Prediction of subsidence effects caused by longwall coal-mining in Pike County, southwest Indiana

The Springfield Coal (No.5) Pike County, Indiana, currently surface mined, will require underground mining to the west. With higher extraction ratios, longwall mining is preferred over room and pillar. Deformation from the longwall yields surface subsidence troughs causing damage to rural and urban areas and disrupting groundwater flow. Damage is caused by change in surface slopes, differential vertical displacement and horizontal strain with tension fractures. Subsidence effects depend on mine layout and existing geological conditions, mainly depth and thickness of extracted coal seam and percentage of hard rock in the overburden. Three specific sites were designated for study based on an expectation of mining and available information; one provided exploratory drilling. Coal depth ranged from 100 to 389 feet and thickness from 3.4 to 10 feet. Overburden consisted mostly of shale with 10 to 70% sandstone/limestone; RQD ranged from 75 to 90%. Subsurface geology was compiled and illustrated by subsurface maps, cross sections and a fence diagram. Geotechnical rock-core logs aided laboratory testing that included unconfined compressive strength, elastic modulus, point load, Brazilian tensile strength and slake durability. Geotechnical properties of rock strata were evaluated regarding response to underground mining and caveability. Caveability of roof rock was classified as self- supporting with little caving. Caved zone for Site No. 1 will be 4 to 5 times the mining height and the fractural zone will extend very close to the ground surface. Three subsidence prediction approaches were applied to this study and to some case histories. Comparisons were made of profile shape and maximum subsidence measured in the field, and that obtained from prediction methods. Maximum subsidence values were predicted fairly well by one or two approaches but shapes of the predicted profiles poorly matched measured profiles. A relationship was established between measured and predicted maximum subsidence. For the study area maximum subsidence ranged from 1.26 to 7.51 feet, and angle of draw from 19 to 30$\sp\circ$. Subsidence deformation indices were calculated using Budryk-Knothe\u27s influence function. Maximum slopes were consistent, 4.25 to 4.94%. Predicted ground surface curvature was greater for the shallower site (No.3). Results show that predicted curvature and horizontal ground strain are directly related to the maximum subsidence and inversely related to depth of coal seam. Relationships for predicted maximum deformation indices with the geologic/mining factors were established. Damage to structures from subsidence was predicted based on two approaches. In the first, only structures of one story, less than 5 meters high, would not sustain damage. For the other, only structures with a length of 5 to 7 meters or less would be safe