Elements of Underground Blasting

 

MIners loading a crosscut round  in the Junction mine.

    Blasting was  the conventional  method used to create underground mine workings in Bisbee. The important parts of blasting in hardrock underground mines are the “round”, order which the holes are detonated, hole depth and hole direction. These are followed by what kind of explosives,how much and hole diameter

     In the underground mines, miners did their own blasting, not a dedicated person . A talented miner could set off a blast, break the amount of rock which was intended and not knock out the timber or creating  too much over break. The importance of good blasting cannot be stressed too much; this ability is one of the qualities of a good miner.  The role of powdermen/powder monkeys in Bisbee, was to take care of the powder magazines. In the powder magazine’s  they were in charge of rotating the explosives (making sure the older explosives were used up first), ordering, transportation of explosives and general housekeeping. Powder monkeys with Copper Queen Mining Company  went to the workings near the end of shift  taking  orders for explosives and delivered them.


A “round” is a series of holes drilled into the rock in special patterns that are to be detonated. The type of round was chosen on what type of working is to be created crosscut, raise, stope, shaft, etc. Character of the rock to be broken was also needed to be considered.  In the early years with hand drilling being done, they placed their holes according to rock features like slips, cracks and crevices. If this style of round failed to break correctly, their answer to the problem was to drill more holes. With the introduction of machine drilling this method was found to the less than effective, it lacked the  capability  to pull a round ( pull=Break the ground to the back of the holes drilled) . In the early years of machine drilling, it was normal to have 6” to 18” bootlegs (remnants of drill holes) in the face. Many miners drilling 5ft rounds would only average 3ft pulled per day by the end of 30 days of mining.

Early machine round “Toe cut”

Copper Queen’s mining engineers experimented with different rounds until they found the most effective. A series of standard rounds was created in the late 1910’s. These patterns continued to evolve and be modified. Rarely, in practices do the drilled rounds look exactly like the drawings, the character of the ground (rock) will prohibit this.  The basic types of rounds used in Bisbee mines were 5 hole burn, pyramid cut, V cut, toe cut, and slab.

The early crosscut standard round V cut

 To begin the process of creating a mine working, a place and what type need to be decided. This is usually done by the mining engineers, mine foreman, shift boss. After this has  determine miners will drill out the correct round for the according mine working.   
 
When the round has been drilled, one of miners will go and fetch the required explosives from the powder magazines*. The other miner will   clean out the holes with a blowpipe and drilling spoon. He will also clear the area of equipment moving it to a safe distance. Also, the timber needed to be wet down and secured so it will not be taken out with the blast.  In the years which hand mucking was done, mucking sheets were used in conjunction with the blast, they are a 4”x8” sheet of  steel boiler plate. These sheets would be installed before or after the drill operation, one sheet would  start 18” from the face going back where it would be overlapped with a second sheet by 6”.  Sheets would have a few boulders placed on them to keep in place during the blast. The lifters would be overloaded so the blast would throw the broken rock on the sheets. This would give the muckers a hard flat surface to work from, making it easier to hand shovel the broken rock into mine cars.

*This is C&A style, Copper Queen miners had their explosives delivered to them. 

Mucking sheet  on ground  at clean face


 The miner retrieving the explosives will go to the powder magazine when the round is ready to load. There are two magazines, one magazine is for blasting caps and fuse, another is for dynamite and Carbamite.  The miner will sign out the required amount of items from the magazines and transport the explosives back to the work place in powder sacks, except Carbamite which came in 50lb paper bags. 
Bisbee mine’s used primarly two kinds of high explosives, dynamite and Carbamite which is Apache powder companies’ brand of prilled ammonium nitrate fuel oil mixture. Dynamite was used  from Bisbee’s mines beginning until the mines closed, it was only in the late 1960’s that Carbamite was introduced. 
The detonators used were usually #6 or #8 fire fused caps. Rarely, were electric blasting caps used underground.   
Loading the holes with explosives is next, the first task is to decide the firing order of the holes; Bisbee’s mines had a standard firing order. Miners varied the order with experience especially the ones that had worked in other mining districts. The drawings of rounds included within the article show the firing orders by numbering the holes.  Holes without numbers are called dummy holes and not to be loaded with explosives. Holes with the same number are fired simultaneously. The ability to do this precisely came with the introduction of electric blasting caps with millisecond delays.  

