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SubTerra, Inc. Twentymile Coal Company 02/I 1/99 <br />Northern Mining District Subsidence Evaluation -Final Report <br />• associated, estimated increase in creek elevation of 2 feet, using the 3-D modeling software. The <br />overall surface area, for conditions following extraction of Panel 16, was estimated to be <br />approximately 210 acres. The expanded surface area is shown on Drawing 6. <br />4.4.2 Potential for Fissure Development and Water Loss <br />Surface cracking or fissuring has previously been confined to areas where the Twentymile <br />sandstone either outcrops or is located relatively close to the surface. In these areas, the blocky <br />nature of the Twentymile sandstone results in strain concentration causing abnormal <br />displacement between adjacent blocks and surface cracking. <br />The combination of a maximum predicted tensile strain of +0.005 and relatively thick cover of <br />marine shale over the Twentymile sandstone in the study area will decrease the potential for <br />surface cracking or fissuring. The potential for water loss under these conditions is extremely low. <br />5. Evaluation of Imaacts to Foidel Creek <br />As shown on Drawing 1, Panel 12-Right will affect a small section of Foidel Creek (Station 380 to <br />Station 410). This is predicted to result in subsidence ranging from 0 to 50 inches and creek <br />bottom gradient changes of less than 1 % (see Table 8). Both subsidence and gradient change <br />values are lower than the creek has already been subjected to due to undermining by panels in <br />the eastern mining district. No adverse impacts are therefore anticipated. <br />r~ <br />L <br />6. Monitorin~Reauirements <br />Visual inspection and conventional or GPS surveys of the expanded Foidel and Fish Creek areas <br />may be used to confirm the predicted effects of mining. This methodology has previously been <br />successfully used where Foidel Creek has been undermined by Panels 9, 8, and 7 Right in the <br />Eastern Mining District. <br />7. Conclusions and Recommendations <br />The evaluation presented in Section 3 predicts that the Energy spur will subside by up to 67 <br />inches and be subject to gradient changes of up to 1% if no mitigation were performed. Partial re- <br />leveling during undermining will therefore be required to maintain the track within the tolerances <br />set by the Union Pacific Railroad. This level of mitigation has previously been successfully applied <br />in the Eastern Mining District without any impacts to rail traffic on the Energy Spur. <br />The evaluation presented in Section 4 predicts that Fish creek will overflow its highbank(s) as a/ <br />result of subsidence from mining in the northern mining district at the Twentymile mine. The <br />average height of overFlow is predicted to be on the order of 0.5 feet, however, in some areas, the <br />highbank is estimated to be as much as 3 feet below the post-subsidence water level. <br />Subsidence is predicted to have the following impacts on Fish Creek and its AVF <br />. 1. A maximum increase in creek bottom gradient of 0.025 (2.5%). ~ <br />F'N: 98-12 <br />