in compliance with any applicable Federal, State or local laws and regulations.

Explosive Materials Disposal:

DO dispose of or destroy explosive materials in accordance with approved methods.  Consult your supervisor, or the manufacturer if you have no supervisor.

DONíT leave explosive materials or their packaging where children, unauthorized persons or livestock can get them.

DONíT allow any explosive materials packaging to be burned in a confined space or to be reused.

DEFINITIONS

                Since we are dealing with explosives and rock blasting, some important terms related to these fields are necessary to be defined:

A- scale:  A sound level measurement scale.  It discriminates against low frequencies.  It approximates the human ear.

Acoustical impedance:  The mathematical expression for characterizing a material as to energy transfer properties.

Air Blast:  A sound pressure, or an airborne shock wave generated by a blast traveling through the atmosphere.

Ammonium Nitrate:  The ammonium salt of nitric acid represented by the chemical formula NH4NO3.

American Table of Distances:  A table showing distances that explosives must be stored from other explosives, inhabited buildings, railroads, highways, and magazines, according to the amount of explosives stored.  Usually called only Table of Distances.

Amplitude:  The height of the vibration trace or wave-form above the zero line on a vibration record.  It usually refers to the maximum value.

ANFO:  An explosive material consisting of  ammonium nitrate and fuel oil mixture.  Used as a blasting agent.

Authorized Person:  An individual approved or assigned by management to perform a specific duty or duties, or to be at a specific location or locations.

Back Break:  Rock broken beyond the limit of the last row of holes in a blast.

Barricaded:  The effective screening of a building containing explosives from a magazine or other building, railway, or highway by a natural or artificial barrier.  A straight line from the top of any side-wall of the building containing explosives to the eve line of any magazine or other building or to a point 12 ft above the center of a railway shall pass through such barriers.

Base Charge:  The main explosive charge in the base of a detonator.

Bedding planes:  Rock formation formed by layering of  rock as it was deposited, as igneous flows or in separated sedimentary deposits.

Bench:  The horizontal ledge in a quarry or mining face along which holes are drilled vertically. Benching is the process of excavating whereby terraces or ledges are worked in a stepped shape.

Bench Height:  The vertical distance from the top of the bench to the floor or to the top of the next lower bench.

Black Powder:  A low-explosive compound of an intimate mixture of an alkali nitrate (usually potassium or sodium nitrate), charcoal and sulfur.

Blast, Blasting:  The operation of  breaking rock by means of  explosives. Shooting or firing of explosive is also used to mean blast.

Blasting Agent:  Any material or mixture, consisting of  a fuel and oxidizer intended for blasting, not otherwise classified as an explosive and in which none of the ingredients are classified as an explosive, provided that the final product, as mixed and packaged for use or shipment, cannot be detonated by means of a No. 8  test blasting cap when confined.

Blast Area:  The area of a blast within the influence of flying rock missiles, gases, and concussion.

Blasting Crew:  A group of persons who assist the blaster in loading, tying in, and firing a blast.

Blasting Cap:  A detonator that is initiated by safety fuse.

Blasting Log:  A written  record of information about a specific blast as may be required by law or regulation.

Blasting Machine:  An electrical or electromechanical device that provides electrical energy for the purpose of energizing detonators in  electric blasting.

Blasthole (Borehole):  A hole drilled in rock or other material for placement of explosives.

Blast Pattern:  The plan of the drill holes laid out on a bench; an expression of the burden distance, spacing distance and their relationship to each other.

Blast Area:  The area where explosive material is handled during loading, including the perimeter of blastholes and 50 ft. in all directions from holes to be loaded.  In underground mines, 15 ft. of solid rib or pillar can be substituted for the 50 ft. distance.

Blaster:  The qualified persons in charge of , and responsible for, the loading and firing of a blast.  The persons who actually fires the blast.

Blockhole:  A hole drilled into a boulder to allow the placement of a small charge to break the boulder.

