Laserfiche WebLink
Subsidence Field Observations <br />West Elk Mine <br />' August 28 and 29, 2001 <br />4.0 CONCLUSIONS <br />' Subsidence cracks observed in the traverses occur well within the limits of the angle of draw and <br />tensile strain projected for the Apache Rocks and Box Canyon permit areas (see Table 2 of <br />Exhibit 60). The projected angle of draw ranges tiom 10 degrees to 20 degrees. The angle of <br />' draw from field measurements ranges from l 1 degrees to 12 degrees. Projected tensile strain in <br />Exhibit 60 ranges from 0.8 to 1.8 percent. Estimated tensile strains, using an estimated increase <br />of 10 to 20 percent over last year, range from 0.3 to 0.6 percent (0.28 to 0.56 percent). <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, because extraction is complete and uniform. Consequently, the various rocks <br />above the longwall mining panels tend to undergo continuum downwarping as multiple plates (in <br />' tluee dimensions) or beams (in two dimensions). <br />' Under the concept of downwatping of multiple plates, tension cracks, such as those observed at <br />Apache Rocks, will narrow with depth and close at the neutral surface of the rock strata behaving <br />' as a plate. The rocks below that neutral surface aze [hen subjected to compression. Therefore, <br />open surface cracks above longwall extraction panels are normally not conduits for any surface <br />' water present to be diverted for any significant distance. Under the conceptual model of <br />downwarping of multiple plates or beams, the water would only flow a distance no greater than <br />' half the thickness of the rock unit 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 />areas. <br />' (Video Enclosed) <br />' C:\831-03?\560do\Subsidence Rpi 9-?001 doc <br />1 <br />831-032.560 Wright Water Engineers, Inc. Page 15 <br />September 2001 <br />i -' ,c. <br />