Drilling & Blasting of Underground Stopes

Drilling & Blasting of Underground Stopes

Drilling in stopes is done with pneumatic-feed stopers, light handheld hammer drills of the jackhammer type and heavier plugger machines, drifters of the Leyner type mounted on crossbars or on columns, and drifters (and, now, only occasionally, piston drills)


mounted on tripods; more recently, with the more extensive use of bort instead of carbon for diamond drilling, diamond drills are employed occasionally for drilling long blast holes, especially in pillar work.

For open stoping in flat or slightly inclined deposits not too thick to be mined from floor to back in one slice, light hand-held or mounted drifter machines usually are employed where the ground is easy to drill and heavier mounted drifters are used in harder drilling ground. In thicker deposits a heading-and-bench system of drilling and blasting may be employed, carrying a 6- to 8-foot advance heading or breast under the top of the ore and then benching the bottom ore behind it (fig. 108). Column-mounted drifter machines usually are employed for driving the heading, and bench holes may be drilled with hand-held machines, as at B in figure 108. Hand-held machines usually are used for stope holes, as at A, and tripod-mounted machines are used for drilling flat “splitter” holes in hard ground, as at D. Down holes usually are preferred because handheld “pluggers” may be employed; it is usually possible to drill a greater footage per machine shift with pluggers if the rock is not too hard, since no time is required for setting up.metal-mining-method-heading-and-bench-stopingUnder some conditions, however, as in the Tri-State zinc and lead district, where the ground is hard, cherty, seamed, and vuggy, flat holes can be drilled and cleaned out more readily, and fewer holes are lost because of stuck steel. The best system to employ in any particular instance therefore will depend on the nature of the rock. The bench system, using down holes, is also employed in large thick deposits for breaking down ore around a central raise (“milling” or “glory-hole” mining). Figure 108 shows the top of a stope of this type at C.

In sublevel stoping the ore is broken partly by slabbing rounds to form the bench, using column-mounted drifters for drilling, and partly by benching (fig. 86), or it may be done by “ring drilling” from and around the sublevel drifts. Bench holes may be drilled upward in the bench above, using stopers or mounted drifters, or pluggers may be employed for drilling downward into the bench below, or holes may be drilled both up and down, as in figure 86. In some mines, only down-hole drilling is done. Where the sublevel interval is large and the benches thick, this may require double or even triple benches, one below the other. By using sectional drill steel, a thick bench may be drilled as a unit. At Flin Flon, Manitoba, bench holes are drilled up to 24 feet in depth. Heavy drifters mounted on a long cross-arm between two 3½-inch columns drill about 40 feet per machine shift in hard, massive sulfide ore. Holes are collared with 3-inch bits and gage reduction is 1/8 inch for each 2 feet of advance.

Drilling practice in shrinkage and cut-and-fill stopes (both overhand systems) is similar. Holes may be drilled vertically or at steep angles upward into the back of the stope with stoper machines, or breasts may be carried in which long, flat holes are drilled with hand-held or mounted hammer drills (fig. 102). The choice between these systems should be governed to a considerable extent by the way in which the ground breaks. This, in turn, may be influenced by a number of factors, prominent among which are the nature and direction of slips, bedding planes, or other planes of weakness in the ore. Another important consideration is the effect that the method of drilling and blasting may have upon the hanging wall and the back of the stope and upon the direction in which the ore is thrown by the blast. Stoper holes are more likely to shatter the back of the stope, and if bottomed close to a wall may shatter and loosen it also. Generally speaking, long, flat breast holes will break more ground per foot of hole and per pound of explosive used than vertical holes, because they have a free underface to break to; also, they will not scatter the broken ore as much as vertical holes. In narrow and moderate-width veins breast holes, if not too long, often can be confined better to the ore, as the trend of the vein can be observed from two sides—the breast and the bottom. There are exceptions to this where sudden changes in the strike of the vein are more pronounced than rolls or changes in dip. In wide stopes the breast-hole system usually is cheaper and often safer because the miner is not standing directly under the ground in which he is drilling. Care must be taken, however, to avoid putting too much burden on the holes, causing the ore to break in large slabs, which will require secondary blasting.

In square-set stoping it usually is desired to break only one set of ground at a time, and care must be taken in pointing and charging the holes to avoid blasting out or breaking the timbers, which are set close to the face. Usually, ground that requires square sets to support the stopes is of such a character that only light blasting is needed, though this is not always the case.

Either breast stoping with mounted machines or steeply inclined holes drilled with stopers may be employed in overhand square-set stopes. At the Bunker Hill and Sullivan mine stoper drills are preferred (fig. 107, D), whereas at the Page mine in the same district breast stoping with light drills is believed to give better control of breaking.metal-mining-method-drilling-with-stopers

Stopers are employed more widely in square-set stopes than are drifters or mounted jackhammers. As in cut-and-fill and shrinkage stoping, breast holes shatter the back less than stoper holes. When drilling horizontal holes, the miner works from inside a timbered set and is fully protected from falling ground by the timber; whereas, with stopers he is often working directly under the ground in which he is drilling vertical holes, or only has partial protection when drilling inclined holes (fig. 109, A).

