Exam Study Material – Blaster Part 2

/ ISEE -Journal of Explosives : En o-ineering . • Vof. 13 No.7 July/August 1996 : Back to Basics: : Electric Initiation Part 4

A blaster, using electric initia­ sense require that whenever an tion, must be aware of certain electrical storm approaches, hazards specific to this type of blasting operations must be sus­ system. Since electric detonators pended. Not only must opera­ are designed to fire when electri­ tions cease, but the blasting crew cal energy is applied to them, any must evacuate the area and clear extraneous source of electric cur­ the site of personnel and equip­ rent represents a potential prema­ ment, just as if detonation were ture initiation source. Therefore, imminent. There are devices such things as lightning, radio available which are capable of transmitters, high voltage power detecting a storm's approach · lines or sources of static electrici­ before it poses a threat to the ty m~rst be avoided. Additionally, blasting crew, and emitting a any condition which would cause warning signal when a thunder­ by Larry Schneider a reduction in the applied current storm and lightning comes within may result in insufficient firing a certain range. current and create a misfire haz­ ard; any condition producing an High Voltage Power excessive current may result in Transmission Lines hangfires. High voltage power lines pre­ sent two distinct dangers to Lightnin9 blasters using electric initiation Lightning is considered to be systems. First, high voltage alter­ the greatest danger associated nating current in the power li~e with electric blasting. Nearly eve,ry has the potential to produce a ,blaster has either experienced this secondary current in the blasting personally, or knows of such circuit. This "induction" of electric instances where lightning has set current can take place in a blast­ off a blast prematurely. Lightning ing circuit without the blasting is not required to strike a blasting wires being in physical contact circuit directly; even a near miss with the energy source. In agdi­ can set off any elec­ tion, such high-voltage power tric detonators. lines may themselves suffer some (Note: lightning rep­ current leakage which can intro­ resents a hazard to ALL types of duce a "stray current" in the blasting, including non-electric ground. There is test equipment systems, while non-electric deto­ available that can be used to nators may not be as sensitive to measure any stray currents in the near misses, a direct hit by a light­ vicinity of a blasting site. Any sig­ ning bolt will almost certainly nificant source of extraneous cur­ cause them to initiate.) There are rent must be eliminated before also instances of lightning travel­ electric detonators are used. ing several miles underground A second and even more along pipes and cables, setting off direct danger to the blaster from electric detonators. high voltage lines is that of elec­ Safety regulations and common trocution. This can occur if the detonation throws the leg wires distances from the specific radio in contact with another conduc­ or lead lineup in the air and they sources. That is, if the frequency tor, or even with a conductive come into contact with overhead and the power of a radio trans­ portion of the ground. Certain high voltage lines. This type of mitter are known, this publication shales and clays are very good accident, which should be so will list a safe working distance. electrical conductors, especially easy to foresee and prevent, has A significant danger can exist when wet. Likewise, areas nevertheless occurred repeatedly when blasting near commercial around the boreholes where in past years. To avoid such an AM, FM, or TV stations because of Ammonium Nitrate-Fuel Oil has accident, the blaster should their large power output. Mobile been spilled, or any puddle of either make sure that the total radio transmitters in vehicles may water containing impurities will length of wires, (leg wires, con­ pose a danger because they may also conduct electricity. necting wire, plus firing line) is approach very near to the blast To prevent current leakage too short to hit the high voltage site. Radar is also dangerous from occurring or to minimize its lines should they be thrown in because of its highly directional impact, the following precautions the air; or the wires must be nature of its beam. can be taken: securely anchored to the ground On every new project using • All bare wire connections so that they would break before electric detonators, the blaster should be kept as far apart as they are thrown in the air and should survey the area for possi­ possible. The wire connections into any high voltage lines. If nei­ ble sources of radio energy. And should be bent in such a fash­ ther of these alternatives is feasi­ even after the blasting operation ion to keep the bare metal up ble, a non-electric system should is under way, he must remain out of dirt and water. be used. aware of the danger from radio • The number of electric detona­ sources. Precautions must be tors in any circuit should be Radio Frequency Energy taken to keep mobile transmitters kept below the recommended Transmission from any two­ away from the area, including maximum per series so that the way type of radio sends out posting the familiar road signs circuit will have power to spare energy in the form of electro­ warning everyone to "Turn Off in the event there is a small magnetic waves. In a very real Two-Way Radios." If a source of leakage of current. sense, the wires of a blasting cir­ radio frequency energy is found, • Only enough insulation should cuit can act as an antennna, pick­ and cannot be eliminated, or be removed from the wires as ing up radio frequency energy complying with the recommend­ necessary to make the connec­ and converting it to electric ener­ ed distance is not possible, then tions. gy in the blasting circuit. switching to non-electric initiation • The blaster should routinely The degree of danger that a is highly recommended. check the insulation on his fir­ radio transmitter poses depends ing cable for cuts or damage. on four criteria: (1) the frequen­ Current Leakage Any splices made to repair cy of the radio waves; (2) the A blasting machine provides breaks in a firing cable should output power (in watts) of the electrical energy in the form of be well insulated. radio transmitter; (3) the distance voltage and current applied to the from the transmitter to the blast­ cap circuit. If a part of this firing Arcing ing circuit; ( 4) the shape and the current is lost during transmission When an excessive current is orientation of the blasting circuit. through the firing cable, leg applied for an excessively long The Institute of Makers of wires, or connecting wires, it is time, an electric detonator will Explosives publishes an excellent described as current leakage. The likely malfunction by rupturing guide, entitled "Safety Guide to remaining electrical energy may before the base charge fires. If the Prevention of Radio be reduced to such a point that arcing occurs and the detonator Frequency Radiation Hazards in the current reaching the detona­ ruptures, the explosive in which the Use of Commercial tors in the circuit is below the the detonator is embedded may Detonators." Whenever any level needed to reliably fire them, catch fire. This is known as a question arises concerning safety resulting in a partial or complete "hangfire" where the explosive in, of blasting in the vicinity of a misfire. the borehole begins burning, and source of radio waves, the blaster This leakage or loss of current eventually may detonate. This should consult the IME publica­ often occurs when bare electrical detonation may take place a few tion. It provides tables listing safe connections are allowed to come seconds after the blast was fired or a few minutes later. Due to result in a partial misfire putting position to fire the shot, he alone this delayed detonation of hang­ unexploded charges in the muck should have control of the blast­ fires, no one should approach a pile. ing machine. Prior to connecting misfired charge for at least 15 Blasting machines must be the firing line to the blasting minutes after a shot is fired elec­ kept clean and in good working machine, he should make one trically. The best way to avert any order with fresh batteries in final check of resistanceand con­ danger from arcing is to use a order to provide their rated out­ tinuity of the blasting circuit. The condenser-discharge-type blast­ put. Except for replacing batter­ firing cable should not be con­ ing machine. These machines ies, a blaster should never nected to the blasting machine deliver their energy in a very attempt to work on his blasting until he is ready to detonate the brief time interval, too short for machine. If it is in need of repair, blast. Upon hearing the final arcing to occur. The use of bat­ it should be returned to the warning signal, the blaster teries, generators, or ordinary manufacturer or an authorized should wait the predetermined electric lines may produce a con­ service facility. The blaster time, connect the firing line, and tinuous high firing current and should never attempt to initiate detonate the shot. Immediately result in arcing. an electric blasting circuit with after the shot is fired, the lead anything other than a power line should be disconnected Firing the Blast source specifically designed. for from the blasting machine and After all the boreholes are electric blasting. The use of shorted.• loaded, the detonators are wired makeshift energy sources, such into a circuit to form an initiation as automobile or flashlight bat­ path. Care must be taken to see teries, is dangerous in that mis­ that all detonators are connected fires or hangfires may occur. into this circuit; omitting any will When the blaster takes his

Back to the Basics: NONELECTRIC INITIATION SHOCK TUBE SYSTEM 0/o PART 1 GENERAL DESCRIPTION

by Larry Schneider Director, Division of Explosives and Blasting Kentucky Department of Mines and Minerals