Loading with dynamite and fire fuse

    Preparing the dynamite, this varies from miner to miner. I personally take each stick and twist it just enough to wrinkle the wrapper breaking up the dynamite inside for tamping. Some miners will roll the stick between both hands to soften the dynamite. Other people take a knife and cut a slit in the wrapper loosening the dynamite inside. This technique is especially useful when loading holes that are overhead or at a steep angle, because it allows very good tamping. Rarely, the wrapper is removed entirely and the dynamite inserted this way. This allows you to put a lot more of the explosive into the hole and gives great tamping.  Some miners do nothing at all and just load them right out of the box.   Each loaded hole will need to have some type of fuse and blasting cap. Bisbee's underground mine nearly always used fire fuse (safety fuse).

     In Bisbee, most of the time the fuses will came from the surface precut to set lengths, 5’, 9’, and 12’. The precut fuses will come with with the blasting caps already crimped on.   Normally the first stick will be primed with the cap and fuse though, I have seen miners who prime the second stick of dynamite.* The proper procedure to prime the dynamite is to make a hole with a wooden skewer (dowel rod sharpened like a pencil) about 2” from one end of the stick. Insert blasting cap and fuse and tie the fuse in place with string. In today’s mines they use the black electricians tape instead of the string. A common method but not approved is to take a stick of dynamite and roll one of the ends between both your hands. This will break up the dynamite inside the wrapper. Insert cap and fuse in the butt end and fold the fuse over the end and insert the dynamite into the hole with the primed end facing the back of hole.

     Place  the primer (primed dynamite) at the entrance to a hole with the primed end facing out. Shove the dynamite to the back of hole with a tamping rod (1” wooden dowel rod 8” ft long). Lightly pound the dynamite in place at the back of the hole with the tamping rod. This is to keeps the dynamite from falling out of the hole. It also allows you to concentrate more of the explosives deeper in the hole where there is more resistance, giving you a better break.  Now the hole will be filled with unprimed dynamite. The amount will vary depending on how well the ground (rock) breaks. Each stick will need to be tamped as they are inserted at the end of the hole. Typically, one stick of dynamite per foot of hole,  so a 6ft hole will have 6 sticks.*  After you are finished loading the hole, tamping (also called stemming) is placed and packed into the entrance of the hole, this helps contain the explosion.  The tamping is usually made of  rock wool placed into narrow and long brown paper bags. There are occasions when you will not use tamping when loading rounds.

When the round is loaded the next step is to trim the fuses to obtain the firing order. The shortest fuse you are ever  to use is 5ft long and that is for breaking boulders, setting off missed holes, etc. Normal rounds will have a 9 ft fuse as the shortest, it being the first hole to be detonated. The next hole to go will have its fuse cut 1/2” to 1” longer than the previous fuse. This continues until all the fuses are cut.  The ends of the fuses will be split back about ½” with a fuse knife to make them easier to light. Then fuses are then either folded and placed in the entrance of the hole it comes from or inserted into a spitter board.  When folding fuses make sure not to kink or damage their outside coating. After spitting the folded fuses pull them out of the hole and let them hang freely while burning.  Fuses placed into a spitter board, will be placed in their firing order starting with the first hole to be detonated.

Loading with Carbamite

Prilled ammonium nitrate

After the holes have been cleaned, a stick of dynamite Amogel # 1 primed with a fuse and blasting cap is placed at the bottom of the hole to be loaded. Then a Carbamite gun is used to pump in the explosives . The gun is a trigger like valve attached to a vacuum pump.  The gun will two attached hoses,  a black and yellow Anti static hose for loading the Carbamite into the hole and a I”ID suction hose to suck the material from the  Carbamite bag.  Near the trigger there will be a spud for a 1”compressed air hose to be connected.

To start  the loading process take the  gun and 50lb bags  Carbamite to the face that is to be loaded.  Make sure any inline oiler is disconnected from air hose. Attached the compressed air hose  to the spud on the gun  making sure the safety chain is  put in place. Ground the gun by attaching ground wire to a rock bolt, wedged into a crack in the rock or lay wire on the exposed ground holding it in place with a boulder.   Blow out the gun to eliminate moisture  inside. Open a bag of Carbamite and place the feed hose into the bag, The insert the loading hose into the primed hole 12”‘to 18” from the the dynamite.  Activate the trigger which causes the Carbamite prill (beads) to get pulled from the bag   and out the end of the loading hose. The hose will be moved slowly out of the hole as it gets filled. When the hole is full the Carbamite will start to spray back out the hole, then move to the next hole.    Raise rounds or holes at a steep angle will require  two sticks of Amogel #1. The second stick is to be tamped to hold the primer in place. Tamping (stemming) will also be used at the mouth of the hole. Fire fused blasting caps were commonly used for detonators timed just like with dynamite, on rare occasions electric blasting caps were used.

