BLOCKHOLING AND MUDCAPPING

Secondary blasting is necessary when rock has broken into boulders too large to handle by the regular means of transportation. In general, two methods are used for breaking up the rock-one called "blockholing" and the other "mudcapping," "bulldozing," "dobying.

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The most economical method of using high explosives is to confine them in a drill hole well-tamped with a stemming material, and this method should be used in secondary blasting wherever possible. Because less dynamite is used and the blast is better controlled, blockholing is preferable to mudcapping. In blockholing a hole is drilled in the boulder and charged with a small amount of explosive which is shot by either fuse and cap or electric detonator. Stemming is recommended.

In mudcapping the explosive is placed on the surface of the rock, covered with a cap of thick mud, and fired with a detonator. The mudcap should be several inches thick and should not contain pebbles or stones that will be thrown by the blast. Several times as much explosive is needed to break a rock by this method as by blockholing, and with most kinds of rock the cost of drilling the hole is less than that of the extra dynamite. Under certain conditions mudcapping may be an advantage, and if conducted with due regard for safety it is a practical method.

Tests made by the Bureau of Mines showed clearly the greater efficiency of the blockhole method.23 On uniform blocks of the same type of rock a charge of two cartridges of 35-percent dynamite without capping produced only a slight crack; an equal charge partly confined by a mudcapping broke another block into many pieces; a charge of 2 ounces of dynamite confined by stemming in a hole 12 inches deep also broke a similar block.

CHUTE BLASTING

Ore broken in metal-mine stopes commonly is dumped or scraped into chutes or raises through which it passes by gravity to haulage levels. Broken ore may hang up in a chute because of packing of sticky material or wedging of boulders. Where there is no manway alongside the chute from which the blocked material can be worked loose with a bar or hammer, blasting may be necessary to bring down the ore. Sometimes the ore can be started by running water into the top of the chute, but this has its disadvantages. Chute blasting generally consists of detonating a bundle of cartridges of explosives under the obstruction. The concussion of a bomb of explosives below the hanging ore frequently starts it running, even if the bomb is not in contact with the ore; however, contact gives better results, The cartridges for a bomb are tied together with stout twine and detonator is inserted in a middle cartridge. The bundle is lashed to the end of a stick, usually from 10 to 12 feet long, and pushed up the raise through the loading gate. If one length of stick is not enough, others are spliced end to end until the obstruction is reached.

Many accidents have happened with this method of blasting in chutes, particularly where fuse and detonators are used for detonating the bombs. The bombs have fallen or have been knocked from

23 Snelling, W. O., and Hall, C., The Effect of Stemming on the Efficiency of Explosives: Bureau of Mines Tech. Paper 17, revised 1928, p. 15.

the blasting sticks down the raises and have exploded on the levels, injuring or killing men. The charge may fall off the stick because of being insecurely tied, or it may be knocked off by lumps of ore falling down and breaking the blasting stick. The explosive cannot be placed to best advantage when it contains a lighted fuse of short length, for under such conditions the blaster is more or less nervous and his main thought is to get out of danger. Once the fuse is lighted there is no way of preventing the bomb from exploding in case the explosive is displaced. Electric blasting is a safer method because the bomb can be fired at will.

Although electric detonators and blasting wires are slightly more expensive than fuse and blasting caps, the added safety of electric blasting and the more efficient use of the explosive makes that method preferable and it may be more economical as a regular practice.

With electric blasting, time can be taken to make sure that the bomb is properly placed and that everyone is safe before the charge is detonated. In some circumstances less explosives may be used because the bomb can be expected to be placed where it is wanted and no allowance need be made for extra cartridges against the chance that a favorable placement will not be achieved.

Burning fuse makes a large part of the smoke that is connected with fuse blasting, and smoke from fuse used in blasting hung-up chutes during the working shift may be a distinct disadvantage.

Safety rules for blasting hanging ore in chutes in some mines of the Lake Superior district are:

When it is necessary to use a blasting staff to blast dirt from a raise or mill, 1. Tie the powder to the end of the staff with marline. Never use the firing fuse to tie the bundle.