ver the past eight to ten years, will all fire within their own delay a path which transmits a "detonation there has been a significant interval. signal" from detonator to detonator. 0 growth in the number of op­ According to the ISEE's Pocket This detonation signal can be ,delayed erations using nonelectric initiation Guide to Explosives Products, there by elements inserted into the path; systems of initiation. While safety fuse are four brands of nonelectric shock and delay elements in the detonators and detonating cord have existed for tube systems currently available in also provide timing delays before fir- many years, and both are cer- ing. tainly nonelectric methods of The plastic tube is made initiation, this modern trend However, it was the shock tube's with sufficient tensile toward nonelectric initiation ability to allow the blaster to design strength and abrasion-resis­ is primarily due to the devel­ tance so it can withstand opment and the widespread a shot with nearly an unlimited the rugged conditions that acceptance of the various number of delay periods that fueled it is routinely exposed to in shock tube systems. the boreholes a1,1d 6n the Shock tube was first intro- their increasing popularity surface. Most manufactur-· duced commercially to the ers also "color-code" this mining and construction industry in the United States and Canada. These tubing as a means of differentiating the United States in the mid 1970s. At are: Austin Powder's Shock Star Sys­ between the various type of products first, these nonelectric systems were tem, Dyno Nobel's Nonel Super Sys­ available, such as surface delays and presented as a safer alternative to the tem, Ensign-Bickford's Primadet Se­ down-hole detonators. familiar electric blasting caps. Since ries, and ICI Explosive's Excel System. The tubing is sealed to prevent they were insensitive to radio fre­ While all these systems are based any contamination from entering and quency energy as well as any source upon the same fundamental prin­ desensitizing the reactive material. of static or stray electric currents that ciples as the original Nitro Nobel sys­ Likewise to prevent the tubing from are found in normal mining or con­ tem, the specific components in the malfunction, the tubing should never struction sites, they could be safely systems vary from manufacturer to be cut open, Nor should it be spliced used in areas where these hazardous manufacturer. The chief differences together to form longer or shorter conditions existed. in these various systems exist in the units unless the manufacturer pro­ However, it was the shock tube's construction of the detonator, the type vides special connections to do so. ability to allow the blaster to design a of surface delay blocks, and the type To provide for the variety of bore­ shot with nearly an unlimited num­ of connections used. hole depths and pattern spacings, · ber of delay periods that fueled their Due to various methods of connec­ the detonators and their attached increasing popularity. Shock tube and tion and the differences in recom­ shock tubes are available in a wide the various associated elements in mended procedures, it is vitally im­ variety of lengths from the manufac­ this system can be utilized in many portant that before a blaster uses any turers. different ways to produce assorted of these systems, he is thoroughly Once the tube is initiated, this reac­ delay patterns. By combining surface trained and familiar with the proper tive material "explodes" and propa­ delays and down-the-hole delay deto­ use of that particular system. gates a shock wave which travels at nators, blasts with a large number of The shock tube itself consists of a between 6,000 and 7,000 feet per sec­ boreholes can be detonated so that small diameter plastic tube coated on ond (1,800-2, 100 meters/ second) each borehole, or even each of sev­ the inside with a very thin layer of down the tube. This shock wave phe­ eral deck charges in the boreholes, reactive material. This tubing acts as nomenon inside the tube is similar in nature to a dust explosion and will propagate reliably though all but the most severe of bends, knots or kinks in the tube. While 6000 feet/ second sounds like a high velocity, in explo­ sives terms this shock wave propaga­ tion velocity is relatively slow. This means that travel time or transmis­ sion time of the shock wave must be considered when designating the de­ lay time for any units with tubing attached. For example this shock wave traveling through 44 feet (13.5 meters) of tubing takes 8 millisec­ onds. The manufacturer must and does incorporate this travel time into the delay time designations of all ele­ ments with a significant length of shock tube. Units cannot be short­ ened without affecting the delay time as listed on the component. For criti­ cal applications, the manufacturer should be consulted for advice on the actual firing times to be expected with detonators of various tubing length. Unlike detonating cord, which acts as a detonator along its entire length and has a disadvantage of being noisy, the shock tube has an extremely small quantity of reactive material in it and the shock wave is confined inside the tubing. Since the outer surface of the tubing is unaffected, there is no noise associated with the detonation of the tubing. Even though the tubing itself wave signal to a detonation. The deto­ Capers, Austin Powder Co.; Dick generates no noise, that is not true of nator contains an ignition charge, a Gotche~ Tbe Ensign-Bickford Co.; the surface delay connections. These delay charge when applicable, and a Bob Hopfer, Dyno-Nobel Inc.; and units can produce significant noise base charge of sufficient strength to David Smith, ICI Explosives USA; for unless they are well-covered with drill initiate any high explosive it contacts their assistance in reviewing this ma­ cuttings or loose earth. (see Figure 1, below). terial,· and Carlos Delgado, Kentucky Since the tube itself remains intact Dept. of Mines and Minerals, for his during initiation, obviously the shock Tbe author wishes to thank: John artwork. - LS. wave traveling inside it can have no effect on any other explosive materi­ als with which it may come in con­ tact. Likewise, this shock wave can­ not be transmitted from one tube to another by merely tying them to­ gether. These two characteristics mark the most obvious contrast with deto­ nating cord used as trunklines or downlines. Since the tubing will not initiate high explosives, a detonator or cap must be used. This detonator is factory-assembled to the shock tube and the elements inside the detonator convert the shock BACK TO THE BASICS NONELECTRIC INITIATION SHOCK TUBE SYSTEMS, PART 2: GENERAL APPUCATIONS

by Larry Schneider

n the simplest application of a shock tube initiation system, the I tubing acts as a "relay line" which passes a detonation signal from bore­ hole to borehole. When the signal arrives at each borehole, it causes the detonator attached to it to fire. Such a single path constitutes a progressive series where each unit must function properly in order to activate the fol­ lowing units. Any application error, malfunction, or mistake in connect­ ing any surface unit will stop the pro­ gression of the detonation signal Figure 1. A single row of bore boles with 1 7 millisecond surface delay through the system and result in a connectors inserted between each. "cutoff" or misfire of the following charges. One of the most basic methods used to create timing delays in such shock tube systems is by means of surface delay connectors. These connectors detain the firing signal for a preset length of time as it travels from one borehole to the next. A second method of creating timing delays is through the use of in-hole delay deto­ nators. These have a pyrotechnic de­ lay element built into the detonator, Figure 2. If an in-hole delay is added to each hole in the samepattem as which produces a specific time delay Figure 1 and shown here, the actualfiring time of the charges will be the between the time the detonator re­ sum of the in-hole delay time plus the time it takes for the firing signal to ceives the initiation signal and the reach the detonator in the borehole. time the base charge in the detonator explodes. ferences, it is vitally important that lay connectors inserted between each. In most practical instances, blasts the blasters be thoroughly trained and As the detonation signal travels down are designed with both surface and familiar with the proper use of the the tubing from the point of initiation in-hole delays in order to produce an system that they will be using. The to each borehole, it is delayed by effective delay pattern. following patterns are described each surface connector. Assuming the As emphasized in Part 1 Ouly j Au­ merely as examples of the fundamen­ in-hole detonators have no built-in gust, 1995) of this series, the various tal principles of shock tube patterns delays, the explosive charges in the manufacturers of the shock tube sys­ and are in no way recommended as borehole will fire at the instant that tems have different components with patterns to be used in actual blasting the detonation signal from the tubing different time delays, methods of con­ with specific systems. reaches the detonator in the bore­ necting, and recommended proce­ Figure 1 shows a single row of bore­ hole. While the example uses a 17 dures and patterns. Due to these dif- holes with 17 millisecond surface de- millisecond surface delay connector, other values are available depending on the manufacturer; such as 25 ms, 42 ms, or 100 ms, etc. If an in-hole delay is added to each hole in the same pattern as Figure 1 and shown in Figure 2, the actual firing time of the charges will be the sum of the in-hole delay time plus the time it takes for the firing signal to reach the detonator in the borehole. If the same in-hole delay period is used in every hole, the duration of the blast is exactly the same as if there were no in-hole delay, though Figure 3. The boreholes are connected with 1 7 millisecond surface delay its starting time will be set back by connectors, and each borehole has a 200 millisecond in-bole delay. the value of the in-hole delay. While the previous patterns showed actual detonation time of the charge changing the progression of the sur­ a single row of only 5 boreholes, this in that borehole. face delays and/ or the point of initia­ pattern and method of delaying the Another significant advantage is tion. Since this is done by placement individual charges could be extended gained by placing a delay in the bore­ of the surface elements, it is possible indefinitely in length and every charge hole. With the in-hole delay, the sig­ to change the direction and move­ would be separated by 17 millisec­ nal, traveling in the tubing on the ment of the rock even after the bore­ onds. surface, has time to move away from holes have been loaded and stemmed To provide a continuous progres­ the vicinity of the rock that may be by rearranging the surface delays. sion through the pattern as well as disrupted by the detonation of the A simple illustration of changing initiate the charges in the boreholes, charge in the borehole. This reduces the rock movement by altering the each element in the "hookup" must the possibility of cutoffs of the deto­ point of initiation in the pattern is initiate at least two following units. nation signal due to ground move­ shown by comparing Figure 4 to Fig­ The shock tubing, carrying the in­ ment breaking the tubing. In the ex­ ure 3. By adding 42 ms surface con- coming detonation signal must be at­ ample shown in tached to an in-hole detonator in or­ Figure 3, the actual der to transmit the signal down the signal in the tub­ borehole to the detonator embedded ing is located at a Packaging for the in an explosive charge. It must also point between the be connected to a second unit, a sur­ twelfth and thir­ face connection which will continue teenth hole when Explosives Industry to carry the detonation signal to the the first charge next borehole. detonates. There- By combining surface delays and fore, it is well in-hole delays with different values, away from any the blaster can create a variety of ground movement blast timing patterns. In the pattern in due to the detona- Figure 3, the boreholes are connected tion of the first with 17 millisecond surface delay con­ charge. In fact, be- nectors, and each borehole has a 200 fore the third millisecond in-hole delay. When the charge fires (234 initiation is started at the first hole on ms), the shock the left in the diagram, the detona­ tube signal has tion signal travels through the system reached the last and each hole receives the firing sig­ hole (221 ms) and nal 17 milliseconds after the previous completed its ini- one. In this example, the tubing is tiation sequence. alternately connected from front to When the same back row to create an echelon-type in-hole delay is delay pattern. The top number in each loaded in each circle representing the borehole is borehole of a shot, the time in milliseconds when the the blaster can di­ firing signal reaches the borehole, the rect the movement bottom number in the circle is the of the rock by mended by an explosive manufac­ turer, the blaster should sit down with a pencil and paper and calcu­ late exactly when each charge is de­ signed to fire. His review of the fir­ ing times should determine whether or not there is an excess number of charges firing within an individual delay interval.

Figure 4. A simple illustration of changing the rock movement ~y altering the point of initiation in the pattern is shown by comparing this figure to Figure 3.

