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SubTerra, Inc. Twen[ymile Coal Company 05/31/00 <br />Northern Mining District Subsidence Evaluation -Final Report <br />4. The start profile for Panel 14-Right was estimated using data from monitoring lines <br />• 1000 and 3000 in the Fish Creek study area (panels 1 LLW and 2LLV~. These data <br />are contained in the semi-annual reports and in prior permit submittals. <br />Subsidence predictions were made for overburden thickness of 800, 1,100 and 1,400 ft <br />representing the range of conditions in the study area. The results of these predictions are <br />contained in Figures 2, 2A, and 3. Summary data include: <br />• Maximum subsidence over 12 Right of 56, 62, and 70 inches for overburden <br />thickness of 1,400, 1,100, and 800 ft, respectively. It is noted that the 800-ft <br />overburden condition only exists at the start of panel 12 Right. <br />• Maximum subsidence over second and subsequent panels of 60 and 67 inches for <br />overburden thickness of 1,400 and 1,100 ft, respectively. <br />• Up-dip and down-dip draw angles of 12° and 21° respectively, which define the limits <br />of subsidence. <br />• Maximum surface strains less than -0.004 (compression) and +0.005 (extension). <br />2.1 Subsidence Monitoring Requirements <br />As noted above, CTCC has developed an extensive subsidence data base from monitoring lines <br />positioned to measure start, transverse, and traveling subsidence profiles. No further <br />systematic subsidence data gathering is therefore warranted. <br />However, it is recommended that CTCC monitor subsidence adjacent to the Energy Spur track, <br />. as part of their remedial activities, and along Fish Creek. Specific recommendations are <br />provided in the following sections. <br />3. Evaluation of Impacts to the Energy Spur Track <br />The Energy spur runs from the mine loading yard, across the eastern mining district to the south <br />of the NMD and thence north-eastwards across Panels 12, 13, 14, 15, and 16-Right. The <br />layout of the Energy Spur track relative to the NMD mine workings is shown on Drawing 1. <br />The track was previously undermined by longwall panels in the Eastern mining district during <br />August of 1996 (Panel 9-Right), during June and July of 1997 (Panel 8-Right), and then again <br />during June, 1998 (Panel 7-Right). Complete re-leveling of the Energy Spur track was <br />accomplished during 9-Right undermining due primarily to concerns over grade steepening. <br />Partial re-leveling, with the approval of the Railroad, was successfully practiced during 8-Right <br />undermining. Panel 7-Right ended immediately beneath the Energy Spur track resulting in <br />limited subsidence (< 22 inches) and temporary and permanent differential gradients that were <br />within the limits established for this type of track and/or the pre-existing conditions. Differential <br />gradients actually improved during undermining and track re-leveling during undermining was <br />not required. <br /> <br />PN: 98-12 <br />