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SubTerra, Inc. Twentymile Coal Company 6/19/2003 <br />19-Right Power Line Study <br />• vertical in the traveling direction but will remain permanently tilted towards the panel center. The <br />estimated transverse tilts at Pylon 146-5 on the Archer-Hayden line and Pylon 143 on the Craig- <br />Hayden-Steamboat line are provided in Tables 3 and 4, respectively. Tilt and displacement in the <br />direction of face retreat will be greatest for structures on the Archer-Hayden line as these <br />structures are closer to the panel center and will therefore experience more subsidence. Table 5 <br />presents the predicted traveling tilt for Pylon 146-5 on the Archer-Hayden line. <br />The effective length of the traveling subsidence profile (approximately 1,200-ft) means that each <br />pole will react to the subsidence wave independently. Thus, as the subsidence atone structure <br />ends, subsidence at the next structure begins. This factor should, for the most part, eliminate the <br />potential for complex interaction allowing the response of each structure to be independently <br />evaluated. <br />3.3 Differential Movements Across the Structures <br />Horizontal surface strain will result in small displacements between the legs of four-legged lattice <br />structures as each structure is undermined. These movements are estimated to be lower in <br />magnitude than experienced by structures overlying the Eastem Mining District that have already <br />been successfully undermined. <br />3.4 Risk Evaluation <br />• Our preliminary evaluation indicates that subsidence risks are lower than for the WAPA line that <br />has already been undermined in the Eastem Mining District. Additional confidence in the <br />acceptability of subsidence related risk might also be gained by reviewing field-monitoring reports <br />for the Eastern Mining District and earlier tests carried out by Twentymile in the Southwest Mining <br />District. <br />4. Field Monitoring Program <br />Subsidence data will continue to be collected as part of Twentymile Coal Company's monitoring of <br />the railroad that runs over the Northern Mining District. These data will continue to be used to <br />verify the predictions for 19-Right at the locations of the power line structures. <br />It is recommended that Twentymile implements the field monitoring protocol previously applied <br />during undermining of WAPA's line that crosses the Eastem Mining District. In this case, a power <br />line contractor was on stand-by during power line undermining but was not mobilized. We <br />understand that the lattice structures on this line were subjected to up to 6-ft of subsidence with <br />only minor damage (Hansen, 2002). <br />For this project we recommend that a detailed field-monitoring program be developed in <br />consultation with WAPA, Tri-State and PSC. <br />LJ <br />3 <br />