Figure 5. When decking is necessary in a blast design, it can be accomplished by placing two different in-hole delays in each borehole, and initiating them with the same incoming signal. nectars to the right side of the pattern more than three rows are used, over­ and initiating the center of the blast lapping delay times will occur. first, the rock movement is directed By use of these principles, blasts from both ends of the pattern toward with hundreds of charges can be deto­ the middle. nated, each in their own individual When decking is necessary in a delay interval. Such delay intervals blast design, it can be accomplished are usually required to have at least 8 by placing two different in-hole de­ milliseconds between the charges in lays in each borehole, and initiating order to comply with blasting regula­ them with the same incoming signal. tions. However, while it is theoreti­ Figure 5 shows such a blast pattern. cally possible for some delay blast Each borehole has three separate patterns to continue with no limit on deck charges, with in-hole delays of the number of holes, many multi-row 150 ms, 175 ms, and 200 ms, and are patterns are often limited in their connected with surface delays of 17 scope. When delay blasting with these ms and 75 ms. The numbers in the non-electric delay elements, the circles representing the boreholes in­ blaster must select the proper combi­ dicate the firing times of the three nation of delays and pattern and decks placed in the boreholes. A pat­ check the progression of the detona­ tern such as the one shown can be tion carefully before firing the shot. extended indefinitely in length, but if Even before using a pattern recom- sives per yard3 of rock) that was too large for the type of rock being shot. A powder factor that is too high can cause enormous movement of the rock. And an excessively large powder factor can actually occur due to any of several of the causes described in the list. For instance, if the burden distance is too small for the borehole diameter, or if not enough stemming In part one of this series on flyrock seven com­ is loaded in the borehole, those factors also result in mon causes of flyrock were listed. The first cause an excessive powder factor and an overloaded blast. was described as an excessive amount of explosives. This may result from poor design when planning When you hear that description, the implication is the burden or spacing. Or sometimes, poor execu­ that the blaster loaded "TOO MUCH EXPLOSIVES IN tion in the field occurs when trying to implement a THE HOLE." In fact, this is what the public often good blast design. For instance, a blaster may load a accuses blasters of, whenever they have a complaint hole where the powder factor has been calculated about the ground vibration, noise or flyrock; that is, correctly for a given burden, spacing and depth, but the blaster deliberately put too much powder in the the boreholes were not drilled accurately according hole. As the blasters themselves are well aware, that to that design. The blaster must also be aware of explanation is too simplistic because there is only a what the true burden is on each borehole, and certain amount of explosives that will fit into a bore­ whether or not that burden is consistent through the hole of a particular diameter and depth. length of the borehole. Note that the true burden is Furthermore, due to the economics of blasting, the determined by the delay timing sequence as well as idea is certainly not to use any more explosives than the blast geometry. necessary. If we look at some of the following diagrams, we So the statement that there was an excessive can see in some cases clearly where the burden turns charge in a blast usually means that there may be out to be less than that which the blaster expected. some other problems, such as the blast was loaded In figure 1, the bench has a good vertical free face, with a powder factor ( as measured in lb of explo- but the drilling was done poorly. If the blaster pro- ceeds to load the borehole for the have an average powder factor of holes are "buffered" in front of 12 foot burden he has at the top, 1.00 lb/yd3 for the borehole. If, the shot, and the blaster drills he will be overloading the bot­ however, the driller errs by 5 near the free face to ensure that tom of the hole where he has degrees from the vertical, his bur­ he "pulls the toe." The profile only 8.5 ft. An inaccuracy such as den at the bottom is only 8.5 ft., may look like the one shown in this, when the drill is off the ver­ and his average burden through­ figure 3. The first row of hole is tical, will be magnified the deep­ out the borehole would be 10.25 drilled close to the crest since er the borehole is drilled. As an feet. His actual powder factor for there is a large buffer that reach­ example, a borehole 40 feet the entire hole will be 1.16lb/yd. es nearly to the top of the deep, which is drilled 5 degrees Even more striking, is the fact bench. Subsequent rows of from the vertical results in an that while the powder factor for holes are set back a reasonable error of 3.5 feet at the bottom of the top one foot of the powder distance, and the blaster the hole. The same borehole column is 1.29, the powder factor assumes that the buffer will con­ drilled 100 feet deep with the at the bottom of the borehole is fine the blast on the outside same 5 degree error, will be off 1. 75 lb/yd, an increase of more boreholes. And it may work as by nearly 9 feet at the bottom of than 35o/o. planned provided the powder the hole. Even in the first Figure 2 shows a perfectly column is not loaded too high, instance, where the burden was drilled vertical hole on a level so that the top portion of the off by 3.5 feet, the powder factor bench, but due to the equipment charge is not near or above the may vary by 20o/o from top to bot­ excavating material at the front buffer. If a charge is loaded such tom of the borehole. of the bench, the blaster may that the charge is unconfined in As an example of this type of encounter a bench with a profile this top portion, it can throw error, consider a blaster who like this. In this example, if the rocks over a long distance. intends to drill a 12 x 15 pattern, blaster does not check the shape Accuracy in drilling is 40 feet deep, using 8 ft of stem­ of the free face, he may load the extremely important when ming in a 51/2 inch diameter, and shot for a 15 ft burden and only angled holes are used. The loads it with a loading factor of have 8 feet of rock confining his setup distance from the face, the 8.25 pounds of ANFO per foot. explosive charge. angle of the drill must be mea­ With this pattern, he expects to A similar instance is where the sured carefully to ensure that the proper burden exists at the toe. Failure to do so can result in charges that are placed too close to the face with less con­ finement than intended. If explosives are loaded into faults, crevices, or mud seams that the borehole passes through, the blaster should expect excessive flyrock. The driller has a cmcial role to play in the preparation of the blast. Information about any unusual conditions such as mud seams or voids encountered in the borehole must be recorded or logged, and the blaster must rely on his driller to communi­ cate any unusual conditions of der column in the tries to blast an excavation to borehole using a grade that is only 8 feet deep. tape or other He may be conscientious method. enough to realize that he only Adequate needs 2 feet of powder in a stemming is borehole drilled on a 12 x 14 required to con­ feet pattern, 8 feet deep to get a fine the high pres­ proper powder factor. However, sure gases such a design produces a bur­ released by the den greater than depth, so that detonation of the the nearest direction for relief is explosive charge. the surface, and rock may be This stemming thrown straight up into the air. must be sufficient Certain operations use blast­ to prevent the ing patterns where the burdens force of these are sometimes close to, or slight­ the boreholes he has drilled. The gases from violently cratering to ly greater than the depth. One driller must be constantly aware the surface. The explosive force of these are the "parting shots" of what is happening as he drills, will always be in the direction of in surface coal mines, where the so he will know if he encounters least resistance, which should be blaster has to break a layer of a significant mud seam or a void the burden distance to the free rock that lies between two coal such as a cavern or an aban­ face at the instant of detonation. seams, this strata of rock may be doned underground mine. However, if the stemming lengthis anywhere from 8 to 20 feet thick If explosives are loaded into a too small, the most direct path for and the blaster may have to use mud seam, the mud does not the gas pressure to vent may be to a large diameter drill with a 12 have sufficient resistance to con­ the surface. In minor occurrences, x 12 pattern. It can result in a fine the energy of the detonation. this results in stemming material great deal of material thrown, As long as the blaster knows in being launched away from the but usually such blasts are a advance that a mud seam inter­ blast. More serious cases occur large distance from houses, and sects the borehole, he can take when material around the collar of roads. steps to eliminate the danger. the borehole can be thrown signif­ Occasionally blasters in quar­ This is normally done by stem­ icant distances. ries are required to shoot "toe ming through the mud with inert Adequate stemming does not shots" where they come back material to make two separate only mean the length of the stem­ after the main blasts have failed charges in the borehole. A cardi­ ming, but also the composition of to pull to grade. On these shots, nal rule for a blaster is that the material used as stemming. the boreholes may be drilled in explosives should never be Drill cuttings or dust are usually a an irregular pattern and very loaded into anything except very poor stemming material, par­ shallow. When they are fired, competent rock. ticularly if the borehole is water­ flyrock is a definite possibility. Likewise, if a borehole is filled. Crushed stone of a suitable So called "lift shots" or any shots drilled so that it cuts into a cav­ size so that it interlocks in the fired to begin an excavation are ern or underground mine, the borehole is a recommended stem­ also prone to generate flyrock. blaster may load huge amounts ming material on most operations. These shots have no free face of explosives, and if detonated Another potential problem and are intended to cause the will create a violent result. If the occurs if a blast pattern is drilled rock to fragment and swell. blaster is careless while loading such that the spacing and burden However, if they are not careful­ the borehole, especially when is greater than the borehole depth. ly loaded and controlled, the using bulk explosives, he may In this situation, the blaster should fragmented rock again is not detect the fact that a large expect flyrock. Blasters occasional­ launched upward. On smaller amount of explosives are going ly drill this type of pattern without scale blasts, such as those for into the borehole. It is always realizing that they are creating a setting electric utility poles, the imp01tant that a blaster constant­ potential hazard. For example, a use of "burn or cut" holes serve ly monitors the rise of the pow- blaster using a 6 3/4 inch drill bit to provide a place for the bro- results in the of the causes of flyrock men­ same effect. In tioned previously can be predict­ this case, a mis­ ed and avoided, it is always pos­ take has sible that there exists some occurred and a unknown condition that will #10 delay is mis­ cause flyrock. For this reason placed in the the blaster must always assum~ shot pattern. the worst possibility and formu­ When this shot is late his plans to clear the area for detonated, the this case. Regardless of how charge in that many "well-behaved" shots he hole will fire has previously detonated, the with no relief at next detonation is always the all and the only most dangerous. possible move- It is the ultimate responsibility ment of the rock of every blaster to oversee the ken rock to move and help con­ around it will be vertical. A blaster safety and well-being of anyone trol the upward throw. On would be incompetent if he laid that could be effected by the these shots, blasting mats may out a shot like thaton purpose, and blast. They must accept this be useful, but a good blast at the very least, careless if he did responsibility and do whatever it design is still critical. it by accident. However, with com­ takes to protect the public, their Poor delay timing can gener­ plex timing layouts, both electric co-workers, and themselves. ate flyrock. Faulty delay time and non-electric, any error in the While the explosive products may be the result of bad design, connections can result in holes fir­ we use today are the safest, most but could also be caused by ing out of sequence. The timing reliable ever produced, they are inaccurate detonator firing time, mistake may not be as blatant as only as safe as the individuals or simply human error in laying the one illustrated to have the same using them. Training and educa­ out the delay detonators. The effect. tion in all aspects of blast safety timing on all shots must allow Regulations usually require that must always take high priority in enough time for the rock to when blasting is done in proximity our industry, as well as account­ move so the muck does not pile to buildings and roads where no ability and responsibility in up in front and prevent the amount of thrown material can be implementing the procedures intended horizontal movement tolerated, blasting mats or other that are learned .• of subsequent charges. Figure 4 protective materials should be used shows the effect that takes place to confine any possible flyrock. But as the detonation proceeds even in those cases, the best deeper into the bench if there is means of controlling flyrock is still insufficient time for the rock in through proper blast front to move. That is, the design and delay movement of the rock from timing. It is therefore each borehole progressively extremely important moves upward at an increased that when a blaster angle until the back rows working with a suc­ become almost a lift shot and cessful blasting pro­ the rock movement is close to gram makes any vertical. Normally this can be change in the bla·st somewhat corrected by provid­ design, he must care­ ing longer delays between the fully consider any rows as the shot progresses changes from the deeper into the pattern. standpoint of its Figure 5 shows a delay pat­ potential effect on tern which illustrates a very sim­ flyrock. While many ple type of timing delay that