Booster:  A chemical compound used for intensifying an explosive reaction. A booster does not contain an initiating device.

Boot-leg:  A situation in which the blast fails to cause total failure of the rock because of insufficient explosives for the amount of burden, or it may be caused by incomplete detonation of the explosives.  That portion of a borehole that remains relatively intact after having been charged with explosive and fired.

Bridging:  Where the continuity of a column of explosives in a borehole is broken, either by improper placement, as in the case of slurries or poured blasting agents, or where some foreign matter has plugged the hole.

Buffer:  Previously shot material, not removed, lying against a face to be shot.

Burden:  Generally considered the distanced from an explosive charge to the nearest free or open face at the time the hole detonates.  Technically, there may be an apparent burden and a true burden, the latter being measured always in the direction in which displacement of broken rock will occur following firing of an explosive charge.

Charge Weight:  The amount of explosive charge in pounds (or kilograms) per delay or per blast hole.

Compressional Wave:  A seismic wave whose motion is compression-dilation, or push-pull, generated by rockís resistance to compression.

Condenser-discharge:  A blasting machine which uses batteries to energize a series of condensers, whose stored energy is released into a blasting circuit.

Connecting Wire:  Any wire used in a blasting circuit to extend the length of a leg wire or leading wire.

Connector:  Refers to device used to initiate a delay in a detonating cord circuit, connecting one hole in the circuit with another, or one row of  holes to another row of holes.

Coupling:  The act of connecting or jointing two or more distinct parts.  In blasting the reference concerns the transfer of energy from an explosive reaction into the surrounding rock and is considered perfect when there are no losses due to absorption or cushioning.

Crest:  The top of the face created by a previous shot.  The maximum amplitude of a wave in the upward direction above the zero line.

Cultural Vibration:  Vibration that is commonplace and familiar to the observer.

Cushion Blasting:  The technique of firing of single row of holes along a neat excavation line to shear the web between the closely drill holes.  Cushion blasts are fired after the production shooting has been accomplished.

Cut Off:  Where a portion of a column of explosives has failed to detonate because of  bridging, or to a shifting of rock formation due to an improper delay system.

Decibel (dB):  The unit of sound level measurement

Deck:  In blasting, a smaller charge or portion of a blasthole loaded with explosives that is separated from the main charge by stemming or an air cushion.

Deflagration:  An explosive reaction that consists of a burning action at a high rate of speed along which occurs gas formation and pressure expansion.

Delay:  The term used to describe a blasting cap which does not fire instantaneously but has a predetermined built-in lag or delay time.

Delay Blasting:  Blasting that uses delays or delay caps.

Delay Element:  That portion of a blasting cap which causes a delay between the instant of impressment of electrical energy on the cap and the time of detonation of the base charge of the cap.

Density:  The mass of an explosive per unit volume.  Expressed in gm/cc. Water has a density of  1.0 gram per cubic centimeter.

Detonating Cord:  A plastic covered core of high velocity explosives used to detonate charges of explosives in boreholes and under water, e.g., Primacord.

Detonation:  An explosive reaction that consists of propagation of a shock wave through the explosive accompanied by a chemical reaction that furnishes energy to sustain the shock-wave propagation in a stable manner, with gaseous formation and pressure expansion following shortly thereafter.

Dip:  The angle at which strata, beds, or veins are inclined from the horizontal.

Displacement:  The amount of motion associated with ground vibration , measured in inches.

Double Priming:  A blasthole containing two priming units, usually on the same time delay.  They are usually placed one near the top and one near the bottom of the blasthole.

Downline:  Detonating cord lines running from the top of the hole.  The primer is attached to the bottom end and additional primers may be slid down the cord in the case of decking and/or multiple priming.

Drop Ball:  Known also as a Headache Ball.  An iron or steel weight held on a wire rope that  is dropped from a height onto large boulders for the purpose of breaking them into smaller fragments.