2. Do not make any tight knots in the fuse.

3. After preparing the powder, but before lighting the fuse, lift the staff into the exact position desired for the blast and block it into that position. Then lower the staff and spit the fuse, and replace it in the same position.

4. If the dirt is hung up too high to be reached with a staff, the pit boss shall decide what is to be done.

FIRING LARGE ROUNDS

When numerous holes are to be blasted as one round, additional precautions are necessary in using fuse. The number of holes that one man can light safely and the length of fuse to be used will naturally depend on local conditions, such as the distance to be traveled to reach a place of safety, wetness of the face, ease of movement, and possibly other conditions. The maximum number of holes which one man may spit may be 12 at one mine and 20 or more elsewhere. The smoke made in spitting a large number of holes may make it difficult to see at the face before spitting is finished; for this and other reasons 2 or more men should work together, and they should have very good lights. A boss should always be present to keep an accurate check on the time so that men will not remain too long. He should see that all men are out before he leaves. The use of master fuse lighters or of "bunch-blasting" will reduce the time required to light the fuse.

For rounds of a hundred or more holes electric blasting is far superior to fuse from both the economic as well as the safety standpoint; in some circumstances this may be true for rounds of even fewer holes. The hindrance and hazard from the smoke of burning

fuse are eliminated, and all holes can be blasted simultaneously where this is desired. The wiring and checking of connections would be a necessary labor, but the hazards of electric blasting are not greatly increased by increase in the size of rounds. Men must be checked out and the usual precautions must be taken to prevent premature detonations as in all electric blasting.

LONG-HOLE DRILLING AND BLASTING

Diamond-drill blast holes are being used to an increasing extent in various types of stoping, and in a few instances they have been used in development work. Long holes may be drilled with mounted hammer drills, but electric or compressed-air-powered diamond drills are being used fairly generally in recent years where long-hole procedure is in effect. The length of holes varies from 10 to 100 feet, with a usual length of about 60 to 70 feet. Except in very hard ore, the holes are charged with 40-percent gelatin dynamite or a similar explosive. Where long holes must be loaded from mine workings that will not admit employment of loading sticks long enough for the depth of the hole, it has been found practicable to load and tamp with jointed sticks or with rubber hose having a plug in the end. Jointed sticks have been made in sections 4 or 5 feet long, joined with leather straps or rubber couplings; in another method the sections are wooden rods hollowed out and strung on strong sash cord or rubber-covered wire. The cord is attached to the end section and can be pulled tight to make a firm rod of the sections in the hole.

Primacord is placed the length of the holes to secure complete detonation, and 2 or 3 feet of clay stemming is used at several mines. Long-hole blasting is particularly adapted to the mining of ore bodies or parts of ore bodies that cannot be mined safely by other methods, such as ore overlying open stopes and pillars in broken or caved ground. A great deal of preliminary development work is generally avoided, and drilling is done from a safe location. Blasting is also safer than in the use of shorter holes, as the shots are fired when men are out of the mine and it is unnecessary to enter the blasted area. There is also a material decrease in the dust thrown into the air when diamond drilling is done.

HANDLING MISFIRES

Misfires or delayed shots are a prolific cause of accidents in blasting, both on the surface and underground. Many accidents occur when persons return too soon after a blast has been fired and are injured or killed by the explosion of a hole that has delayed firing; or, more frequently, a missed hole is exploded by some work done subsequent to blasting.

The best method of reducing the hazard is to reduce the number of misfires by proper storage and correct methods of priming, loading, and firing the explosives.

No one should return to the face after a blast until the smoke, dust, and fumes have been cleared away. If there is reason to suspect a missed hole, the return should not be made until a safe interval has elapsed; where electric detonators are used a wait of at least 15 minutes is advised; where fuse and detonators are used the interval should be as long as possible, preferably until the next day. Records of misfires show that charges have exploded several hours after the fuse was lit. Any misfires should be reported to a designated person

and subsequent steps should be taken by him or under his direction. When a misfire is found or suspected no other work should be done except that necessary to remove the hazard, and only those persons necessary to the work of removal should remain in the danger zone. If the missed hole does not have more than 2 feet of stemming over the charge a new primer can be inserted against the stemming and the hole fired.24 The stemming can be removed by washing it out with water under pressure from a hose. This is probably the safest

FIGURE 28.-Starting fan after blasting to clear smoke and gases from working place. method of removing stemming from a borehole and the one most strongly recommended. No picking or digging should be done about a missed hole unless it is done with wooden tools, and then only with extreme care under supervision.