Drift or stope round loaded with Carbamite (no tamping) 

Carbamite is very sensitive to water, once wet it will not detonate. In wet ground  dynamite was used instead, it was typical in Bisbee mines to load the lifter holes with dynamite and the rest with Carbamite. Another source of problems, are hidden cracks in the rock which while loading get filled with the explosive thus overloading the hole causing a much larger explosion than planned for.  

Raise round load with Carbamite

Detonation


  Blasting time is Bisbee mines were at the end of each shift. At lunch time small pops, less than 10 boulders could be shot.  Rarely, blasting could be done at other times and only with special permission form the general mine foreman.  The miners stayed underground for the blast unlike some other mines were everyone goes to the surface or designated place like a shaft station. After the loading is finished the area is secured by moving away equipment, protecting immobile equipment and removing left over explosives. A common practice was to destroy the left over explosives instead of returning to the magazine, like the rules state.  When blasting, two men were required to be present and they could spit (light) up to 35 individual  fuses.  More fuses than this required and additional personal for safety.  Two spitters were required to be used when blasting with fire fuse. Both spitters (hot wire igniters) are spit at the same time. Fuses are spit in the firing order, until all the fuses are ignited or a spitter goes out.  After the fuses are spit the miners will yell “fire in the hole” and the leave the area to a safe distance. 200ft or so and around a corner is good for a normal size round. The only time you need to get a great distance away is when you are in a straight shot from the blast.  Every entrance into the blast area will need to be guarded to prevent others from walking into the blast. The miners will find a good position, insert there earplugs and then wait until the round has finished going off.  It is a good idea to put your hands over your ears and open your mouth to prevent ear damage from the concussion.  Safety rules required the miners to stay for  30 minutes guarding the blast after  the detonation before leaving.  
In a normal fire fuse round the first hole will detonate 6 minutes after the fuse was light the proceeding hole will go off every 1.6 to 3.3 seconds.  When close to the blast, the miners will feel the concussion, this sensation which overpowers the noise. The noise is a series of loud booms which the miners will count during the blast and correlate it with the number of holes they loaded. This will allows them to be cognizant of any missed holes that have occurred. The miners will report any missed holes or suspected missed holes to their shift boss who will post them.  When using electric blasting caps the miners will be unable to count the explosion. This is caused by the use of millisecond delays with this style of caps, which cause the holes to go off so quickly that it sounds like and elongated boom.  
  The task of the first holes exploding  is to make an area in the rock face for the proceeding holes to break into.  This is where the hole pattern shows it’s importance.  A correctly chosen pattern gives the end results of material of an easily handled size, little over break, good advance and timber and other nearby items in suitable condition.  In the early years before mechanized mucking, it was also important get the blasted muck a few feet away from the face. This allows the miners to setup to drill before the muckers were finished mucking. To do this the lifters are detonated in a way that allow them to flip the muck pile. The miners will need to wait at least 30 minutes before returning to the freshly blasted workplace. This allows time for hang fires to detonate and clear the smoke (gases) from the area. The normal situation is the miners who blasted will leave the mine because there shift is over. They will need to report any missed holes to the shift boss who will inform the opposite shift. When the opposite shift gets to the work place they will first inspect the area for hazards loose timber, rock etc. Then they wash down the area with water, allowing them to give a better safety inspection. The area is then barred down; boot legs inspected and prepared to be mucked out.

Missed holes

Doug Graeme standing by four missed holes in drift round  that have been reloaded and ready for blasting Golden Queen tunnel California