EBC (Electric Blasting Cap):  An electrically initiated blasting cap that may be instantaneous or contain a delay element.  Used to initiate primers or detonating  cord.

Energy Ratio:  A standard for damage caused by vibration from blasting.  Also written as  ER and defined as (acceleration in ft /sec/frequency).

Explosion:  A  thermo-chemical process whereby mixtures of gases, solids, or liquids react with the almost instantaneous formation of gaseous pressure and near sudden heat release.  There must always be a sources of ignition and the proper temperature limit reached to initiate the reaction.  Technically, a boiler can rupture but cannot explode.

Explosive:  Any chemical mixture that reacts at high speed to liberate gas and heat thus causing tremendous pressure.  The distinction between high and low explosives are twofold:  the former are designed to detonate and contain at least one high explosive ingredient; the latter always deflagrate and contain no ingredients which by themselves can be exploded.  Both high and low explosives can be initiated by a single No.8 blasting cap as opposed to blasting agents which can not be so initiated.

Explosive Charge:  The quantity of explosive that is to be detonated.

Explosive Decks:  Explosive placed in certain areas of the hole separated by drill cuttings.

Face:  The end of an excavation toward which work is progressing or that which was last done.  It is also any rock surface exposed to the air.

Fatigue:  The weakening or failure of material because of repeated  vibration or strain.

Fuse, Safety Fuse:  A flexible cord containing an internal burning medium by which fire or flame is conveyed at a continuous and uniform rate from the point of ignition to the point of use, usually a fuse detonator.

Fuse Cap (Fuse Detonator):  A detonator that is initiated by a safety fuse; also referred to as an ordinary blasting cap.

Fire:  In blasting, it is the act of initiating an explosive reaction.

Floor:  The bottom horizontal, or nearly so, part of excavation upon which haulage or walking is done.

Flyrock:  Rock that is propelled into the air by the force of an explosion. Usually comes from pre-broken material on the surface or the upper open face.  Flyrock is an indicator of wasted energy.

Fracture:  The breaking of rock without movement of broken pieces.

Fragmentation:  The extent to which rock is broken into small pieces by primary blasting.

Free Face:  A rock surface exposed to the air or water that provides room for expansion upon fragmentation, sometimes called open face.

Frequency:  The number of vibrations or complete oscillations occurring within one second.  The unit of measurement is the hertz (Hz) or cycle per second (cps).

Fuel:  A substance that may react with oxygen to produce combustion.  In explosive calculations, it is the chemical compound used for the purpose of combining with oxygen to form gaseous products and cause a release of heat.

Galvanic Action:  Current caused when dissimilar metals come in contact with each other or through a conductive medium.  This action may create sufficient voltage to cause premature firing of an electric blasting circuit, particularly in the presence of salt water.

Galvanometer:  A device containing a silver chloride cell which is used to measure resistance in an electric blasting circuit.

Grade:  In excavation, it specifies the elevation of a roadbed, rail, foundation, and so on.  When given a value such as percent or degree grade it is in the amount of fall or inclination compared to a unit horizontal distance for a ditch, road, etc.  To grade means to level ground irregularities to a prescribed elevation.

Gelatin Dynamite:  A type of high water-resistant dynamite characterized by its gelatinous consistency.

Geology:  A description of the types and arrangement of rock in an area; the description usually includes the dip and strike, type and extend of preexisting breaks in the rock, and the hardness and massiveness of rock as these effect blast design.

Ground Vibration:  Shaking of the ground, by elastic waves emanating from a blast, usually measured in inches per second of velocity.

High Explosives:  The explosives that are characterized by a very high rate of reaction, high pressure development, and the presence of a detonation wave in the explosive.

Initiation:  The act of detonating a high explosive by means of mechanical device or other means.

Initiator:  A device or product used to transmit or supply heat or shock wave to start an explosion.

Joints:  Planes within a rock mass along which there is no resistance to separation and along which there has been no relative movement of material on each side of  the break.  They normally occur in sets, the planes of which are generally mutually perpendicular.  Joints, like stratification, are often called parting.