BACK TO THE BASICS Nonelectric Initiation Shock Tube Systems Part 3: Safety Considerations

by Larry Schneider

ne of the incentives that just as the leg wires are in electric The blaster should follow the led to the development of caps. manufacturer's directions closely in 0 non-electric shock tube These products are explosives order to prevent this flying debris initiation systems was the desire and as one major manufacturer from causing cutoffs or damaging within the industry to improve states in their disclaimer: "Use of other components in the initiation detonator safety. And to the extent these products by anyone who system. Often this entails simpl.:x that nonelectric initiators are im­ lacks adequate training, experi­ covering these surface units with.· mune to the hazards of stray cur­ ence, and supervision may KILL or adequate dirt or drill cuttings so as rent and radio frequency energy INJURE." to confine the shrapnel. found on normal blast sites, an ad­ The need for suitable training and Due to some of the reasons men­ vancement in safety has been ac­ experience with nonelectric initia­ tioned previously, there are a num­ complished. However, as a caution­ tors is especially true because of ber of operations that have ary note, it is important to realize the wide variety of systems avail­ switched from electric blasting to that there are still many hazards able and the procedures and meth­ nonelectric initiation, and have associated with any initiation sys­ ods unique to each one. The fact found that misfires have become tem. While some of these hazards that a blaster is knowledgeable more frequent. And, unfortunately, may differ from those associated about the particular nonelectric sys­ some misguided blasters may treat with electric detonators, others are tem he is using does not necessar­ these misfires as a routine prob­ identical to those associated with ily qualify him with any other sys­ lem. Such blasters may have a ten­ any type of initiation system. For tem. dency to become complacent with instance, contrary to what some un­ Likewise, the various systems are misfires using nonelectric systems informed blasters may believe, not compatible with one another because when such misfires are due lightning remains a hazard when and components from different to an application error on the sur­ using nonelectric initiation as well types of systems should never be face, they usually can simply re­ as electric initiation. A direct light­ mixed in a single blast unless spe­ connect the charges and refire the ning strike will provide enough cifically approved by the manufac­ shot. However, misfires can have energy to detonate shock tube, and turers. very serious consequences and it this has been clearly demonstrated Particular care must be exercised is important that the blaster ·be. in a number of cases on actual during the hookup procedures, aware of those consequences. Re- blasting sites. The rules regarding since any error in making surface gardless of the type of detonators blasting and thunderstorms remain connections will result in a "cut­ used, the blaster should take all the same regardless of the type of o££." Since the only checks for con­ steps possible to eliminate the initiation used-that is, whenever tinuity of these nonelectric "hook­ chance of misfires. an electric storm approaches, blast­ ups" is through visual inspection, On a blast site, care must be taken ing operations must cease and the the success of the nonelectric ini­ so that vehicles, such as bulk area must be cleared. tiation system depends heavily on trucks, do not drive over the tub­ Likewise, since the detonator at­ the blaster's qualifications and skill. ing, connectors, or any surface tached to the end of the tubing Additionally, it is vital that all the components. At a minimum, such contains sensitive ignition and base hookup work is done in a system­ action can damage the shock tub­ charges, it is susceptible to heat atic and orderly fashion. As noted ing and may result in a misfire. At and impact and must be protected earlier, omitting a charge or failing worst, it can result in a premature from abuse just as any other type to make a connection will result in detonation causing injury or death. of detonator. The plastic tubes a misfire, and handling misfires is There has been at least one recent which carry the firing signal are dangerous. serious accident that occurred as a subject to damage from equipment Some surface delay components result of a blaster driving over and mishandling during loading produce metal and plastic shrap­ shock tube on the surface at a blast nel when detonated. site causing the explosive charge in the borehole to detonate prema­ signed; attempts to modify or alter through double firing loops or turely. any of these parts should never be paths. Furthermore, the plastic tub­ When the blaster prepares to con­ attempted. If any components are ing on the surface is essentially nect the nonelectric system, no found to be defective, damaged, or noiseless when contrasted to the tools should ever be used to pry incompatible, they should not be older detonating cord trunklines. on any component containing a used, but rather returned to the Despite all these improvements, detonator. Nor should any tool be manufacturer. these materials are detonators and used to open, close, fasten, or clean In the past few years, nonelec­ must be handled with the same care out any connector containing a tric systems have been refined by and respect as any explosive. Prod­ detonator or detonating device un­ the explosive manufacturers to im­ ucts which are designed to deto­ less specifically designed for that prove the simplicity of the hook­ nate can never be treated casually. purpose by the manufacturer of the ups, increase the tensile strength They will always represent a po­ detonating system. Components of of the tubing, reduce the necessity tential hazard that must be care­ these initiation systems should be of maintaining large inventories of fully handled and closely con­ used as originally manufactured different delays, and provide some trolled. and for the purpose they were de- measure of misfire prevention >JEE< much harder and has a higher density. Additionally there are different types and amounts of coatings used on the blasting grade prills to add a certain amount of water resistance, thereby preventing cak­ ing and allowing them to flow freely. The caking of this prilled material is due to the AN prill's tenden­ cy to absorb moisture from its environment. Without such a coating, AN will attract moisture from the humidity of the air and in effect dissolve itself. Information from the U. S. Department of Interior showed that in 1993, ANFO blasting agents and unprocessed ammonium nitrate accounted for 83% of the total industrial explosives sold for consump- tion in the United States. Of the more than 800 mil­ lion pounds of explosives used annually in the state of Kentucky, over 90% of it is ANFO which is deto­ nated primarily in the surface coal mines. As referenced earlier, the reasons for ANFO's widespread use are twofold:

1. It is safer than high explosives to handle, trans­ port, and store. And because of its reducecj. sen­ sitivity, regulations governing the storage and transportation of ANFO are not as stringent as Since its development in the mid 1950's, ANFO, those governing high explosives. which is a mixture of 94% Ammonium Nitrate and 2. It is economical, beginning with a much lower 6% Fuel Oil, has become the explosive of choice for cost per pound than any other explosives. In many blasters working on large scale blasting oper­ addition to its low cost, it has good rock break­ ations. The stability and economy of this blasting ing ability with a relatively high detonation veloc­ agent has brought it to the forefront in both mining ity in large diameter boreholes, and large gas pro­ and heavy construction. Additionally, the advantages duction during detonation. Under many condi­ of the bulk mLxing of the ammonium nitrate prills tions, its performance/ cost ratio makes other and liquid fuel oil on site, and loading directly into explosive materials uncompetitive. the boreholes, provides safer handling and quicker loading. However, its performance as an explosive does While ammonium nitrate (AN) is also used exten­ vary based on a number of factors, some of which sively in the formulation of some high explosives, are controllable, while others are not. These factors emulsions, and water gels, the discussion in this arti­ are: cle centers on its most common form, that of a prill to be mixed with fuel oil and used as a blasting 1. The physical characteristics of the prills, such as agent. These "prills" are porous, spherical shaped size, shape, porosity and density. pellets, typically between 6 and 20 mesh U.S. stan­ 2. The distribution and proportion of the fuel oil dard screen size. The porosity of the prills is care­ when mixed with the prills. fully controlled during the manufacture of the AN 3. The diameter of the borehole and degree of con­ prills to ensure that they will absorb the correct finement. amount of fuel oil. This porosity is one difference 4. The size and type of primer and the priming pro­ between the blasting grade AN prill and the more cedure used. common fertilizer grade AN used in agriculture. 5. The exposure to water in the boreholes. Another distinction is that the agricultural prill is When the performance of the explosive is nega­ the plant. tively affected, the blaster is very likely to know by The detonation velocity of ANFO is determined observation of orange-brown smoke generated by both the borehole diameter and the degree of during the detonation. A blast which exhibits large confinement. As can be seen from Figure 1, the quantities of such smoke or fumes may have been detonation velocity is highly dependent upon bore­ rendered inefficient for any one of the reasons in hole diameter. While crushed prills can be detonat­ the list above. ed in boreholes as small as 1 inch in diameter, the The initial physical characteristics of the AN priUs normal ANFO prill requires a borehole diameter of are determined during the manufacturing process, 2 inches or greater to reliably detonate.As the bore­ and the blasters must rely on their suppliers for hole diameter increases to about 10 inches, the det­ good product. However, once the prills are on site, onation velocity also increases until it is approxi­ they are subject to a phenomena called cycling. mately 14,700 feet per second ( 4480 meters/sec­ Cycling is a process whereby AN undergoes a ond). Any increase in borehole dimensions beyond change in its crystal structure in response to a this, has little or no effect on the detonation veloc­ change in temperature. There are two temperatures, ity. at which AN goes through this cycling, 0° F and 90° Likewise the confinement of the explosive dur­ F ( -18° C and 32° C). Both of these temperatures ing detonation is crucial in maintaining the detona­ are readily reached by products stored in outdoor tion velocity. Any premature loss of pressure in the magazines during the winter and summer in most borehole, due to rock movement or venting, will areas. The actual "cycling" occurs when the tem­ result in a decrease in detonation velocity. Most perature fluctuates above and below these critical blasters feel that the detonation velocity is one of temperatures. the most important considerations in selecting an The effect of this change in crystalline structure explosive. This is especially true when trying to is to break the prills down into smaller and smaller fragment hard, massive rock strata. particles. This increases the density of the ANFO, By its definition as a blasting agent, ANFO is not and also diminishes the prills' water resistance, "cap sensitive" and therefore requires a high explo­ which is supplied by its coating. Over time, this will sive "primer" to initiate it. Such a primer must sup­ result in the AN drawing moisture from the humid­ ply adequate energy, both as shock and pressure, ity in the air, dissolving and reforming as larger crys­ to reliably initiate the blasting agent. An efficient tals. Significant caking may occur in the bagsor bins primer will force the blasting agent to reach its if it is stored for long periods of time as the tem­ steady state velocity in as short a time as possible. perature fluctuates around 0° or 90°. ANFO that has Steady state velocity refers to the detonation veloc­ "caked" will detonate unreliably or inefficiently, and ity at which an explosive or blasting agent typical­ will present problems when loading. ly detonates. After the primer is fired, a certain time The energy released by the detonation of ANFO passes, during which the detonation travels through is markedly effected by the percentage of fuel oil in a distance in the borehole, before the blasting agent the mixture. The energy release of ANFO is maxi­ reaches this steady state velocity. In this area of the mized when fuel oil content is 5.7 % by weight. borehole, which can extend from the primer to a Since the prill is porous, voids in it to allow it to distance of several borehole diameters, the detona­ readily absorb that amount of fuel oil. To further tion wave will travel at a "transient velocity." This ensure the performance of ANFO, it is critical that transient velocity may be significantly less than its the right amount of fuel oil be added uniformly and steady state velocity, and will represent a loss of distributed throughout the mixture, regardless of energy. whether it is done on site, in the bulk trucks or at While the final steady state velocity is not affect­ sive means that it will have a low "loading factor", ed by the primer, the time and distance required to which is the pounds of explosives that can be reach it depends on the primer and the priming pro­ loaded per foot of borehole. In operations where the cedure. Figure 2 is a chart of detonation velocity blaster desires to maximize the explosive charge in versus distance from the primer for three different the powder column, a low loading factor does cre­ types of primers. ate a problem. Line 1 represents a 1 pound cast primer in a ten However, in other cases, blasters may wish to get inch diameter borehole, which produces steady state more widespread distribution of the explosive velocity in 3 charge diameters, or approximately 30 throughout the boreholeand the blast pattern. Alow inches. In deep boreholes with a long powder col­ density explosive often times, will accommodate this umn, such priming would probably be satisfactory much better than a high density one. Therefore, the unless the toe was very difficult to pull. However, in density and loading factor of ANFO may be either an shallow boreholes, or boreholes loaded with a rela­ advantage or disadvantage, depending upon the tively short powder column, this 30 inch region of specific blast design. It is one of the basic charac­ lower velocity may represent a significant loss of teristics of an explosive that the blaster must consid­ energy. For example, in a 20 foot powder column, er when selecting which explosive to use. It may nearly 12. 5 % of the powder is detonating below also determine the number and size of the boreholes steady state velocity. that need to be drilled and loaded. Line 2 represents a high energy primer, such as Finally, there are several other points to consider a combination of a cast primer imbedded in several when priming ANFO, such as: feet of water gel in the borehole. This will cause the ANFO column to reach steady state of velocity in 1. The detonation pressure of the high explosive, approximately one charge diameter or 10 inches. used as a primer, is as important as its detonation This method is generally accepted as the most effec­ velocity when determining in its effectiveness as tive method to prime ANFO in order to release the a primer. At least one explosive manufacturer rec­ maximum amount of useful energy. ommends a primer with a detonation pressure of Line 3 shows a condition known as overdrive, at least 80 kilobars. which normally does not occur in actual field blast­ 2. The diameter of a primer should approximately ing: However, test shots, with extremely high ener­ match the borehole diameter so that it produces a gy primers, have shown these results. The ANFO stable, flat pressure wave entering into the ANFO. detonates at a higher velocity for a brief time before decreasing to its steady state velocity. This curve is 3. Multiple primers may be needed in very deep illustrated to disprove the notion that the use of an boreholes, especially in holes where the geology . extremely energetic primer could cause the ultimate or burden dimension may cause rock movement steady state detonation velocity of ANFO to be that could vent the borehole pressure before the increased. powder column completely detonates. Multiple Any discussion of ANFO as a blasting agent must primers may also be appropriate in boreholes note its one major drawback, that is, its lack of water with multiple zones of difficult breakage. It is gen­ resistance. If ANFO is exposed to water in the bore­ erally accepted practice that the primer shouldbe hole for any length of time, it will show a decrease placed in the area of most difficult breakage. in detonation velocity. The greater the amount of 4. Care must be taken when using large detonating water, the ANFO is subjected to, and the longer the cord downlines in small boreholes with ANFO. time of exposure, the worse the performance of the This may have a detrimental effect on the ANFO explosive becomes. Tests have shown that a water by desensitizing it or even causing low order det­ content of 10% can result in detonation failure, even onation. after only a few minutes exposure. It is essential that either the ANFO be used only in dry conditions, or Over the past forty years, ANFO has retained its steps be taken to keep the blasting agent dry. These leading position in the explosive market, in spite of can include dewatering of the boreholes, or using the introduction of m~ny newer products and blast­ plastic borehole liners. ing techniques. And it is very likely that ANFO will One physical property, that is sometimes consid­ continue for the foreseeable future to remain the ered as a disadvantage to ANFO, is its density. It has standard, against which all performance versus cost a relatively low density or specific gravity, typically considerations will be compared.• from 0.82 glee to .88 glee. Low density of an explo-

