IC9149
COAL MINE BUMPS: FIVE CASE STUDIES IN THE EASTERN UNITED STATES
By Alan A. Campoli(Mining engineer), Carla A. Kertis(Geologist), and Claude A. Goode(Manager of Test Facilities. Pittsburgh Research Center. Bureau of Mines, Pittsburgh, PA.)
UNITED STATES DEPARTMENT OF THE INTERIOR
Donald Paul Hodel, Secretary
BUREAU OF MINES
David S. Brown, Acting Director
ABSTRACT
This Bureau of Mines study was conducted to obtain a better understanding of the coal mine bump problem and its effect on underground coal mining in the Eastern United States. To accomplish this, information was collected on the geologic conditions, mining techniques, and engineering parameters at five bump-prone mines. Two geologic conditions have been found to cause the occurrence of bumps in the Eastern United States: (1) relatively thick overburden and (2) extremely rigid strata occurring immediately above and below the mine coalbed. Additionally, the probability of bump occurrence is increased by certain mining practices that concentrate stresses during retreat mining, in areas where geologic conditions are conducive to bumps. Mining plans that permit the development of pillar line points or long roof spans that project over gob areas should be avoided because these features may contribute to the occurrence of bumps.
INTRODUCTION
A review of literature and accident reports on violent failures in coal mines reveals confusion as to the definition of the type of failure involved. Violent failures in coal mines may be classified as bounces, bursts, and outbursts. A bounce is the sudden forceful impact or vibration of a coal pillar, which may be accompanied by rib or face sloughage. A burst is the instantaneous explosive failure of coal or associated strata. An outburst is the spontaneous ejection of coal and gas from the solid face. The coal is pulverized in the process. The gas released is a mixture of predominantly methane and carbon dioxide. Outbursts result in a cavity ahead of or to one side of the entry. During an outburst, large quantities of gas are emitted. Subsequently, there is a rapid reduction in the gas emission rate with time. This paper deals with bursts encountered during retreat coal mining. Because «bump» is the term applied to this type of failure in the Eastern United States, the term will be used throughout this paper. Retreat coal mining concentrates stresses on the pillars directly outby gob areas. This stress situation is made worse when mining is conducted in areas encased in rigid associated strata. Overlying strata form cantilever beams over adjoining gob areas that transfer pressure onto adjacent outby pillars. Available data show bumps have caused 49 accidents from 1978 to 1984 and resulted in 14 fatalities from 1959 to 1984 in the eastern States of Kentucky, West Virginia, Pennsylvania, and Virginia.
ACKNOWLEDGMENTS
This work could not have been accomplished without the help of many people well acquainted with coal mine bumps. Cloyde Blankenship, mining engineer, MSHA, Princeton, WV; Anthony Zona, chief, roof control division, MSHA, Bruceton Safety Technology Center, Pittsburgh, PA; Harry Harmon, district engineer, Frank Bacho, senior engineer, and Robert Pawlowski, geologist, U.S. Steel Co., Gary District Office, Gary, WV; A. R. Christian, administrative manager, Charles Couch, superintendent, Rick Bonham, chief engineer, and Gerald Lucas, safety director, Milburn Colliery Co., Burnwell, WV; Kenneth Cooper, general manager, John T. Clark, chief engineer, and Francis Oliver, safety director, W-P Coal Co., Omar, WV; James R. Vilsek, general manager, Leonard P. Mokwa, manager of engineering, Island Creek Coal Co., Virginia Pocahontas Div., Oakwood, VA; Dan Ashcraft, director of coal mines, K. V. Rao, chief mining engineer, LTV Steel Co., Pittsburgh, PA; Martin Valeri, general superintendent, Dwight Strong, superintendent, Don Winstone, chief engineer, Chandra Sharma, senior mining engineer, Olga Coal Co., Coalwood, WV; and Martin Hayduk, mining consultant, Peterstown, WV, provided the valued information, insight, and advice that made this publication possible.
