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SubTerra, Inc. Twentymile Coal Company 04/07/09 <br />Foidel Creek Mine <br />Permit Revision PR09 -08 Subsidence Evaluatio <br />1. INTRODUCTION <br />Permit Revision PR09 -08 covers Twentymile Coal Company's (TCC) expansion of its operations into <br />the Western Mining District (WMD) of the Foidel Creek mine near Steamboat, Colorado (Drawing 1). <br />Plans call for mining to be completed in six (6) 1,000 -ft wide longwall panels (Panels 12L through 17L) <br />during this period (see Drawings 2 and 3). Current plans also involve extracting the panels in reverse <br />numerical order retreating each panel from the west towards the east. Coal extraction will result in <br />subsidence to County Road 27 which crosses five of the six panels and Fish Creek and its Alluvial <br />Valley Floor (AVF) that meanders over 15L, 14L and then 13L as shown on Drawing 3. <br />TCC originally studied potential impacts of subsidence on Fish Creek associated with workings in the <br />southwest mining district located south of the current panels. In addition, Permit Revision PR03 -06 <br />presented our original analysis of potential impacts to Fish Creek and its AVF due to subsidence from <br />Panels 12 to 16 -Right in the Northern Mining District (SubTerra, Inc., 1999 and Panels 17 and 18- <br />Right (SubTerra, Inc., 2002 <br />Subsidence effects on the county roads that cross the Foidel Creek Mine have been evaluated in <br />several prior reports including our evaluation of impacts associated with rock -fall hazards from the <br />Twentymile Cliff (SubTerra, Inc., 1996 and predictions for the Eastern Mining District. <br />2. Subsidence Prediction <br />Twentymile Coal Company (TCC) has developed an extensive subsidence database from <br />monitoring longwall panel extraction in the Southwest, Eastern, and Northern mining districts. This <br />data has been submitted to the Colorado Division of Reclamation, Mining and Safety (CDRMS) in <br />semi - annual subsidence reports and in reports attached to permit documents. <br />TCC has supplemented these published data with results from monitoring the county road, power <br />lines, the Energy Spur track, and both Foidel and Fish Creek AVFs which interacted with the <br />Southwest Mining District and crossed the Eastern and Northern Mining Districts. <br />The process of incorporating these prior 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 Southwestern <br />and Eastern mining districts and shows the relationship between maximum subsidence <br />and the width /depth ratio for the Foidel Creek mine. This figure was used to predict <br />maximum subsidence values for the 1000 -ft wide panels at depths of 1,100 feet and <br />1,400 -feet, which represents the range in conditions for the study area. <br />2. Time phased data from the initial surveys of Panels 1, 2, and 3 in the Southwestern <br />Mining District (SubTerra, 1996 representing the surface response to the first, <br />second, and third panel extraction, were used to predict the effects of 17 -Left, and <br />subsequent panel, extraction. These data are contained in the semi - annual reports and <br />in prior permit submittals. <br />SubTerra, Inc., 1999. Northern Mining District AVF Subsidence Prediction, Project 99 -03. <br />2 SubTerra, Inc., 2002. Twentymile Coal Company, AVF Subsidence Study, Project 2002 -06. <br />. 3 SubTerra, Inc., 1996. Evaluation of Potential Rockfall Impacts to County Road 27, Project 96 -26 <br />1 <br />