Geologic Structures That Affect Coal-Mine Roof Stability

Abstract

The US Bureau of Mines has investigated several geologic structures that plague underground coal mining. These structures have caused numerous roof falls and, consequently, many miners have been hurt or fatally injured. Hazardous geologic anomalies that can be encountered in the eastern and western US coal fields include slips, hill seams, slickensides, sedimentary dikes, kettlebottoms, and paleochannels. Slips occur throughout the United States and are high-angled fractures in the roof rock. Typically, the rock adjacent to slips is highly slickensided and striated. Hill seams are weathered fracture zones and usually are found in areas where the overburden is shallow. Due to their nature and regional mining conditions, hill seams usually are found only in the southern Appalachian coal basin. Slickensides are lower angled planes of weakness found in coal-mine roof rock throughout the country. Slickensides usually are found in clusters and the adjacent rock is highly polished and striated. Sedimentary dikes (also clastic dikes and clay veins) are the remnants of ancient fissures that have been filled through sedimentary processes, Sedimentary dikes normally are found in coal basins that were slowly subsided. Kettlebottoms are the fossilized remains of trees that once grew in and above the swamp forest and generallymore » are found in coal basins that were rapidly subsided. Paleochannels are the lithified remains of rivers and frequently are found in all coal basins except the Black Warrior.« less