PILLAR LOADING AND BUMP MECHANISMS
Coal and adjoining rock, when subjected to an increasing load, such as is imposed by an approaching pillar line, adjust by deformation and fracturing of the roof, floor, and coal pillars. Occasionally the ground failure is catastrophic. When this occurs, coal may be expelled violently from the pillar. In some areas the floor may heave suddenly. The failure is usually accompanied by a very loud report, and tremors or vibrations that can be detected some distance away are set up in the surrounding earth and in the mine atmosphere. A failure of this kind may involve only a single pillar, part of a pillar, or several pillars, with varying degrees of violence. Such failures usually occur in the vicinity of a pillar line in a room-and-pillar mining panel, or at or near the face in an advance or retreat longwall mining panel. Several geological conditions are believed to cause bumps in the eastern U.S. coalfields. The overburden is 500 ft or more thick. A strong, overlying stratum, usually a massive sandstone or a conglomerate, occurs immediately above or close to the coalbed. The floor is strong and does not heave readily. These assumptions were drawn from an examination of 117 bump incidents during the period from 1925 to 1950, performed by Holland and Thomas (4). The case studies that follow reaffirm many aspects of their work. The size and configuration of coal mine pillars are determined by the function they are to fulfill. They may be required to support the overburden to minimize surface subsidence or to prevent the ingress of water from adjoining workings. In these cases the pillars are usually wide and exceed the width required to support the overburden. Oversized pillars may also be required to provide a barrier to shield important main underground roadways from structural damage. Ventilation or haulage requirements on advance may force the pillar geometry away from the optimum design for retreat mining. Mining under heavy cover with strong, competent adjacent strata that may cause coal bumps to occur is better accomplished using a yield pillar design to prevent dangerous accumulations of stress (2). When an opening is developed in a coalbed, a portion of the natural ground support is removed, and the load of the roof over the mined out area must be carried by the coal that remains. The floor also reacts to that added load through the coal. The natural tendency of the roof, floor, and coal pillars is to close this opening. In actuality, coal pillars bearing substantial load will deteriorate, resulting in perimeter yielding and sloughing. This widens the unsupported span and transmits an additional load onto the remaining structurally competent coal. Figure 1 is an idealized illustration of the adjustment of the stress field to the loss of equilibrium and the creation of high loading at the edge of the coal pillar because of stress concentration.
Figure 1. Adjustment of stress around a single entry.
The load transferred to a pillar is determined by the percent of extraction and the thickness of the overburden. The stress distribution in the pillar, however, is governed by the physical properties of the roof, floor, and coalbed, along with pillar design geometry. The probable stress distribution on a wide pillar is idealized in figure 2. Idealistically, the pillar has enough roof contact area to carry the load without failure and sufficient floor bearing area to resist the load. It is further postulated that the roof and floor are very resistant to yielding. Since coal generally is a friable material, the edges of the pillar yield. Thus, the stresses are low at the yielding edges of the pillar and increase rapidly over a short distance into the core of the pillar. The state of stress in the core zone of the pillar is a function of its width and the length of time it has been supporting the roof. In a wide pillar it is postulated that the stress level is substantially lower in the pillar core than near the edges (3).
Figure 2. Adjustment of stress around a wide pillar.
Figure 3 indicates the idealized stress pattern over a narrow pillar. As a narrow pillar takes load, the pillar yields and the roof and floor tend to converge. Under this condition, the yield pillar is incapable of carrying subsequent loadings. As a result, solid coal bears the additional weight. The formation of a secondary arch as shown in figure 4 is time dependent, being a function of the nature of the strata (3). The pillar loading hypotheses just presented for development of a pillar section are similar for retreat mining, with the addition of abutment zone forces. While the stress distribution in the gob is difficult to measure, the effect of the associated abutment pressures on the active pillar section is indicated by convergence directly outby the pillar line. Roof-to-floor convergence, brought on by the nearing pillar line, represents the total movement of the roof, floor, and pillar system. Depending on the physical properties of the coalbed, adjacent strata, and the depth of cover, the lateral extent of the zone of convergence may vary from a few tens of feet to hundreds of feet. In the Pocahontas No. 4 Coalbed, to be discussed in two of the case studies that follow, massive sandstone roof, combined with a friable coalbed,leads to cantilever loading and zones of convergence 300 ft outby the pillar line. Coal pillars exposed to high abutment zone pressures will yield or support the load, depending on their size and strength.