When stopers are used, no time is lost rigging up on mountings, but in some cases this may be more than offset by the necessity of rigging staging. Several methods of drilling in square-set stopes with stoper machines are shown in figure 109, A.

Steel breakage is generally greater with stopers than with mounted machines, and in hard ground automatically rotated stopers entail greater maintenance and repair expense.

In block-caving, most of the ore is broken by natural forces and drilling is confined to development work (driving of boundary or cut-off drifts, raises, and shrinks), to undercutting operations, and to bulldozing boulders on the grizzly levels. Mounted machines or hand-held jackhammers are used in boundary drifts and stopers in boundary raises and shrinkage stopes. Pluggers are employed for funneling raises on the undercutting level; where undercutting is done by checkerboarding with a series of drifts and blasting out the pillars, hand-held jackhammers are used for drilling the pillars and stopers for drilling the backs of the drifts. Ground suitable for block-caving is usually easily drilled, and mounted machines seldom are required in undercutting operations.

Top-slicing in soft ore requires little drilling, and only a few holes are required to break a 5- or 6-foot round in a large slice (fig. 109, B). Soft ores often can be drilled by hand with auger steel fashioned with a fishtail bit. In recent years light, hand-held machines and auger bits (fig. 109, C) often are employed. In harder ground mounted or unmounted machines and standard steel and bits are employed to drill regular rounds similar to drift and crosscut rounds previously discussed under Development. Rounds in sublevel caving slices are similar to those for top-slicing under comparable ground conditions.

Stope drilling often is not as efficient as it might be; actual drilling time commonly occupies but 2½ to 3½ hours during an 8-hour shift, even when much more drilling could be done if there were time for it. This is due partly to the necessity of barring down the back and making it safe before drilling, an operation that may require 1 to 2 hours. In some stopes, drills, mountings, and hose have to be dragged in over a rough, uneven muck pile and connected and machines rigged up for drilling (when mounted machines are employed) or staging installed. In timbered stopes much of the shift may be used for timbering. Upon completion of the round, drills, steel, and gear have to be removed and the holes charged and blasted. All these operations are essential, and the resultant loss of drilling time is inherent in some stoping methods. With other methods, such as sublevel stoping and underhand bench or “glory-hole” stoping or in drilling pillars, it is often possible to drill for several shifts before blasting, thus increasing the ratio of actual drilling time to total time. Even with shrinkage, cut-and-fill, or square-set stoping, by making changes in practice to permit drilling over a considerable area before blasting, the actual drilling time per shift often can be increased.

Delays occasioned by shortage of drill steel, taking steel into the stopes, going out during the shift for hose or pipe couplings or other small items of equipment or for tools are common in many mines. Such delays often can be reduced by more careful attention on the part of the foremen and stope bosses to distribution of materials and supplies or by providing better facilities for taking timber and supplies into the stopes.

Where detachable drill bits have been found suitable for drilling, their use may save much time in handling drill steel. One company that uses about 6,500 steels per day expects to save $50,000 per year in “nipping” charges alone by substituting detachable bits for conventional forged-steel bits.

Published data correlating speed of drilling or rate of penetration, actual drilling time, and tons of ore broken per machine shift in stopes are meager. However, a few typical data on feet drilled per shift and tons of ore broken per machine shift are given in table 33.

Considerable dry drilling is still done in some mines, but this practice is dangerous to health and should be discouraged.



Blasting in stopes is designed not only to break the ore from the solid but to obtain its proper fragmentation, and the explosive should be selected and the holes loaded with this in mind. Methods of pointing and loading the holes that result in throwing down large slabs or blocks of ore without proper fragmentation are not efficient. Unless the ore is broken small enough by the blast to pass the grizzlies and ore chutes, secondary breaking by plugging and reblasting boulders or by sledging must be resorted to. “Mud-capping” of boulders is sometimes employed but is not considered good practice. Blasts that break the ore into large slabs are also more likely to loosen other slabs and so produce a dangerous condition.

As a rule, blasting in stopes is done at the end of the shift. Where the second shift follows the first immediately, blasting is done only at the end of the second shift; there is then a period of several hours for smoke to clear and dust to settle before men reenter the stope. Regulations at some mines provide that blasting shall be done only at the end of the shift. At others, great loss of efficiency would result from such a rule. In any event, where blasting must be done during the shift, adequate ventilation for quick removal of smoke and dust should be provided. Secondary blasting on grizzlies usually requires that blasting be done during the shift, whereas bulldozing of blocks in the stopes usually can be done at the end of the shift, although occasionally it may be done at lunchtime.

Gelatin dynamites usually are employed for stope blasting because of their low production of noxious gases, and ammonia dynamites frequently are employed for dry work. Straight dynamites should never be employed in confined stopes, and it is best not to use them underground even under most favorable conditions of ventilation on account of the fume produced.