 Missed holes are the blasted holes that have failed to detonate, these can be dangerous if left unaddressed. There are several ways they can be found,  Miners returning to a blasted area look for bulges of unbroken rock that are left behind  in the blasted face., unexploded dynamite strewn in the muck pile Is another sign . Missed holes can are often found in bootlegs, which are the short remains of the holes that did not break during the blast. Unexploded explosives can be hidden inside the bootlegs or inside bootlegs that are buried beneath the muck pile. All the bootlegs will need to be cleaned out and inspected before proceeding with other work
     The disarming of the missed hole is done in varying ways depending the situation.  Missed holes were dealt with before starting any other work. In Bisbee this was a firing offence if a boss caught you doing anything else. If a bootleg is found fairly intact with explosives it is best to reblast it.  Clean out the  missed  hole entrance. Insert a  fresh stick of dynamite that has been primed into the hole entrance, then re detonate.  With a badly damage hole, it will be best to wash the dynamite or Carbamite out.   Get a water hose with a small enough diameter to fit inside the hole. Turn the water on and start washing the explosives out of the hole. If there is a  piece fuse, left in the hole,  give it gentle tugs.  After a while  the dynamite will  start to float out of the hole  hopefully fairly intact.     Sometimes  the  sticks will fall apart and  come out as pieces of wrapper with small bits of the dynamite. When doing this  make sure and do not touch the water coming from the hole  it is full of nitroglycerin. Severe headaches occur from contact with the nitroglycerin.  When dynamite is strewn amidst the muck pile, the procedure is to dig through it slowly  either by hand or mechanical  means. Removing the dynamite and blasting cap as they are found. With Carbamite just wash the pile with water dissolving it look for and recover the primed dynamite with the blasting cap.  Note the most dangerous part of any missed hole is the blasting cap. Dynamite and Carbamite and other secondary explosives are very difficult to detonate other than with  primary
explosives like blasting caps. Blasting caps are sensitive to rough handling, especially when smashed or pinched  and can be detonate by these means. If the blasting cap goes off it is likely any explosives nearby will detonate. When electric blasting caps are involved, start by stripping the two end wires, of insulation and twisting the bare wires together. This will help keep they from being set off by stray electricity.

Spittercord (Thermalite Ignitacord)

Type "B" spitter cord and    safety fuse with a Thermalite connector

The use of spittercord  was another method used to light  fire fuse. This method was most commonly used when blasting inside raises. Spittercord is a small diameter (about 1/8” ) bendable incendiary cord made of a wire coverd with a combustible material that has been waterproofed with a plastic coating.   The spittercord used in bisbee came in two burinng rates. “A” type which is coiled on a green  plastic   reel burn at 10 seconds per ft. “B” type is found a red reel burns at 20 seconds per ft. The cord burns with a strong and robust external flame. To use spittercord  the ends of the fire fuse must be fitted with Thermalite connectors . The connectors look like a  # 6  copper blasting cap with a slot cut at one end. The connectors are crimped on the end of the fuse and the spittercord is run throught the slot. The slot end of the connectors is filled with a combustiable material. as the cord burns through the slot it ignites the connectors which inturn lights the fire fuse. In rounds that use spitter cord all the fuses will be the same length, at least  18” of fuse exiting the hole. The timing will be done with the cord. You start inserting the cord into the connectors following  the firing order. As the cord is inserted lightly crimp the cord inplace by pushing down  on the end of the connector .  When hooking up the cord keep it at least 4” away from it’s self to avoid cross ingnition. Also time the cord hook ups so that all the fuses will be burning inside the collar of the holes, before the first hole detonates. Kinks in the spitter cord should be repaired by twisting the double cord that those areas. Splices should be made using a six-wrap western union knot. For safety it is necessary to be careful with the cord it is susceptible to ingation from falling rocks, sparks, from hitting it and open flames. The cord is lit with a spitter or  match. Starting at the end which is closest to the first hole to be detonated. 

Diagram showing  how to wire a round using spitter cord

Electric Blasting

The use of electric blasting caps was rare in Bisbee’s underground mines. The current  Queen Mine Tour guides do not recall ever seeing them used except in the Lavender pit. Safety rules were in place by 1947 for their use underground in Bisbee. The most probable explanation for their lack of use, is concern of premature detonation caused by the power from the 240 D.C. trolley line and other stray currents.   Special precautions had to be made when using electric blasting caps. All the pipe lines, track (rail) and electrical equipment had to be grounded.  Battery locomotives could only be used during blasting. Trolley motors were only allowed to be used to deliver supplies and then taken elsewhere before loading. All the D.C. power (trolley Lines) is shut off during loading.  Special care is taken when electrical storms are occurring on the surface because of stray currents.   With electric blasting the holes are prepared in the same fashion as other blasting methods. Loading will began with checking the blasting caps with a cap tester or galvanometer. This requires the plastic covering on the end of the wires to be removed. After checking replace the covering or twist the wires together this keeps the cap short circuited for safety. The primers will be made using the method in the figure below.

The holes will be loaded in the normal technique for the type of high explosive.  Care will need to be taken not to damage or kink the wires.