On the top right side of the chart, High Explosives and Blasting Agents are separate classes of explo­ sives. The difference between these is their dissim­ ilar sensitivity to a blasting cap. A blasting agent will not initiate when placed in contact with a detonator (blasting cap) and the detonator is fired. Blasting agents require more energy to initiate, normally a primer cartridge of high explosive must be detonat­ ed in contact with the blasting agent. To initiate a high explosive, requires only a detonator fired in contact with it. The decreased sensitivity of a blasting agent offers several obvious safety advantages when blasting agents are being transported, stored, and/or han­ dled. Indeed, a number of regulatory agencies rec­ ognize that the stability of blasting agents require less stringent standards for the storage and trans­ portation of blasting agents as compared to those for high explosives. When selecting an explosive to be used on a par­ ticular job, the blaster must know the conditions that he will be working under, and must choose a type of explosives which are most appropriate for those circumstances. Economic considerations will always A fundamental responsibility of blasters, often enter into the selection of materials, including explo­ defined in regulations as well as assigned to him by sives, to be used on any project. However, the their employer, is to choose which type of explosive blaster must insist on explosives ·of a type that is to use on a particular job. In order to make an compatible with the job conditions and equipment informed choice, they must be familiar with more he is using. than just the brand name on the explosive, they Cost should never take priority over proper appli­ must also recognize the various classes of and types cation of explosives, particularly when safety may be of explosives, and their characteristics. The chart on affected. It is also important to note that the priceof page 29 illustrates an overall summary of the classi­ the explosives is only one factor to consider in eval­ fication of explosives, and contains a very brief uating the cost of blasting. Inexpensive explosives description of each type basedupon its composition that do not serve the purpose for which they are and properties. intended are no bargain. High energy explosives At the top left hand side of the chart, low explo­ may have a higher initial price, but savings in sives are separated from all other types of explo­ drilling, loading, and mucking the rock may offset sives by the fact that a low explosive does not gen­ this cost. A thorough study of the overall costs erate a shock wave when initiated. The reaction in involved in a blasting program is essential before a a low explosive, such as black powder, is properly judgement can be made on the economic benefits of called a deflagration, which is a rapid burning that any particular explosive. produces large quantities of gases. A detonation, on To ensure that he selects the proper explosive, the other hand, produces a large amount of high the blaster must have a basic understanding of the temperature and high pressure gases, but also is properties and characteristics of explosives. These accompanied by a shock wave. The existenceof this are: shock wave is the primary distinguishing character­ 1. Density istic between high and low explosives. 2. Water Resistance 3. Detonation Velocity DESCRIPTIVE CLASSIFICATION OF EXPLOSIVES

LOW EXPLOSIVES Deflagrate rather than detonate BLASTING AGENTS highly namable, very sensitive HIGH EXPLOSIVES Combination of an oxidizer most common example: Black Can be detonated by a and a fuel, cannot be Powder blasting cap alone. detonated by a cap alone.

BINARY GELATIN DYNAMITES SEMI-GELATIN BULK MIXED (Two component) Each COMPOUNDS PRE-MIXED NCN Combination of Combination of Nlbocarbonitrate mixture DYNAMITES nitroglycerine and ammonium gelatin and individual component is Ammonium Nitrate and Composed of non-explosive, can be prepared by nitrocellulose to fonn a ammonium dynamite. Fuel Oil can be mixed on manufachJrer. Low nitroglycerine and tiller rubber-like consistancy. Lower strength than shipped by any method. site by the user. Very materials. Beoomes a high economical and efficient If density, no water Excellant water gelatin dynamite. Good resistance. Commonly resistance. water resistance. explosive only when mixed and used property. mixed. No water resistance. packaged in 50 pound Fume production papel' bags and poured dependant on conditions by hand into the used. boreholes.

STRAIGKT DYNAMITE Nitroglycerine Is only STRAIGHT GELATIN expk)sive material very No Ammonulm Nitrate in sensitive commercial mix. Excellent water explosive. High sensitivit resistance. WATER GELS, SLURRIES, and sensftiveness. May Very cohesive. High EMULSIONS produce large quantity of detonation velocity. Can be classed as a high explosive or fumes. blasting agent depending on their cap sensitivity and composition. Mlxturas contain oxidizer, fuel, and sensttizer. Available in large range of densities, packaged In polyethylene b.Jblng for cartridge use. Also available for bulk AMMONIUM DYNAMITE use. Excellent water resistance. Water Ammonium nitrate SPECIAL GELATIN gels contain significant quantity of water substituted for some of Some nitroglycerine the nitroglycerine. Lower replaceD by ammonium (~J8d~~~~~~1n:r;1~o;~:s density and less resistant nHrate. which are surrounded by an oil forming to water. Moderate Good fume class. an oil in water mixture and an emulsifying detonation velocity Less resistant to water. agent Is added to prevent separation. (6-11,000 fps).