Explosives of 25- to 60-percent equivalent (based on straight dynamite) strength are employed, 40-percent strength being by far the most common. Detonation is by fuse and cap or electric detonators. Most mines cling to the fuse and cap except for large blasts— that is, blasts in which a large number of holes are shot simultaneously or in rapid sequence.

The sequence of firing many shots in a single blast can be controlled more certainly with electric delay denotators than with fuse and cap, and no fume is produced by the firing medium, as there is with fuse. For firing a large number of shots simultaneously, as in blasting out posts in a top slice, or firing a single row of holes around a bench, some favor the use of Cordeau.

Under some conditions space blasting is used to obtain better fragmentation, as in cut-and-fill stopes in massive sulfides at Cananea. The use of 40-percent explosive gave a heaving action that threw down many huge boulders, and substitution of 60-percent explosive helped this somewhat. Results of space blasting with 60-percent explosive were still better. In loading the holes, individual sticks of explosive were separated by 1-inch round wood sticks 10 inches in length. Thus, a stick of explosive was followed by a wood stick, on top of which the next stick of explosive was tamped, and so on. The hole was then sealed with two or three sticks of stemming. Since the wood sticks were only 1 inch in diameter and the hole diameter was 1 5/8 to 2 inches, detonation was complete, yet the force of the explosion was distributed along the hole instead of being concentrated in the bottom.

A similar result may be achieved by employing an explosive of low bulk strength, using more cartridges distributed along the length of the hole rather than fewer sticks of high bulk strength concentrated in the lower part of the hole.

Fragmentation, of course, can be improved in many instances by spacing the holes more closely, drilling more holes, and loading lighter, but the additional cost of drilling must be balanced against the additional cost of secondary breaking with fewer holes.

In cut-and-fill stoping it usually is necessary to lay a tight flooring on top of the fill before blasting to keep the ore from mixing with the filling material. This becomes especially important if the ore is high-grade or if the mineral is concentrated in the fines.

In deposits where square-set stoping must be employed, the ground usually is of such nature that comparatively light blasting is required for fragmentation, and the same is true of soft-ore mines employing top-slicing and sublevel caving. Care must be taken, however, to avoid blasting out or breaking the timber, as previously pointed out. Often it is necessary to brace the timbers to prevent blasting them out and to cover the floor (in square-set stopes) with loose ore or lagging to prevent breakage by chunks of ore. In square-set stoping, holes usually are placed and loaded to take advantage of seams and slips in the ore rather than according to a standard system.

The use of stemming in all holes is advocated by explosives manufacturers to achieve better detonation and greater efficiency, and to abate fume production. Tests made some years ago by the Bureau of Mines indicate that “The use of stemming helps decidedly to prevent formation of boulders, does not increase the cost of blasting perceptibly. The tests show that one stick of stemming to a hole is not enough and that at least two should be used.” Further tests showed that stemming will almost always result in some saving of powder, and less poisonous gas is produced when it is used. At present (December 1938) exhaustive tests are being made at the Mount Weather testing adit of the Bureau of Mines to check the earlier work. The extra danger and difficulty inherent in firing missed holes when employing stemming partly offset the advantages claimed for is use. Whatever the virtues of stemming may be, the fact remains that most stope holes are loaded without stemming and, in the judgment of many operators, the effect of stemming on both blasting efficiency and gas reduction is small.metal-mining-method-bulldozing-chamber

Considerable secondary blasting is required in connection with block caving, forced caving, and sublevel stoping in hard ore. It is usually done on grizzlies or in bulldozing chambers situated between the undercutting level and haulage level (see figs. 90 and 97). Blasting directly on the grizzlies is destructive to the grizzly bars and settings, and the recent trend is toward blasting in chambers on one side of the grizzly and transferring the reblasted ore by scrapers to the grizzlies.

Figure 110 shows a bulldozing chamber and details of grizzly construction at the Alaska-Juneau mine in 1928. In that year, 71 percent of the explosive consumed was used in secondary breaking, 13 percent in development, and 16 percent in stope blasting (in powder drifts, see fig. 89).

In shrinkage and cut-and-fill stoping, large boulders usually are reblasted in the stope. If they become covered with finer muck in shrinkage stopes they may be missed, however, and finally show up at the chutes, where blasting may result in injury to the chute gates and timbers.

Boulders may be either block-holed by drilling with hand-held machines (or stopers, if boulders are wedged behind the brow of a draw raise) and loaded with a light charge and blasted; or they may be broken by “mud-capping,” which consists in placing a charge of explosive on top of the rock to be blasted and plastering it down with clay or wet fine material.

Block-holing is the approved method, from a safety standpoint, and powder consumption usually is appreciably less than when breaking is done by mud-capping. Furthermore, if several mud-capped boulders are blasted together the concussion from the first one to go may throw the charge off one or more of the others.

Data on explosives consumption in stoping are presented in table 34.