When wiring, all non-cap lamp lighting such as a stope lights will need to be removed from the area.  In wiring of the round should be done by no more than two people to avoid confusion. First run lead wire from the face to the main lead do not connect and keep lead wire ends twisted (shorted) at both ends. Keep all the wires dry to keep them from shorting out. Next wire the round. Take one wire from a hole and wire it to one wire in the next hole. Now take the left over wire and move to the next.  The connections will be made using the Western Union Twist. Keep wiring until finished  all the connection will need to be tight the wires will need to be out of the way to keep them from being pulled apart. 

After the round is wired connected the two left over blasting cap wires to the lead wire. Make sure the ends of the lead wire do not touch. Test the round using a Galvanometer. To test, touch or hook up lead wires to the galvanometer post. The meter needle should move to a reading of 15 for 32 holes loaded. 18 for 16 holes etc. If the needle does not move or not enough there is a loose connection or broken wire.  When there is a problem the meter can be used similar to a voltmeter to detect the problem. If it is found that a wire is broken behind the tamping inside a hole, treat it as missed hole.

After the circuit has checked out, wire the lead wire into the main lead. Making sure wires the do not touch each other and are not grounded. The shift boss will send the miners to their areas to guard.  The boss will open the short circuit switch at the workplace and leave. The boss will then ensure everyone is in a safe place and the area is guarded. Then they plug in the blasting box.  The safety switch on the box is unlocked and the throw switch is put into the “on” position which detonates the round. Hold switch in the “on” for less than 2 seconds then throw to the “off” position. Unplug the blasting box and put away.

If the round miss fires, lock the safety switch and put away the blasting box. Check and see if main power source had power. Then back track along the wiring looking for breaks and short circuits. Once at the main lead and lead connection disconnect the leads from the main leads twisting the lead wires together.  Trace the wires back, using the meter to check for problems.  Continue to do this until the problem is found.

 

Nitramon and Nitramon S

 

Nitramon is a type of high explosives that is very insensitive, it requires a special detonator and cannot be detonated with a blasting cap or primacord.. Nitramon and Nitramon S saw use in Bisbee’s underground mines in mining near the mine fires. The burning sulphide ore would increase the rock temperatures to a point where conventional high explosives could not be used. Nitramon is made up of a mixture containing paraffin wax, dinitrotoluenebuit and ammonium nitrate, secured in cans or paper cartridges (sticks).  If using the can variety, the cans are screw together as they placed into the holes. A special detonator is attached to the last can placed in the hole from which a safety fused cap is used. The stick type is loaded as usual, but with the special detonator at the beginning of the hole.

Nitramon S cans

Old explosives

 

When remining older mine workings the discovery of old explosives is common. Miner’s often hid left over dynamite and blasting caps behind timber in gob walls etc. instead of taking it  back to the powder magazines.

Dynamite does not have a good shelf life inside the underground mine environments. In just a few years dynamite becomes unreliable and unable to detonate.  Old dynamite should be destroyed, this can be done by taking it back to the surface, piling it up in secure location with straw or kindling and burning it with kerosene. Destruction of it underground is done by taking to a secure location, adding fresh explosives and detonating it. If only a few sticks are found, unwrapping the dynamite and dissolving it in the water ditch works.   

Blasting caps are best destroyed by grouping the old caps together around a stick of dynamite and it’s then detonated.

Old stick of dynamite inside the Southwest mine

Abandon tool room with full case of powder on left  bottom shelf. 4th level Southwest mine

References

Code of Safe Practice for Drift Mining 1955, Phelps Dodge Corporation pp 21-33

 Code of Safe Practice for Stope Mining 1968, Phelps Dodge Corporation pp 80-104

Code of Safe Practice for Raise Mining 1963, Phelps Dodge Corporation pp10-11 Fig.9

Richard Graeme IV 2010, pers. comm., 8 February

Richard Graeme III  2010, pers. comm., 2 February

Standardization of Mining Methods Charles A. Mitke 1919 McGraw-Hill book Company pp.19-29

“Procedure of Safe Practice for Electric Blasting 1947”, Phelps Dodge Corporation

“Procedure of Safe Practice for transporting explosives from shaft collar to underground magazines  Operation of Magazines and issuing explosives 1949”, Phelps Dodge Corporation

“Procedure of Safe Practice for The Handling and Use of Explosives In Underground Mining 1960”, Phelps Dodge Corporation

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© 2013 by Doug Graeme