4. Strength case) to the mass or weight of an equal volume of 5. Fume Characteristic water. Materials with specific gravities less than 1.00 6. Sensitivity are lighter than water; materials with specific gravi­ 7. Sensitiveness ties greater that 1.00 are heavier than water. Therefore, an explosive used in a water-filled bore­ When loading a borehole, the density of the hole should have a specific gravity greater than explosive will determine how many pounds of 1.00. explosives can be loaded into a certain size bore­ The water resistance of an explosive is also an hole. A dense explosive is one which is concentrat­ important property to be considered in such a case. ed and will enable the blaster to load more explo­ It is a measure of how much exposure to water, an sives per foot of borehole than a low density one. explosive can withstand before becoming desensi­ ANFO, the most common explosive used, has a rel­ tized or at least before losing some effectiveness. atively low density. However, it is possible to blend This property is generally specified in qualitative ANFO and emulsions in a bulk truck, which gives terms such as good, fair, poor, etc. Emulsions, slur­ the blaster the ability to increase the density of the ries, gelatins, and high density explosives are said blasting agent, or even vary the density loaded to have "good to excellent" water resistance. Low within a single borehole. density, porous explosives, such as ammonia dyna­ When loading in boreholes containing water, it is mites usually have poor water resistance. ANFO important to use an explosive with a density greater loaded in bulk has extremely poor water resistance. than the density of water. If not, the explosive will The detonation velocity of an explosive is con­ float, and the charge column can become separat­ sidered by many blasters to be a very important ed, resulting in a misfire. Density is measured in indicator of the rock-fragmenting ability of an terms of specific gravity, which is the ratio of the explosive. Detonation velocity is the actual speed mass or weight of a material (an explosive in this with which the detonation wave travels through a the cartridge or case does not reflect the actual powder column. It is measured in terms of feet per amount of nitroglycerine in the explosivebut is used second or meters per second; with ranges of 5,000- for comparison only. For instance, a cartridged 6,000 ft/sec for slow velocities, up to 24,000 ft/sec explosives listed as "40% gelatin" does not contain for very high velocity explosives. When detonated, 40% nitroglycerine, but it is considered to have a high velocity explosives expend a greater percent­ strength equal to a straight dynamite thatwould have age of their energy in the creation of a shock wave. contained 40% nitroglycerine. Care must be taken This shock wave provides more shattering effect on because the percentage ratings are not linear. That is, the rock, which is especially useful in hard, massive 60% gelatin is not 11/2 times stronger than 40% rock formations. Explosives with lower velocities gelatin. concentrate more of their energy in the formation of Today, manufacturers of explosives precisely mea­ gas pressure, which create a heaving or lifting action sure and report the actual energy of their explosives on the rock. This type of explosive has the best in terms of calories per gram, foot-lb per pound, or results when used in softer rock or that which has other units of energy per quantity of explosive. A thin seams or layers. switch to a higher strength explosive, should provide The strength of an explosive can be expressed a blaster with better fragmentation without a change and measured in several ways, one of which is by in pattern, or enable him to expand his blasting pat­ listing a percentage. Years ago, 60% straight dyna­ tern with no loss of efficiency. It is the explosive mite actually meant that the dynamite was com­ energy, designated as strength, that is doing the work posed of 60 % nitroglycerine and 40% other materi­ of breaking and moving the rock. als. However as the chemistry of explosives A blast produces large quantities of gases, some advanced, other ingredients were substituted for, or innocuous, such as carbon dioxide, carbon monox­ combined with nitroglycerine to provide the same ide, and steam; but also some that are very toxic, amount of energy. Today, the percentage listed on such as the oxides of nitrogen. The fume charac- teristics of an explosive is a measure of the quan­ cannot be so sensitive to shock, heat, or friction that tity of poisonous fumes produced by detonation. becomes unsafe to handle. The sensitivity of a Such fumes can be very hazardous even in small blasting agent will depend upon its composition, concentrations. When blasting on the surface, the which will determine what type of primer must be fumes are usually dispersed rapidly by the air. used to initiate it. However in a confined area, such as a tunnel, it is Sensitiveness is the ability of an explosive to important to use an explosive which produces a propagate continuously once it has been initiated, minimum amount of toxic gases, in addition to pro­ that is, the detonation will not die out in the pow­ viding adequate ventilation. The Institute of Makers der column. Explosives with high sensitiveness, may of Explosives and the U. S. Bureau of Mines both detonate when the shock wave from an adjacent have methods of classifying explosives according to hole reaches them. This hole-to-hole propagation the amount of toxic fumes they produce when det­ will most likely occur when very sensitive high onated. explosives are used in boreholes drilled very close The conditions under which an explosive is det­ together or in wet conditions. Such propagation can onated will also effect the quantity of fumes it pro­ be a serious problem because it disturbs the delay duces. For example, ANFO, when mixed in proper pattern planned for the shot, and can result in proportions, well confined, and detonated in a dry excessive flyrock or high vibration levels. borehole produces a minimum amount of fumes. To evaluate which explosive is best in any situa­ However, if detonated when poorly confined, when tion, the blaster must rely on information from the wet, or with an unbalanced mixture of fuel oil, explosive manufacturers, and often on their advice ANFO can produce large quantities of oxides of in order to make the right choice. However, he must nitrogen. understand the criteria to use for choosing explo­ The sensitivity of a high explosive must be such sives and recognize when any change in field con­ that it will be reliably initiated by a detonator, yet it ditions necessitates a change in explosives' proper- ties.O (SEE

that he or she must be able to do. The most funda­ mental calculations that a blaster must be able to do involve the distribution of explosives throughout the rock mass. These are the calculations of: 1) volume of rock broken, 2) powder factor, and 3) loading factor. Determining the Volume of Rock Broken Before anything else in a blast plan is considered, the blaster must determine how much rock must be broken by the shot, and how much will be broken / by each borehole. The volume of rock to be fr~w. mented is determined by the geometry of the blast pattern. The layout of the blast can be very simple, such as a single row breaking to an open face as shown in Figure 1. In the diagram the solid line would indicate the existing face of rock before the shot is detonate?. The dashed line indicates where the rock face w1ll be after detonation. The area between the two lines represents the fragmented rock. To calculate the vol- ume of the broken rock, multiply the area times th'? < height of the rock or use the formula for volume of: a rectangular solid which is:

Volume = Length X Width X Height Some blasters will strenuously argue that blasting is an art, one which can be mastered only after years In this case, length is the length of the shot, the of experience, and then only by certain select, and width is equal to the burden (the distance from the ... talented individuals. These "artists'~ are able to borehole to the original free face), and the height is "read" the rock, and intuitively design a pattern that the depth of the rock. Subdrilling is not considered will break it effectively. I would contend that blast­ in this calculation. The height of the rock mass to be ing is very much a science, or engineering disci­ broken is the depth of the borehole minus any sub­ pline. The results of a blast can be accurately pre­ drilling, which should correspond to bench height. dicted and controlled provided the blaster accounts Often trench blasting involves drilling through the for all the variables that determine the outcome of a dirt and loose material overlaying the rock, and then shot. Unfortunately, there are a very large number through the rock to grade. When boreholes are of factors that must be considered, some of which drilled through this unconsolidated "overburden" to are difficult to measure and some of which may be the rock, the depth of this "overburden" is not completely hidden from the blaster. For example, it included in the height of the rock. is not always possible to detect potential geological In a more complicated or irregular shot pattern problems, such as mud seams, caverns or voids, such as in Figure 2, it is not as easy to calculate the unless the boreholes happen to be drilled so that total volume of the rock broken. Rather, the method they intersect these areas. Even in cases where a for finding rock broken in such a blast is to calculate borehole intersects a mud seam or void, unless the the volume broken by a single borehole, and multi­ driller is particularly vigilant, he may not be able to ply by the number of holes. This can be used in both provide much information 'to the blaster. simple and complex patterns and will give good Assuming that a blaster can obtain all the data approximations of the rock produced by a blast. that he needs to design a safe and efficient blast, there will necessarily be some basic mathematics Assuming the burden, spacing, and depth are Example 2: A blast consists of 80 holes drilled on measured in feet, the volume of rock in cubic yards a square 10' X 10' pattern. The depth of the bore­ is given by the following formula: hole is 30 feet; the subdrilling is 3 feet. How many Volume (yd3)= Burden (ft) X Spacing (ft) X Depth (ft) cubic yards of material will be broken by this 27 blast? The 27 in the denominator is a conversion factor to change cubic feet to cubic yards, Solution: Volume = 10 X 10 X 27 = 100 yd3 ( 1 yd3 = 27 ft. 3). 27 Example 1: A borehole is drilled on a 10' X 12' Total volume of shot = # of holes X volume per pattern 30 feet deep. How many cubic yards of borehole material will this hole break? Total volume of shot= 80 X 100 Total volume of shot = 8000 yd3 Volume = Burden X Spacing X Depth 27 Calculating Powder Factor Volume = 10' X 12' X 30' = 3600 Powder factor is defined as a ratio of the quanti­ 27 27 ty of explosives needed to break a quantity of rock. Volume = 133.3 yd3 By this definition powder factor could be expressed as pounds of explosives per ton of rock (lb/ton), 3 pounds of explosive per cubic yard of rock (lb/yd ), or in any other units of measure that would specify Powder factor can also be used as a method of explosives and rock amounts. Pounds of explosives blast design or at least as a way of determining the per cubic yard of rock is a widely accepted form of amount of explosives to be used in a borehole. The expressing the powder factor, and the one on which formula above can be rewritten as : the following examples are based. Powder factors vary for different blasting opera­ Potmds of Explosives =Powder Factor X Volumeof Rock Broken tions depending on how hard the rock is, the densi­ ty and strength of the explosive used, the blast pat­ This assumes that the blaster has experienceand/or tern, the geology, the degree of fragmentation knowledge about the type of rock to be blasted, the required as well as a host of other factors. method of blasting, and the correct powder factor to On large scale operations such as quarries, mines, be used. and highway excavation, powder factors may vary 3 3 from 0.5 lb/yd to 2.0 lb/yd3• For smaller operations, Example 3: A borehole will break 100 yd of materi­ powder factors may range from 1.0 lb/yd3 for wide al. If 130 pounds of explosives are loaded into the trenches and easy shooting to 8.0 lb/yd3 for narrow hole, what is the powder factor? trenches and hard shooting. The usual way of calculating powder factor is to Solution: find the cubic yards of rock a borehole will break Powder Factor = 130 lb = 1.3 lb/yd3 and divide this into the pounds of explosive loaded 100 yd3 in that borehole: Powder Factor (lb/yd3) = amount of explosive (lb) 3 volume of rock broken (yd ) Example 4: A blast is drilled on a 12' X 15' pattern, actual value of the loading factor is dependant on the 40 feet deep. Each hole contains 300 pounds of diameter of the explosive column and the specific explosive. What is the powder factor? gravity of the explosive loaded into that borehole. When using a free flowing or bulk explosive such Solution: as ANFO or emulsion, the diameter of the explosive Volume of rock broken = burden x spacing x depth column will match the borehole diameter. A rigid car­ 27 tridge will normally have a diameter smaller than the borehole diameter. Volume of rock broken = 12 X 15 X 40 = 7200 The numerical values for loading factors are read­ ily available on charts with hole diameter and specif­ 27 27 ic gravity as the two variables, and listing the pounds of explosives per foot of borehole for every combi­ Volume of rock broken = 266.67 yd3 nation of those variables. The table to the right is a small part of a typical chart. To find, for example, the Powder Factor = pounds of explosive loading factor for a 5-inch diameter explosive column cubic yards of rock using an explosive with a specific gravity of 1.00, read down the left hand column to 5 and under the Powder Factor = 300 lb top row to 1.00. Where the row for 5 inch diameter 266.67 yd3 intersects with the column for 1.00 specific gravity, the loading factor of 8.50 pounds per foot will be Powder Factor = 1.125 lb/yd3 found.

