Blasting unconfined, sheathed explosives in coal mines: safe, fast, and effective way of clearing falls on longwall faces.
SAFE, FAST, AND EFFECTIVE WAY OF CLEARING FALLS ON LONGWALL FACES
Mud blasting, the handy method of breaking boulders by placing an explosive charge on their surface, tamping it into place with a lump of mud, and firing, is banned in many coal mines, including those of the United States. The danger of the unconfined charge igniting an explosive atmosphere is too great. The lack of recourse to this inexpensive, simple method means that breaking boulders and clearing other obstructions is much more time-consuming and expensive in most coal mines than it is in hard-rock mines.
Sheathed explosives, those surrounded with an inert powder that, on ignition, forms a barrier between the flame front and the atmosphere, have long been used for safe shot-hole blasting in coal mines. Since 1981, the U.S. Bureau of Mines has been involved in the development of a sheathed-explosive charge that can be safety fired unconfined in underground coal mines. This charge has a variety of applications, such as clearing large rocks from roof falls, bringing down loose roof slabs and hanging brows, and removing cribs[1-6].
The Bureau's sheathed-charge design is a 7-in.-dia disk roughly 1-in. deep. There is a damp salt (NaCl) layer coating the outer surface of the charge. This salt spreads out as a dust cloud ahead of the explosion shock-wave and serves as a barrier between the flame and the atmosphere.
Based on its prior experience with the sheathed charge, the Bureau has worked with the Mine Safety and Health Administration (MSHA) on the development of a test schedule for the approval of sheathed-explosive charges as permissible explosives and on regulations governing their safe use. Federal regulations providing for the approval and use of sheathed charges are now in effect. At present, one explosives company has submitted a sheathed explosive charge for approval by MSHA. This charge has passed all the tests for approval, and is expected to be available on the market this year.
In the fall of 1988, the Bureau entered into a cooperative agreement with Jim Walter Resources, Inc. (JWR) to evaluate the use of the sheated charge to deal with problems encountered in JWR's Blue Creek No. 4, No. 5, and No. 7 mines in Brookwood, Ala.
The Blue Creek seam, 70-95 in. thick, is mined by eight longwalls. Above the Blue Creek seam is a thinner seam of about 12 in. in thickness separated by a rock binder of a variable thickness. Ideally, JWR mines the Blue Creek seam but leaves the upper seam and the rock binder in place. Occasionally, the rock binder thins out to the point where it and the upper coal seam cannot be held in place, and both fall. The face conveyor can usually carry the fallen rock to the feeder-breaker where it is crushed. But sometimes, the pieces of rock are so big that they jam the feeder-breaker and shut down the longwall. In extreme cases, the rocks are so large that the face conveyor cannot carry them or they damage the longwall supports.
The traditional practice for dealing with this problem is to drill and shoot the rock, a process that takes 1-2 hours. The purpose of our research was to determine whether use of the sheathed charge in this application would be more efficient and safer than drilling and shooting.
It was found that the use of the sheathed charge reduced the longwall downtime for clearing oversize boulders to 15-30 min--a big saving compared to the hour or more needed to drill and shoot. Firing the sheathed charge at the face caused no significant damage to longwall equipment. Use of the sheathed charge also represented an improvement in safety by eliminating the need for the miner to climb onto the rock pile on the face conveyor and expose himself to the hazards of the face area for an extended period of time while drilling the rock. Another benefit which is difficult to quantify, is the elimination of the temptation to shoot illegal mudcaps or adobes.
At the time this research was conducted, regulations permitting the use of the sheathed charge in underground coal mines were not in effect. MSHA and the Alabama state authorities gave special clearance to conduct the experimental tests as long as Bureau personnel supervised the firing of all charges.
All trials with the sheathed explosive charge were conducted according to standards that formed the basis of the eventual regulations. These included the use of a separate detonator for each charge, rock dusting of roof, ribs, and floor for a 40-ft radius around the blast, no more than three charges to be set off at the same time, and no charges to be fired in contact with one another.
TESTING AT BLUE CREEK
At Blue Creek, the sheathed charge was used in applications such as breaking large rocks on the face, breaking large boulders left from an overcast shot, shooting off the corner of an overhang left from an overcast shot, breaking large boulders from a rib slump, crevice shooting of ribs about to slump, and knocking out a variety of roof supports.
As a preliminary, a series of underground shots were fired at the site of a previous overcast shot in the Blue Creek No. 5 mine, mainly to familiarize the miners with the use of the sheathed charge and to observe how well the charge would break rock of the type found in the JWR mines. The first rock broken with sheathed charges measured about 8 x 4 x 2 ft. The debris was cleared from the surface of the rock prior to placing the charges to ensure good contact with the rock surface. As shown in Fig. 1, three charges were placed atop the rock and primed with instantaneous detonators. The area was well rock-dusted so additional rock-dusting was not necessary. In this trial, bags of rock dust were placed on top of the sheathed charges to determine whether the additional confinement would improve their effectiveness. The three charges broke about two-thirds of the rock; one large piece measuring about 4 x 3 x 1.5 ft remained.