Jumbo:  A machine designed to contain two or more mounted drilling units which may or may not be operated independently.

Lead Wire:  The wires connecting the electrode of an electric blasting machine with the final leg wires of a blasting circuit.

LEDC:  Low Energy Detonating Cord. It is used to initiate a nonelectric cap at the bottom of a borehole.

Linear Scale:  A sound level measurement scale that is non-weighted so that there is little or no discrimination at low frequencies.

Longitudinal Component:  That component of vibration which produces motion in the direction of a line joining the vibration source and the seismograph.

Longitudinal  trace  The line on the vibration record that records the longitudinal component of motion.

Low Order:  Used to describe a condition of detonation that is not as rapid or complete as it should be.

Millisecond Delay Caps:  Delay electric caps which have a built-in delay element, usually

25/1000th of a second apart, consecutively.  This timing interval may vary from manufacturer to manufacturer.

Misfire:  A charge or part of a charge, which for any reason has failed to fire as planned.  All misfires are to be considered extremely dangerous until the cause of the misfire has been determined.

Mud Cap:  Referred to also as an adobe or plaster shot.  A charge of explosive fired in contact with the surface of rock after being covered with a quantity of mud, wet earth or similar substance, no borehole being used.

MS Connector:  A nonelectric millisecond delay device used with detonating cord for delaying shots from the surface.

Open Pit:  A surface operation for the mining of metallic ores, coal, clay, and so on.

Overbreak:  Excessive breakage of rock beyond the desired excavation limit.

Overburden:  The material lying on top of the rock to be shot; usually refers to dirt and gravel, but can mean another type of rock; e.g. shale over limestone.

Overpressure:  The pressure generated by a sound wave which produces vibration in atmospheric pressure.  Overpressure is measured in psi or decibels.

Over Shot:  A condition resulting from using more than the necessary amount of explosive.  Usually characterized by excesses of fragmentation, flyrock, and noise.

Oxidizer:  A supplier of oxygen.

Particle Velocity:  The velocity at which the earth vibrates, measured in inches per second.

Peak Particle Velocity:  The maximum particle velocity.

Powder:  Any of various solid explosives.

Premature:  A charge which detonates before it is intended to.

Presplitting:  Stress relief involving a single row of hole, drilled along a neat excavation line, where detonation of explosives in the hole causes shearing of the web of rock between the holes.  Presplit holes are fired in advance of the production holes.

Primary Blasting:  The main blast executed to sustain product.

Primer  An explosive unit containing a suitable firing device that is used for initiation of the entire explosive charge.  A primer accepts initiation from a detonator or detonator cord, the resulting detonation is then transmitted to an equal or less sensitivity explosive.

Propagation  Velocity:  The velocity at which a vibration or seismic wave travels outward from source.  It is measured in thousands of feet per second.

Quarry:  An open or surface mine used for the extraction of rock such as limestone, slate, building stone, and so on.

Round:  A group or set of blastholes constituting a complete cut in underground headings, tunnels, etc.

Safe Limit:  The amount of vibration that a structure can safely withstand.  Vibrations below this limit have a very low probability of causing damage.  Vibrations above this limit have a reasonable probability of causing damage.

Scale Distance:  A relationship between weight of explosive detonated per delay and distance from a receptor that relates to seismic disturbance at the receptor.

Secondary Blasting:  Using explosive to break up larger masses of rock resulting from the primary blast, the rocks of which are generally too large for easy handling.

Seismic Velocity:  The same as propagation velocity.  The velocity at which a seismic wave travels outward from its source.

Seismic Waves:  Waves that travel through the earth

Seismograph:  An instrument that measures, and supplies a permanent record of, earth-born vibration induced by earthquake, blasting and so on.

Seismograph Trace:  A line on the seismograph record showing the vibration of the seismic wave.