Example 5: A blaster has drilled a pattern of 18 ft To compute the total amount of explosives in the by 21ft and 60 feet deep. He wants to use a powder borehole, the loading factor is multiplied by the 3 factor of 1.20 lb/yd , how much explosives must he height of the explosive column. The explosive col­ put in each borehole? umn corresponds to the borehole depth minus the stemming length. Subdrilling is included in the explo­ Solution: sive column since subdrilling is loaded with explo­ Volume of rock to be broken = burden x spacing x depth sives. 27 Example 6: A borehole is 30 feet deep, and 6 inch­ Volume of rock to be broken 18 X 21 X 60 es in diameter. Stemming is held to 7 feet. Using 27 ANFO with a specific gravity of 0.8, how many pounds of explosive are loaded into this hole?

Volume of rock to be broken = 840 yd3 From the table, the loading factor is 9.81lb/ft

Pounds of explosives = powder factor x yd3 of Rock to The height of the explosive column = 30 feet - 7 be broken feet = 23 feet

Pounds of explosives = 1.2 x 840 = 1008 lb/hole

For this blast design to be practical, the borehole must be of sufficiently large diameter in order to hold 1008 pounds of explosives and still have space for an adequate amount of stemming. Obviously, a blaster designing a pattern with a relatively large burden and spacing such as above, must have a large diameter drill.

Loading Factor The loading factor for a borehole is defined as the weight of explosives loaded per foot of borehole. The Borehole Specific Gravity of Explosive

Diameter (in) 0.80 0.90 1.00 1.10 1.20 1.30 1.40 3.5 3.33 3.75 4.16 4.58 5.00 5.42 5.83 4.5 5.51 6.19 6.88 7.57 8.26 8.95 9.63 5 6.81 7.65 8.50 9.35 10.20 11.05 11.90 6 9.81 11.01 12.24 13.46 14.68 15.91 17.13

Pounds of explosive = height of explosive column X gravity of 1.3. If the explosive column is 12 feet loading factor high, how many pounds are loaded into the hole? Pounds of explosive= 23 feet X 9.81lb/ft Pounds of explosive= 225.6 lb Solution: Loading factor is 5.42 lb/ft Example 7: A 4 inch diameter hole is loaded with 3.5 Pounds 0f explosive= 12ft X 5.42 lb/ft = 65.04lb inch rigid cartridges of explosive having a specific

Neda Mine Bat Habitat (continued from page 12) been successful, a clean cut of the mark with virtually no access by humans yet allow the batsto freely pass. At the damage beyond. The ore car was retrieved with no dam­ request of the site manager, Dr. Reinartz of UWM, the as age, much to the delight of the local historical society. drawn design of the gate was changed to allow for a larg­ er entry in case the need for a stretcher removal of an Final Installation injured person was required. Once installed the mine w:as A total of ninety-six feet of five foot diameter galvanized closed. The total time to completed installation of the culvert was installed in the cut, extendingboth into the adit tube was nine days, well under schedule requirements. and out of the face. Wooden shoring was put in place to The mine now has had extensive monitoring equip­ enhance the strength of the culvert during the inevitable ment installed for airflow, temperature and population settling that would occur. Backfilling was completed and a densities. This is now ,one of the two largest known riprap surface finished with on site material. Fifteen other colonies of bats of this type. The installation will serve far areas were filed and covered with site generated riprap to into the future both as habitat and study information for prevent entry to the works. This was vital not only for the generations .• protection of the bats, but to end the intrusions of local adventurers and the legal liabilities inherent. A tube steel For further information, please contact Robert Ederat (608) 634- 2702 or Bat Conservation International at (512) 327-9721. gate was built in my shop that would prevent unauthorized

Eve1yone in the blasting · However, even if cast blasting industry knows what is meant by may meet d1is definition, it is not flyrock, but there are several per­ the nightmare referred to earlier, spectives or ways to view fly­ because it is both expected and rock. In some cases, flyrock has controlled. When done correctly,· simply been described as ANY cast blasting is a positive out- rock thrown in the air by a blast. come of the blast design. Using this definition, even rock Howev~r, the flyrock that the. that rose only three feet off the blaster must be concerned about · ground, could technkally be is neither controlled nor · considered flyrock. Obviously, planned, and would normally be such flyrock is not . the kind a considered as excessive. blaster needs to be concerned When d'l.e blaster is concerned with. with flyrock and safety,there are · In certain other texts, · flyrock two possible -problems: either is defined as "any dirt, mud, the amount of material thrown, stone, fragmented rock or other or the distance the material trav- · material i:hat is displaced from els is excessive. When either or the blast site by being d1rown in both of these dungs exceed what the air or cast along the ground." most experienced blasters would Several points to note about this reasonably expect from a blast, definition are: there may be serious problems. Some · laws are written to specify what is considered to be excessive dis­ tance for flyrock. They may say that "flying rocks 1. Flyrock can be mud or dirt, it shall not be allowed to fall does not necessarily inean greater than one-half the dis­ only rock or stone. tance between the blast and a 2. The material can be displaced' dwelling house, public building, from the blast site by traveling school, church, commercial or along the ground as well as institutional building." This is .a truough the air. That is, fly­ quantitative· ·way of evaluating rock doesn't necessarily have the degree of flyrock, whlch has to fly. the advantage of flexibility in that it does not set a specific dis­ If you also examine this defin­ tance and apply it to all blasting ition of flyrock, it says nothing operations. about flyrock being dangerous. It would simply not be feasi­ Therefore, cast blasting could fit ble to set a pa1ticular distance as this definition since the material the limit for flyrock for every is displaced from the blast site by case. For instance, if 300 feet the force of the detonation. was chosen as a universal limit on flyrock, it obviously would not tion, the blaster's most critical that control of access to the site be appropriate when blasting in concern must be that the area is be maintained continuously; if the midst of a residential area. completely clear and access to such control is lost or removed However, when blasting on a the site is controlled. He should for even a few seconds, a mine site in an isolated area have a pre-determined plan for repeated inspection of the area' which may be three miles from safeguarding all personnel and is mandatory to be sure no one your nearest neighbor, 300 feet is the public. This is a matter about has entered the area during the unnecessarily restrictive. The dis­ which the blaster can make no time that access control was advantage to this type of law is assumptions, he must be lost. that it does mean that flyrock can ·absolutely sure that the area is The need for protection from remain within the boundaries of a cleat. Flyrock has been known to flyrock is most serious for the job site or riline permit and still be travel remarkable distances from blaster himself, since he is usu­ in violation. a · blast, and a plan to protect ally the person closest to the In most jurisdictions, there will against flyrock must take into detonation. Even though most be laws and regulations that account the worst case scenario. blasters want to fire the blast restrict flyrock in the vicinity of Furthermore, when blasting in a from a position with a clear highways, waterways, that endan­ public area, the blaster must real- vantage point to see the entire ger property or constitute a haz­ ize that the public is both curious blast area and the blast itself, it ard to_ employees or the public. and uninformed about the use of is imperative that they take ade­ Many of these laws are more explosives. They will sometimes quate precautions to protect directly concerned with securing place themselves in dangerous themselves. This means that the the area around a blast site. situations in an attempt to see blaster must be at a safe loca.- Provided a blaster does a good what i~ happening. In the surface · tion, and under substantial job of clearing and securing the coal mines of Appalachia, there cover. Light buildings, pickup area, even if a blast generates a have been a number close calls trucks, and other vehicles larger amount of flyrock than where people who ride "four­ which are often used as cover, usual, it should have no effect. If wheelers" in these isolated areas, have also often been penetrated the blaster is lax in clearing the drive directly up on a blast site. by flyrock. The use of specially area or blocking access, even a There are also reported instances designed blast shelters are well controlled shot can result in of hunters or back-packers walk­ becoming more widely avail­ enough flyrock to injure ·or kill ing in wooded areas near blast able and used, and represent a someone. In fact aU.- S. Bureau of sites and who deliberately remairi step forward in providing for Mines study done in the late nearby in attempt to watch a blast the safety of the blaster. 1980's, showed that the two lead­ being set off. A safe location and sufficient ing causes of fatalities in blasting All -roads leading into the cover is critical to the blaster's operations are : vicinity of the blast area must be protection because lead lines or physically guarded, and all per­ the shock tube is seldom long 1) Failure to secure the area sonnel and equipment must be enough to allow· the blaster to 2) Excessive Flyrock. removed from the area to a loca­ be b_eyond flyrock range. tion which is both at a safe dis­ Another critical point is that no Therefore, in a discussion of tance and well-protected. A thor­ one, including the blaster, flyrock and safety, there should ough visual inspection of all pos­ should ever be located in. front be some consideration given to sible areas that could be affected of the shot. Since so many securing the area. is essential prior to sounding the injuries are caused by flyrock, Immediately before detona-. warning signals. It is imperative- ariy discussion of safety should emphasize the means to prevent 5) Inadequate amou~t or type of into the rock so that the energy it. Some ·of the common ·causes stenuning. is nearly all used to fragment the rock, and whatever energy is left of flyr:ock are as follows: 6) Lift shots or shots which are over displaces the rock in a con-· over-co:hfmed. 1) Excessive amount of explo­ trolled manner. A poor blast sives used. 7) Poor delay timing in the pat­ design can result in a blast which. tern or detonators firing out of the explosive is ·under-confined, 2) Inadequate B~rdl':':n. sequence. that is, the strata of rock is ·not 3) Explosives loaded into voids, strong enough to contain the crevices, mud ·seams, or any From this list, it should be force of the detonation .• incompetent material. obvious that the best precaution 4) Spacing and burden exceed against flyrock is a good blast the depth of the borehole. pattern, one which effectively distributes the explosive energy·

