Effects of coal mine subsidence in the western Powder River basin, Wyoming

Abstract

Analyses of the surface effects of past underground coal mining in the western Powder River Basin suggest that underground mining of strippable coal deposits will damage the environment more over long periods of time than will modern surface mining, provided proper restoration procedures are followed after surface mining. Subsidence depressions and pits are a continuing hazard to the environment and to man's activities 5-20 km north of Sheridan, Wyo., above mines that were abandoned 25-80 years ago where the overburden is weak and is less than about 60 m thick. In addition, fires commonly start by spontaneous combustion when water and air enter the abandoned mine workings via subsidence cracks and pits. The fires can then spread to unmined coal as they create more cavities, more subsidence, and more cracks and pits through which air can circulate. In modern surface mining operations the total land surface underlain by minable coal is removed to expose the coal. The coal is removed, the overburden and topsoil are replaced, and the land regraded and revegetated. The land, although disturbed, can be more easily restored and put back into use than land underlain by abandoned underground mine workings in areas where the overburden is less than about 60 m thick. The resource recovery of modern surface mining also is much greater than that of underground mining procedures. Although present-day underground mining technology is advanced as compared to that of 25-80 years ago, subsidence resulting from mining of thick coal beds beneath overburden less than about 60 m thick by underground methods can cause greater damage to surface drainage, ground water, and vegetation than can properly designed surface mining operations. This report briefly discusses the geology and surface and underground effects of former large-scale underground mining in a 50 km2 area north of Sheridan, Wyo. and describes some environmental consequences and problems caused by mining.