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SubTerra, Inc. Twentymile Coal Company 05/28/03 <br />PR6 AVF Subsidence Evaluation <br />• 1. Introduction <br />Permit Revision six (PR6) covers Twentymile Coal Company's (TCC) expansion of its operations <br />in the Northem Mining District (NMD) of the Foidel Creek mine near Steamboat, Colorado. <br />Current plans call for mining to be completed in five (5}, additional, 1,000-ft wide longwall panels <br />(Panels 17 through 22) during this period. <br />A prior permit revision presented our original analysis of potential impacts to Fish Creek and its <br />AVF due to subsidence from Panels 12 to 16-Right in the Northem Mining District (SubTerra, Inc., <br />1999'). The original predictions resulting from this initial study are shown on Drawing 1. Panels <br />12 and13-Right have already been mined out and Panel 14-Right is currently in production. At the <br />time of writing, both 13 and 14-Right had successfully undermined Fish Creek and its AVF. <br />There are three principal structures and natural features that will be affected by subsidence from <br />the new panels, the Energy Spur track, two sets of Power Lines, and Fish Creek. Two reports <br />(SubTerra, Inc., 2002(a)Z and SubTerra, Inc., 2002(b)3) that cover subsidence impacts to the <br />Energy spur rail line and the power lines over 19-Right have already been submitted under <br />separate cover. This third report covers the prediction of subsidence effects on Fish Creek and its <br />Alluvial Valley Floor (AVF). <br />2. Subsidence Prediction <br />Twentymile Coal Company (TCC) has developed an extensive subsidence database from <br />• monitoring longwall panel extraction in the West, Southwest, Eastern, and Northem mining <br />districts. This data has been submitted to the Division of Mines and Geology in semi-annual <br />subsidence reports and in reports attached to permit documents. <br />TCC has supplemented these published data with results from monitoring the county road, <br />power lines, and the Energy Spur track, which crosses both the Eastern and Northern mining <br />districts during mining Panels 9, 8, and 7-Right in the Eastern mining district and Panels 12, 13, <br />and 14-Right in the Northern Mining District. <br />The process of incorporating these data in the current predictions is described in the following <br />paragraphs: <br />1. Figure 1 was compiled from records of maximum subsidence from the Western, <br />Southwestern, and Eastern mining districts and shows the relationship between <br />maximum subsidence and the width/depth ratio for the Foidel Creek mine. This <br />figure was used to predict maximum subsidence values for the 1000-ft wide panels <br />at depths of 1,100 and 1,400 feet, which represents the range in conditions for the <br />study area. <br />2. Time phased data from the initial surveys of Panels 1, 2, and 3 in the Western <br />Mining District (Subsidence Line 12000E), representing the surface response to the <br />first, second, and third panel extraction, were used to predict the effects of 13-Right, <br />and subsequent panel, extraction. These data are contained in the semi-annual <br />reports and in prior permit submittals. <br />• ~ SubTerra, Inc., 1999. Northem Mining District AVF Subsidence Prediction, Project 99-03 <br />Z SubTerra, Inc., 2002(a). PR6 Railroad Evaluation <br />' SubTerra, Inc„ 2002(b). 19 Right Power Line Study <br />PN: 2002-06 <br />