20

sives per yard3 of rock) that was too large for the type of rock being shot. A powder factor that is too high can cause enormous movement of the rock. And an excessively large powder factor can actually occur due to any of several of the causes described in the list. For instance, if the burden distance is too small for the borehole diameter, or if not enough stemming In part one of this series on flyrock seven com­ is loaded in the borehole, those factors also result in mon causes of flyrock were listed. The first cause an excessive powder factor and an overloaded blast. was described as an excessive amount of explosives. This may result from poor design when planning When you hear that description, the implication is the burden or spacing. Or sometimes, poor execu­ that the blaster loaded "TOO MUCH EXPLOSIVES IN tion in the field occurs when trying to implement a THE HOLE." In fact, this is what the public often good blast design. For instance, a blaster may load a accuses blasters of, whenever they have a complaint hole where the powder factor has been calculated about the ground vibration, noise or flyrock; that is, correctly for a given burden, spacing and depth, but the blaster deliberately put too much powder in the the boreholes were not drilled accurately according hole. As the blasters themselves are well aware, that to that design. The blaster must also be aware of explanation is too simplistic because there is only a what the true burden is on each borehole, and certain amount of explosives that will fit into a bore­ whether or not that burden is consistent through the hole of a particular diameter and depth. length of the borehole. Note that the true burden is Furthermore, due to the economics of blasting, the determined by the delay timing sequence as well as idea is certainly not to use any more explosives than the blast geometry. necessary. If we look at some of the following diagrams, we So the statement that there was an excessive can see in some cases clearly where the burden turns charge in a blast usually means that there may be out to be less than that which the blaster expected. some other problems, such as the blast was loaded In figure 1, the bench has a good vertical free face, with a powder factor ( as measured in lb of explo- but the drilling was done poorly. If the blaster pro- ceeds to load the borehole for the have an average powder factor of holes are "buffered" in front of 12 foot burden he has at the top, 1.00 lb/yd3 for the borehole. If, the shot, and the blaster drills he will be overloading the bot­ however, the driller errs by 5 near the free face to ensure that tom of the hole where he has degrees from the vertical, his bur­ he "pulls the toe." The profile only 8.5 ft. An inaccuracy such as den at the bottom is only 8.5 ft., may look like the one shown in this, when the drill is off the ver­ and his average burden through­ figure 3. The first row of hole is tical, will be magnified the deep­ out the borehole would be 10.25 drilled close to the crest since er the borehole is drilled. As an feet. His actual powder factor for there is a large buffer that reach­ example, a borehole 40 feet the entire hole will be 1.16lb/yd. es nearly to the top of the deep, which is drilled 5 degrees Even more striking, is the fact bench. Subsequent rows of from the vertical results in an that while the powder factor for holes are set back a reasonable error of 3.5 feet at the bottom of the top one foot of the powder distance, and the blaster the hole. The same borehole column is 1.29, the powder factor assumes that the buffer will con­ drilled 100 feet deep with the at the bottom of the borehole is fine the blast on the outside same 5 degree error, will be off 1. 75 lb/yd, an increase of more boreholes. And it may work as by nearly 9 feet at the bottom of than 35%. planned provided the powder the hole. Even in the first Figure 2 shows a perfectly column is not loaded too high, instance, where the burden was drilled vertical hole on a level so that the top portion of the off by 3.5 feet, the powder factor bench, but due to the equipment charge is not near or above the may vary by 20% from top to bot­ excavating material at the front buffer. If a charge is loaded such tom of the borehole. of the bench, the blaster may that the charge is unconfined in As an example of this type of encounter a bench with a profile this top portion, it can throw error, consider a blaster who like this. In this example, if the rocks over a long distance. intends to drill a 12 x 15 pattern, blaster does not check the shape Accuracy in drilling is 40 feet deep, using 8 ft of stem­ of the free face, he may load the extremely important when ming in a 51/2 inch diameter, and shot for a 15 ft burden and only angled holes are used. The loads it with a loading factor of have 8 feet of rock confining his setup distance from the face, the 8.25 pounds of ANFO per foot. explosive charge. angle of the drill must be mea­ With this pattern, he expects to A similar instance is where the sured carefully to ensure that the proper burden exists at the toe. Failure to do so can result in charges that are placed too close to the face with less con­ finement than intended. If explosives are loaded into faults, crevices, or mud seams that the borehole passes through, the blaster should expect excessive flyrock. The driller has a crucial role to play in the preparation of the blast. Information about any unusual conditions such as mud seams or voids encountered in the borehole must be recorded or logged, and the blaster must rely on his driller to communi­ cate any unusual conditions of der column in the tries to blast an excavation to borehole using a grade that is only 8 feet deep. tape or other He may be conscientious method. enough to realize that he only Adequate needs 2 feet of powder in a stemming is borehole drilled on a 12 x 14 required to con­ feet pattern, 8 feet deep to get a fine the high pres­ proper powder factor. However, sure gases such a design produces a bur­ released by the den greater than depth, so that detonation of the the nearest direction for relief is explosive charge. the surface, and rock may be This stemming thrown straight up into the air. must be sufficient Certain operations use blast­ to prevent the ing patterns where the burdens force of these are sometimes close to, or slight­ the boreholes he has drilled. The gases from violently cratering to ly greater than the depth. One driller must be constantly aware the surface. The explosive force of these are the "parting shots" of what is happening as he drills, will always be in the direction of in surface coal mines, where the so he will know if he encounters least resistance, which should be blaster has to break a layer of a significant mud seam or a void the burden distance to the free rock that lies between two coal such as a cavern or an aban­ face at the instant of detonation. seams, this strata of rock may be doned underground mine. However, if the stemming length is anywhere from 8 to 20 feet thick If explosives are loaded into a too small, the most direct path for and the blaster may have to use mud seam, the mud does not the gas pressure to vent may be to a large diameter drill with a 12 have sufficient resistance to con­ the surface. In minor occurrences, x 12 pattern. It can result in a fine the energy of the detonation. this results in stemming material great deal of material thrown, As long as the blaster knows in being launched away from the but usually such blasts are a advance that a mud seam inter­ blast. More serious cases occur large distance from houses, and sects the borehole, he can take when material around the collar of roads. steps to eliminate the danger. the borehole can be thrown signif­ Occasionally blasters in quar.,. This is normally done by stem­ icant distances. ries are required to shoot "toe ming through the mud with inert Adequate stemming does not shots" where they come back material to make two separate only mean the length of the stem­ after the main blasts have failed charges in the borehole. A cardi­ ming, but also the composition of to pull to grade. On these shots, nal rule for a blaster is that the material used as stemming. the boreholes may be drilled in explosives should never be Drill cuttings or dust are usually a an irregular pattern and very loaded into anything except very poor stemming material, par­ shallow. When they are fired, competent rock. ticularly if the borehole is water­ flyrock is a definite possibility. Likewise, if a borehole is filled. Crushed stone of a suitable So called "lift shots" or any shots drilled so that it cuts into a cav­ size so that it interlocks in the fired to begin an excavation are ern or underground mine, the borehole is a recommended stem­ also prone to generate flyrock. blaster may load huge amounts ming material on most operations. These shots have no free face of explosives, and if detonated Another potential problem and are intended to cause the will create a violent result. If the occurs if a blast pattern is drilled rock to fragment and swell. blaster is careless while loading such that the spacing and burden However, if they are not careful­ the borehole, especially when is greater than the borehole depth. ly loaded and controlled, the using bulk explosives, he may In this situation, the blaster should fragmented rock again is not detect the fact that a large expect flyrock. Blasters occasional­ launched upward. On smaller amount of explosives are going ly drill this type of pattern without scale blasts, such as those for into the borehole. It is always realizing that they are creating a setting electric utility poles, the important that a blaster constant­ potential hazard. For example, a use of "burn or cut" holes serve ly monitors the rise of the pow- blaster using a 6 3/4 inch drill bit to provide a place for the bro- results in the of the causes of flyrock men­ same effect. In tioned previously can be predict­ this case, a mis­ ed and avoided, it is always pos­ take has sible that there exists some occurred and a unknown condition that will #lO delay is mis­ cause flyrock. For this reason placed in the the blaster must always assum~ shot pattern. the worst possibility and formu­ When this shot is late his plans to clear the area for detonated, the this case. Regardless of how charge in that many "well-behaved" shots he hole will fire has previously detonated, the with no relief at next detonation is always the all and the only most dangerous. possible move- It is the ultimate responsibility ment of the rock of every blaster to oversee the ken rock to move and help con­ around it will be vertical. A blaster safety and well-being of anyone trol the upward throw. On would be incompetent if he laid that could be effected by the these shots, blasting mats may out a shot like that on purpose, and blast. They must accept this be useful, but a good blast at the very least, careless if he did responsibility and do whatever it design is still critical. it by accident. However, with com­ takes to protect the public, their Poor delay timing can gener­ plex timing layouts, both electric co-workers, and themselves. ate flyrock. Faulty delay time and non-electric, any error in the While the explosive products may be the result of bad design, connections can result in holes fir­ we use today are the safest, most but could also be caused by ing out of sequence. The timing reliable ever produced, they are inaccurate detonator firing time, mistake may not be as blatant as only as safe as the individuals or simply human error in laying the one illustrated to have the same using them. Training and educa­ out the delay detonators. The effect. tion in all aspects of blast safety timing on all shots must allow Regulations usually require that must always take high priority in enough time for the rock to when blasting is done in proximity our industry, as well as account­ move so the muck does not pile to buildings and roads where no ability and responsibilitY in up in front and prevent the amount of thrown material can be implementing the procedures intended horizontal movement tolerated, blasting mats or other that are learned.• of subsequent charges. Figure 4 protective materials should be used shows the effect that takes place to confine any possible flyrock. But as the detonation proceeds even in those cases, the best deeper into the bench if there is means of controlling flyrock is still insufficient time for the rock in through proper blast front to move. That is, the design and delay movement of the rock from timing. It is therefore each borehole progressively extremely important moves upward at an increased that when a blaster angle until the back rows working with a suc­ become almost a lift shot and cessful blasting pro­ the rock movement is close to gram makes any vertical. Normally this can be change in the blast somewhat corrected by provid­ design, he must care­ ing longer delays between the fully consider any rows as the shot progresses changes from the deeper into the pattern. standpoint of its Figure 5 shows a delay pat­ potential effect on tern which illustrates a very sim­ flyrock. While many ple type of timing delay that