One more sheathed charge shattered the remaining piece of rock. For this trial the charge was covered with loose rock dust to evaluate the effect of this partial confinement on performance. Two sheathed charges were also placed atop another rock measuring approximately 6 x 3 x 2 ft; no rock dust was placed on top of the charges in this case. All three charges were wired in series and fired simultaneously, shattering the rocks quite effectively.
It appeared that the two bare sheathed-charges were more effective than either charges covered with bags of rock dust or the charge covered with loose rock dust. There is no guarantee that this will always be the case, but the results do suggest that covering the charges with rock dust may be unnecessary and possibly inefficient. This practice did seem to reduce airblast though.
The next series of shots were fired along the longwall in the Blue Creek No. 4 mine. These shots represented the main goal of clearing large rocks that jammed the face conveyor or the feeder-breaker as illustrated in Fig. 2.
The face area was rock dusted within 40 ft of the feeder-breaker, and two sheathed charges were placed on the large rock causing the jam. The charges shattered the rock and longwall operations promptly resumed (Fig. 3). The entire operation of placing and firing the charges, including rock dusting, took about 15 min. Some concern was expressed about the possibility of damage to the roof supports or pan conveyor; no damage was observed other than damaging one light on the longwall supports.
The next shot was very similar to that described above except that the face conveyor was stopped before the rock had a chance to get stuck at the feeder-breaker. The rock in this case measured about 10 x 3 x 3 ft. Fig. 4 shows the rock with two charges in place ready for firing. The charges shattered the rock very effectively (Fig. 5). Again the entire operation required only about 15 min and caused no damage to any equipment.
The third trial involved two large rocks (approximately 5 x 4 x 2 ft and 8 x 3 x 2 ft) that had fallen onto the conveyor. In this case, two sheathed-charges were placed on the larger rock and one on the smaller. Again the rocks were effectively broken with no damage to equipment and with minimal expenditure of time.
Since the sheathed charge had no difficulty breaking any of the rocks tried so far, shots were fired to test the limits of its breaking power. A large rock was located measuring 4.5 x 4.5 x 18 ft, which had slumped from the rib. Three charges were placed on the rock in an attempt to break about two-thirds of it. After firing, inspection of the rock showed that although it had not fallen apart, it was fractured clean through. Two more charges placed on the remaining one-third of the intact rock fractured the rock so that with minimal handling it would break apart into manageable pieces.
Several other shots essentially the same as those described above were also fired on longwalls and in other situations with similar results. The sheathed charge was effective, safe, and minimized lost operating time.
PHOTO : Fig. 1--In the first test of sheathed blasting at Blue Creek, these three charges were
PHOTO : covered with bags of rock dust. The 8 x 4 x 2 ft rock was only partially broken in this
PHOTO : firing. A single charge was enough to break the remaining large piece.
PHOTO : Fig. 2--This oversize jamming the feeder-breaker presented the first chance to try
PHOTO : sheathed charges instead of drilling and blasting.
PHOTO : Fig. 3--The results of two charges on the rock shown in Fig. 2 indicate successful breakup
PHOTO : and significant time savings: what would have taken two hours to blast was completed in 15
PHOTO : min.
PHOTO : Fig. 4--This rock was blasted on the pan conveyor where it fell, to most closely simulate
PHOTO : procedures used in the past at Jim Walter Resources.
PHOTO : Fig. 5--After the two-charge blast the oversize is eliminated, with no significant
PHOTO : equipment damage and little time lost.
Mainiero, R. J., and J. E. Hay. Design Criteria for Sheathed Permissible Explosive Charge for Open Shooting in Flammable Atmospheres. BuMines RI 8703, 1982. Evaluation of a Sheathed Permissible Explosive Charge for Open Shooting in Flammable Atmospheres. BuMines TPR 118, 1982. Prototype Sheathed Explosive Rock-Breaker Charge for Open Shooting in Flammable Atmospheres. Paper in Proceedings of the 13th Annual Institute on Coal Mining Health, Safety and Research. Virginia Polytech. Inst., Blacksburg, Va., 1982, pp. 239-257. U.S. Bureau of Mines. Sheathed Rock-Breaker Explosive Charge. Technol. News 177, July 1983. Verakis, H. C., R. J. Mainiero, and J. J. Mulhern. Safe Practices for Use of the Sheathed Charge in Underground Coal Mines. Paper in Proceedings of the 13th Conference on Explosives and Blasting Technique. Soc. Explosives Eng., Montville, Ohio, 1987, pp. 115-130. Mainiero, R. J. Non-Incendive Rock-Breaking Explosive Charge. U.S. Pat. 4,537,133, Aug. 27, 1985. Federal Register. U.S. Mine Safety and Health Administration (Dep. Labor). Requirements for Approval of Explosives and Sheathed Explosive Units; Final Rule. V. 53, No. 223, Nov. 18, 1988, pp. 46748-46764. U.S. Mine Safety and Health Administration (Dep. Labor). Safety Standards for Explosives and Blasting; Final Rule. V. 53, No. 223, Nov. 18, 1988, pp. 46768-46789.
Richard J. Mainiero, U.S. Bureau of Mines, Research Supervisor Lon Santis, U.S. Bureau of Mines, Mining Engineer
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|Author:||Mainiero, Richard J.; Santis, Lon|
|Publication:||E&MJ - Engineering & Mining Journal|
|Date:||Apr 1, 1990|
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