Laserfiche WebLink
<br />58 <br /> <br />MCC's predictions of possible subsidence are based on historical observation <br />from past mining, conceptual analytical modeling (relation between extraction <br />height and workings depth, adjusted for lithologic variation), and numerical <br />modeling (computed influence function). MCC predicts the angle of draw for <br />longwall mining in the E seam in the Dry Fork lease area will be 21 degrees. <br />(The angle of draw is the angle between a vertical line at a panel edge and a <br />line extending from the panel edge to the point of zero subsidence at the <br />ground surface.) MCC predicts the angle of draw for longwall mining in the <br />E seam in the South of Divide area will be between 10 and 20 degrees. MCC <br />predicts 95 percent of subsidence will have occurred at a location in the Dry <br />Fork lease area when the longwall face has moved from the location a <br />distance equal to 1.0 to 1.2 times the depth of mining. The depth to mining in <br />the Dry Fork lease area will range between 800 and 1,400 feet, with maximum <br />vertical displacement on the land surface of 7.0 feet. The depth to mining in <br />the South of Divide area will range between 375 ft. and 1,300 ft. Maximum <br />surface crack depth is predicted to occur in brittle sandstone ridges, as <br />observed elsewhere in the permit area, with maximum crack depth of 50 feet. <br />Maximum crack depth is predicted to be 5 to 15 feet on gently sloping land <br />(«30%). Surface cracks are predicted to not occur where mining depth is <br />several hundred feet and alluvium is more than a few feet thick. <br /> <br />Possible subsidence effects on ground water has been previously discussed in <br />this Findings document under the heading “Probable Hydrologic <br />Consequences”. <br /> <br />MCC predicts the mining nearest State Highway 133 (600 feet horizontal <br />distance) will probably not re-activate existing landslide deposits in the area <br />because the mining there consists of room-and pillar development entries <br />which have a relatively small subsidence potential. The angle of draw of <br />longwall mining activity does not intersect landslide bodies in the area. MCC <br />monitors monuments it has installed on a landslide mass in the mine’s surface <br />facilities area. <br /> <br />Monument Dam and Minnesota Reservoir – MCC predicts the dam or <br />reservoir will not be subsided by mining because the nearest mining will be <br />800 ft. away in panel E9. The angle of contact between the reservoir is 69 <br />degrees, significantly more than the 20-degree predicted maximum angle of <br />draw in the South of Divide area. MCC has committed to monitoring the <br />angle of draw of panel E9 in order to verify the prediction. MCC predicts that <br />ground vibration (seismicity) created by longwall mining could affect the <br />dam, the reservoir, and the landslide abutting the dam because the static safety <br />factor of the dam has been found to be less than 1.0, as shown in an analysis <br />conducted by MCC’s consultant and contained in Exhibit 72 of the permit <br />application. The suspected cause of the relatively low factor of safety is a <br />landslide that underlies the dam’s south abutment. Possible effects from <br />mining-induced seismicity are: rockfall into the reservoir; overtopping of the <br />dam by water in the reservoir; catastrophic failure of the dam; flooding, <br />sediment deposition, and erosion downstream from the dam; flood damage to