Sensitizer:  The ingredient used in explosive compounds to promote greater ease in initiation or propagation of the reaction.

Sensor:  A device that senses or measures the vibration of the seismic wave.

Shot Firer:  Also referred to as the shooter or blaster.  The person who actually fires a blast.  A powder-man, on the other hand, may charge or load blastholes with explosive but may not fire the blast.

Shunt:  A piece of metal connecting two ends of leg wires to prevent stray currents from causing accidental detonation of the cap.  The act of deliberately shorting any portion of an electrical blasting circuit.

Sinking-cut:  A round drilled, loaded and timed to be lifted vertically, due to the fact that no open face is available.

Slope:  Used to define the ratio of the vertical rise or height to horizontal distances in describing the angle a bank or bench face makes with the horizontal.  For example, a 3/2 slope means there would be a 3 ft. rise to each 2 ft. of horizontal distance.

Sound Level:  The value of sound level pressure in psi or decibel.

Spacing:  In blasting, the distance between boreholes or charges in a row.

Stemming:  The inert material, such as drill cuttings, used in the collar portion (or elsewhere) of a blasthole so as to confine the gaseous products formed on explosion.  Also, the length of blasthole left uncharged.

Strength:  Refers to the energy content of an explosive in relation to an equal amount of  ANFO.

Stratification  Plane:  Within sedimentary rock deposits, formed by the interruption in the deposition of sediments.

Strike:  The course or bearing of the outcrop of an inclined bed or geologic structure on a level surface.

Subdrill:  To drill a blasthole beyond the planned grade lines or below floor level.

Swell Factor:  The ratio of the volume of material in its solid state to that when broken.

Tamping Process:  The action of compressing the stemming of explosive in a blasthole.

Tamping Bags:  Cylindrical bags containing stemming material and used in the borehole to confine the explosive material charge.

Theft-resistance:  Construction designed to deter illegal entry into facilities used for the storage of explosive materials.

Toe:  The burden or distance between the bottom of a borehole to the vertical free face of a bench in an excavation.

Trace:  A line on a vibration record.

Trace Amplitude:  The amplitude of a seismic wave on any of the traces of the vibration record.

Transverse:  A direction of motion at right angles to another direction of motion.

Trunk, Trunkline:  Detonation cord line on the surface, to which the downline is tied prior to firing.

Unbarricade:  The absence of a natural or artificial barricade around explosive storage areas or facilities.

Unconfined Detonation Velocity: The detonation velocity of an explosive material without confinement, for example, a charge fired in the open.

Under Shot:  A condition resulting from not enough explosive being used, or the pattern size too large for the amount of explosive used.  Usually characterized by poor fragmentation and lack of movement.

U.S. Bureau of Mines (USBM):  A now defunct bureau of the Department of Interior which was active in promoting safety in coal mines and in carrying out broad programs in mining and related fields.

Velocity:  The rate of change of distance with time. The rate of detonation.

Warning Signal:  A visual or audible signal that is used for warning personnel in the vicinity of the blast area of an impending explosion.

Water Gel:  An explosive material containing substantial portions of water, oxidizers, and fuel, plus a cross-linking agent.

Water Stemming Bag:  Water-filled plastic bags with a self-sealing valve classified as a permissible stemming device by the Mine Safety and Health Administration (MSHA).

Weight Strength:  The energy of an explosive material per unit of weight expressed as a percentage of the energy per unit of weight of a specified explosive standard.

REFERENCES

       Atlas Powder Company,  Field  Technical  Poeration, Dallas, Texas, USA ďExplosive and Rock BlastingĒ  1987, pp. 662

       E.I. du Pont de Nemours & Co (Inc.), Expolsive Productions Division, Sales Development Section, Wilmington, Delaware 19898, ďBlastersí HandbookĒ 1980, pp. 494.

       Bauer, A. and Crosby, W.A., ďBlastingĒ: Surface Mining 2nd edition, 1990, pp. 540-581.