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<br />SubTerra, Inc. Twentymile Coal Company 611912003 <br />19-Right Power Line Study <br />1. Introduction <br />This report presents the results of a preliminary evaluation of subsidence effects on Western Area <br />Power Administration's (WAPA's) Archer-Hayden power line and TriState's (formerly Colorado- <br />Ute's) Craig-Hayden-Steamboat power line due to mining in the eighth panel in the Northern <br />Mining District (Panel 19-Right) at the Twentymile Mine located southeast of Steamboat Springs, <br />Colorado. Public Service Company of Colorado (PSC} also routes power through lines <br />suspended from the Structures that will be undermined by 19-Right. Longwall panels in the <br />Eastern Mining district previously undermined a power line owned by WAPA during the late <br />1990's. <br />The layout of the Northern Mining District in relation to the Archer-Hayden and Craig-Hayden- <br />Steamboat power lines is shown in Figure 1. Mining will proceed from the east towards the west <br />in a direction that is almost parallel to the alignment of the power lines. The 1,000-ft wide longwall <br />panel will retreat between 60 and 80-ft per day affecting each structure for from 20 to 25 days. <br />1.1 Scone of Work <br />The scope of work covered by this report included predicting transverse and traveling subsidence <br />associated with the extraction of the 19-Right longwall and determining the effects of these <br />predicted movements on the power line. As noted above, this work followed on from previous <br />work covering subsidence prediction, planning, and observation of power line response when <br />. WAPA's line crossing the Eastern Mining District was undermined. <br />Tasks included: <br />• Digitizing topography along the power line route (for input to clearance calculations). <br />Twentymile Coal Company supplied the topographic map and aerial photographs for <br />the area. Inputting pylon location coordinates to prediction tables. <br />• Estimating time phased surtace and pylon foundation subsidence using the traveling <br />subsidence curve and retreat rates of 60, 70, and 80-ft per day. <br />• Reviewing prior pylon subsidence data (from the eastern mining district) and dummy <br />pylon foundation test (from the southwest mining district) for input to this study. <br />Preparing a draft report for discussion with WAPA. <br />1.2 Approach <br />Our approach included updating the subsidence prediction for the Northern Mining district using <br />data collected during the extraction of Panels 12 and 13-Right. Predicted transverse and traveling <br />subsidence profiles were subsequently used to estimate the time phased movements along those <br />portions of the Archer-Hayden and Craig-Hayden-Steamboat lines that cross Panel 19-Right. <br />2. Subsidence Prediction <br />• A substantial quantity of subsidence data has been collected by Twentymile Coal Company over <br />more than a decade during the extraction of longwall panels ranging from 625-ft to 1,000-ft wide <br />