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SubTerra, Inc. Twentymile Coal Company 04/07/09 <br />Foidel Creek Mine <br />Permit Revision PR09 -08 Sub sidence Evaluation <br />4.1. Scope of Work <br />The scope of work covered by this section of the report involved an evaluation of potential impacts <br />that may result from lowering the Fish Creek AVF during coal extraction in the Western Mining District. <br />The following issues were investigated and resolved: <br />1. Will subsidence cause Fish Creek to overtop its existing high bank(s)? <br />2. If overtopping is likely, what remedial measures are required? <br />3. What will be the time - phased increase in the surface area of Fish Creek due to <br />subsidence, and /or how much of the Fish Creek AVF will be flooded? <br />Several assumptions were made to simplify the analysis: <br />• Fish Creek elevation will rise, relative to the surrounding ground, a distance equal to <br />predicted ground subsidence at that point. <br />• The effects of spring run -off on creek elevation would be in addition to the effects of <br />subsidence. <br />4.2. Subsidence Prediction <br />Subsidence prediction followed the methodology successfully applied in 1999 to evaluate the possible <br />impacts to Fish Creek when panels in the Northern Mining District undermined it. The approach <br />involved superimposing estimated subsidence on over 100,000 data points representing Fish Creek <br />and the ground surrounding Fish Creek and comparing the subsided elevations with existing (i.e., un- <br />subsided) creek levels. The following tasks were performed: <br />1. Generating a grid of data points from the detailed, local topographic map of Fish Creek <br />and the immediate, surrounding area. <br />2. Developing an excel spreadsheet that contained each of the data points for approximately <br />28,000 linear feet of Fish Creek. <br />3. Importing approximately 100,000 data points, representing the ground surface over the <br />eastern end of Panels 13, 14, and 15 -Left from AUTOCAD and adding these data to the <br />AVF data in the spreadsheet. <br />4. Numerically subsiding this data set in the spreadsheet. <br />5. Exporting the numerically subsided data set back to AUTOCAD and creating a new <br />subsided topography (see Drawing 4). <br />6. Expanding the 3 -D plane, representing the un- subsided water surface of Fish Creek, to <br />meet the subsided ground surface and determining the new area of this surface. <br />The results of the predictions are shown in Drawing 4. Summary data include: <br />Subsidence ranging from 0 to 69 inches. <br />• Maximum gradient change of 0.7% along the creek bottom. <br />The estimated impacts of this level of predicted subsidence are provided in the following section. <br />(0 <br />3 PN: 2009 -04 <br />