Figure 3. Adjustment of stress around a narrow pillar.
Figure 4. Adjustment of stress due to the yielding of a narrow pillar.
A bump hazard may develop in a pillar of intermediate size, especially when the pillar is surrounded by smaller yielding pillars. The intermediate-sized pillar in the Pocahontas No. 4 Coalbed is generally 160 by 160 ft square (2). A pillar of this size may yield around its periphery. The yielded coal around the perimeter confines the pillar core. Figure 5 is an idealized plan view of the conditions in such a pillar. The lateral forces exerted by the pressurized core are counterbalanced by the lateral confinement provided by the yielded perimeter.
- Yield zone
Figure 5. Idealized diagram of core confinement loading of a critical size pillar.
REFERENCES
1. Holland, C. T., and E. Thomas. Coal-Mine Bumps: Some Aspects of Occurrence, Cause, and Control. BuMines B 535,1954, 37 pp.
2. Hayduk, M. (US. Steel). Private communication, 1985; available upon request from A. A. Campoli, BuMines, Pittsburgh, PA.
3. Goode, C. A., A. Zona, and A. A. Campoli. Controlling Coal Mine Bumps. Coal Min., v. 21, No. 10, 1984, pp. 48-53.
4. Hennen, R. V. Wyoming and McDowell Counties. WV Geol. Surv. County Rep., 1915, 783 pp.
5. Blankenship, C., and A. T. Castanon. Multiple Fatal Bump Accident (Outburst). MSHA (4015 Wilson Boulevard, Arlington, VA 222031, 1983, 17 pp.
6. Miller, T. C., and R. Sporic. Development of a Hydraulic Device for Measuring Relative Pressure Changes in Coal During Mining: A Progress Report. BuMines RI 6571, 1964, 13 pp.
7. Talman, W. G., and J. L. Schroder, Jr. Control of Mountain Bumps in the Pocahontas No. 4 Seam. Min. Eng. (Littleton, CO), Aug. 1958, pp. 877-891; Sept. 1958, pp. 982-1004B.
8. West, M. L., and C. E. McGraw. Report of Fatal Coal-Mine Bump Accident. MSHA (Drawer AA, Richlands, VA 24641), 1974, 12 PP.
9. Diamond, W. P., and J. R. Levine. Direct Method Determination of the Gas Content of Coal: Procedures and Results. BuMines RI 8515, 1981, 36 pp.
10. Kissell, F. N. The Methane Migration and Storage Characteristics of the Pittsburgh, Pocahontas No. 3, and Oklahoma Hartshorne Coalbeds. BuMines RI 7667, 1972, 22 pp.
11. Davis, J. E. Fatal Outburst of Coal Accident. MSHA (P.O. Box 112, Mt. Hope, WV 25880), 1983, 26 pp.
12. Hennen, R. V. Fayette County. WV Geol. Surv. County. Re.p. , 1919, 1002
13. Grose. H. S. Nonfatal Fall of Rib Accident (Coal Outburst). MSHA (p.0: Box 112, Mt. Hope, WV 258801), 1984, 9 pp.
Library of Congress Cataloging-in-Publication Data
Campoli, A. A. (Alan A.)
Coal mine bumps.
(Information circular ; 9149)
Bibliography: p. 34.
Supt. of Docs no: I 28.27:9149.
1. Coal mines and mining-United States-Accidents.
2. Rock bursts.
I. Kertis, Carla A. II. Goode, Claude A. III. Title. IV. Series: Information circular (United States. Bureau of Mines);9149.