Coal mine bumps as related to geologic features in the northern part of the Sunnyside District, Carbon County, Utah

Abstract

Coal mine bumps, which are violent, spontaneous, and often catastrophic disruptions of coal and rock, were common in the Sunnyside coal mining district, Utah, before the introduction of protective-engineering methods, modern room-and-pillar retreat mining with continuous mining machines, and particularly modern longwall mining. The coal at Sunnyside, when stressed during mining, fails continuously with many popping, snapping, and banging noises. Although most of the bumps are beneficial because they make mining easier, many of the large ones are dangerous and in the past caused injuries and fatalities, particularly with room- and-pillar mining methods used in the early mining operations. Geologic mapping of underground mine openings revealed many types of deformational features, some pre-mine and some post-mine in age. Stresses resulting from mining are concentrated near the mine openings; if openings are driven at large angles to small pre-mine deformational features, particularly shatter zones in coal, abnormal stress buildups may occur and violent bumps may result. Other geologic features, such as ripple marks, oriented sand grains, intertongued rock contacts, trace fossils, and load casts, also influence the occurrence of bumps by impeding slip of coal and rocks along bedding planes. The stress field in the coal also varies markedly because of the rough ridge and canyon topography. These features may allow excessively large stress components to accumulate. At many places, the stresses that contribute to deformation and failures of mine openings are oriented horizontally. The stratigraphy of the rocks immediately above and below the mined coal bed strongly influences the deformation of the mine openings in response to stress accumulations. Triaxial compressive testing of coal from the Sunnyside No.1 and No.3 Mines indicates that the strength of the coal increases several times as the confining (lateral) stress is increased. Strengths of cores cut from single large blocks of coal vary widely. Although the strengths of coal cores increase slowly at high levels of confining stress, the coal in Sunnyside No. 1 Mine is slightly stronger in laboratory tests than coal in Sunnyside No.3 Mine. The coal in No.1 Mine probably can store larger amounts of stress than coal in the No.3 Mine, which may account for the apparently greater number of violent bumps in No.1 Mine. The strength of coal, and its ability to store stress before failure, may correlate in part with chemical composition, particularly with the amounts of benzene ring compounds in vitrain; coal with relatively large amounts of benzene ring compounds is stronger than coal with lesser amounts of these compounds. Alternatively, the chemical composition of coal may affect its response to stress. Increasing contents of kaolinite in coal appear to reduce its compressive strength at low confining stresses, resulting in easy failures of pillars and ribs in mine openings. Applications of the geologic factors outlined in this report, carefully coupled with advanced modern engineering methods, have markedly reduced the hazards from coal mine bumps and related failures of mine openings at Sunnyside. Similar studies probably could aid in reducing bump-related hazards in other coal mining areas.