All places that have been blasted should be examined carefully for missed holes, and any bootlegs should be inspected for the presence of explosives in them. If explosives are found they should be treated as missed holes. Some companies require that any bootlegs must be 24 Agnew, Wing C., Johnson, John A., and Mosier, McHenry, Firing Through Stemming: Bureau of Mines Rept. of Investigations 3646, 1942, 4 pp.

loaded and shot before any other work is undertaken in the place where they are found.

GASES FROM EXPLOSIVES

All explosives give off toxic gases when fired. These gases may be produced in deadly amounts or may be relatively harmless, depending on the type and quantity of explosive used, the degree to which the explosive charges are detonated rather than burned, the conditions of confinement, and the composition of the material being blasted. Dynamites used in underground metal mining usually are chosen for their favorable fume properties, but the detonation of rounds involving one or more cases of explosives usually will produce enough toxic gases or may so reduce the oxygen content of the air that it is dangerous to breathe. The quantities of gases produced normally require a moderately strong ventilating current to dilute and remove them if the return of men to the face is not delayed for several hours. In the absence of adequate ventilation a lapse of several hours may still find dangerous quantities of gases in the working face and in the broken muck pile. A blower fan installed for clearing out blasting fumes from an advancing face is shown in figure 28. Blowing with a compressed-air hose may clear only a small space, even in 1 or 2 hours time. The results of field tests of the fumes produced by various types of explosives were published by the Bureau in Bulletin 287 and Technical Paper 482, containing the following conclusions:

When an explosive is properly detonated in inert country rock the only toxic gas produced in dangerous quantities is carbon monoxide. Some sulfur dioxide and oxides of nitrogen were frequently produced under these conditions, but the quantities were harmless. Blasts in massive sulfides produced dangerous quantities of hydrogen sulfide and sulfur dioxide.

The oxygen balance of the explosive appears to be of the most importance in the amount of carbon monoxide produced. The condition of the explosive also affects the gases from blasting. Deteriorated insensitive explosives give more carbon monoxide than fresh.

Although less toxic gases are made by some explosives than by others, with all the explosives tested efficient ventilation is required to remove the gases that follow blasting from the mine workings before men return to work.

The composition of the gases from blasting of the different holes of a round is not uniform, and no definite relationships were found to explain the variations. However, in hard rock the use of stemming reduces the amount of carbon monoxide.

Sprinkling muck piles removes most of the sulfur dioxide, hydrogen sulfide, and oxides of nitrogen in the trapped gas. It has no appreciable effect on the carbon monoxide, which is only slightly soluble in water.

Fuse equivalent in amount to that used in blasting an average round was burned. The amount of carbon monoxide given off was too small to have any appreciable effect. The smoke was dense and sickening, but poisonous gases were not present in large enough quantities to be noticed.

When a charge of dynamite burns, oxides of nitrogen are formed and give the characteristic burned powder odor. The oxides of nitrogen can be detected by smell; they corrode the respiratory passages and the breathing of relatively small amounts may cause death. Their effect is unlike carbon monoxide poisoning in that a patient may apparently recover and then suddenly die several days later. One-hundredth of 1 percent may cause dangerous illness If breathed for a short time, but 0.07 percent is fatal if breathed for 30 minutes.

A typical accident that was due to fumes from blasting occurred when two miners were suffocated in a drift being started from the bottom of a 250-foot winze. A round in the drift face had been blasted at 4 p.m. when the day shift left the mine, and blasting on the level above broke the compressed-air line. When the miners went to the drift at 7 p.m. they found heavy "powder" smoke but left