Mining Publication: Geologic Factors Causing Roof Instability and Methane Emission Problems: The Lower Kittanning Coalbed, Cambria County, Pa
A geologic study of Mines 32 and 33 in Cambria County, Pennsylvania, was conducted as part of a methane control research program. Areas of high methane emission and roof instability are encountered; these are related to the structural and stratigraphic features that characterize the geologic setting of these mines. Locally the folds trend N 20 deg E +/-2 deg, the direction of the butt cleat in the Lower Kittanning coal. The face cleat strikes at N 69 deg W +/-2 deg and the systematic rock joint in the surface bedrock strikes N 67 deg W +/-2. This suggests that a directional stress oriented N 70 deg W produced the folds and influenced the directions of coal cleat and systematic joint sets. Analysis of side-look airborne radar (SLAR) indicates three directions of prominent regional lineation: N 39 deg E, N 31 deg W, and N 70 deg W. Little gas was encountered during development mining; large volumes of gas are encountered when roofs fall. Most of the gas comes from strata above the mined coal; therefore, to control methane, roof instability must be controlled. Roof falls and high rates of methane emission are most likely where less than 30 feet of many thin layers of limestone, shale, and sandstone occur between the Lower Kittanning coal and the next coal and in zones along the margins of a sandstone channel.