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II <br />II <br />' S.0 <br />CONCLUSIONS <br />Subsidence Field Observations <br />West Elk Mine <br />August 27-29, 2002 <br />Subsidence cracks observed in the traverses this year occur well within the limits of angle of <br />draw and tensile strain projected for the Apache Rocks and Box Canyon Mining Permit areas <br />(see Table 2 of Exhibit 60). The projected angle of draw ranges from 10° to 20°. The angle of <br />draw, based on field measurements, ranges from 11 ° to 12°. Projected tensile strain in Exhibit 60 <br />ranges from 0.8 to 1.8 percent. Estimated tensile strains-using an estimated increase of 10 to 20 <br />percent over last yeaz-range from 0.3 to 0.6 percent (0.308 to 0.616 %). <br />Subsidence effects, such as tension and compression, caused by longwall mining, are more <br />predictable and have less impact on the overburden rocks than subsidence resulting from room- <br />and-pillar mining. This is because extraction is complete and uniform, compared to the less <br />uniform room-and-pillar mining procedures. Consequently, the various rocks above the <br />longwall mining panels tend to undergo continuum downwarping as multiple plates (in three <br />dimensions) or beams (in two dimensions). <br />Under the concept of downwazping of multiple plates, tension cracks-such as those observed at <br />Apache Rocks and above the western part of longwall Panel 14 (Location 8~vvi11 narrow with <br />depth and close at the neutral surface of the rock strata behaving as a plate. The rocks below that <br />neutral surface aze then subjected to compression. Therefore, open surface cracks above <br />longwall extraction panels aze normally not conduits for any surface water present to be diverted <br />for any significant distance. Under the conceptual model of downwarping of multiple plates or <br />beams, the water would only flow a distance no greater than half the thickness of the rock unit <br />that is behaving as a plate. <br />Landslide features-such as cracks, bulges, and steps-that were observed in the Minnesota <br />Reservoir area, are more extensive and may cause a greater impact on the ground surface than <br />any of the subsidence features observed during the field traverses above the longwall mining <br />azeas. The landslide features observed in this azea this yeaz aze similaz to those observed last <br />yeaz because of very dry conditions this spring and summer. <br />D:\Work\W WE\831-032\570ha\Subsidence Rpt 10-2002.doc <br />831-032.570 Wright Water Engineers, Inc. Page 23 <br />December 2002 <br />