Laserfiche WebLink
Subsidence Field Observations <br />West Elk Mine <br />July 22-24, 2003 <br />5.0 CONCLUSIONS <br />In the 17 traverses made during the 2003 study, the area of subsidence cracks observed above the <br />mined-out longwall panels occur within the limits of the angle of draw and the strain projected <br />for the Apache Rocks and Box Canyon Permit areas (see Table 2 of Exhibit 60, dated May 1996 <br />and revised February 1998). The range in angle of draw projected in Exhibit 60 was 10 to 20 <br />degrees; the angle of draw, based on field observations, ranges from 11 to 12 degrees. The <br />tensile strain forecast in Exhibit 60 ranges from 0.8 to 1.8 percent. Field estimates of strain <br />(including increases of 10 to 20 percent in 2002) range from approximately 0.3 to 0.6 percent. <br />As stated in the 2002 report, the effects of tension and compression caused by longwall mining <br />are more predictable and have less impact on [he overburden rock than does subsidence caused <br />by room-and-pillar mining. The major reason for this is that longwall extraction is complete, and <br />therefore, more uniform than room-and-pillaz mining practices. In longwall extraction, therefore, <br />the rock units undergo continuum downwarping as multiple plates (in three dimensions) or as <br />beams (in two dimensions). <br />Applying the concept of downwarping of rocks as multiple plates, tension cracks, for example, <br />will close at the neutral surface of the rock unit behaving as a plate (in three dimensions) or a <br />beam (in two dimensions). Below the neutral surface, the material in this plate undergoes <br />compression. Under the conceptual model of the overburden rocks downwarping as multiple <br />plates or beams, no surface water can flow a vertical distance greater than the depth of the neutral <br />surface. This vertical distance commonly is one-half the distance of the thickness of the rock <br />unit behaving as a plate or beam. <br />The landslide on the west side of Deep Creek, and the debris flows on the southeast side of West <br />Flatiron, appear to have been unaffected by tongwatl mining beneath them. No recent cracking <br />or bulging was observed as of July 22-24, 2003. The cracks, bulges, and steps observed on the <br />landslide terrain around Minnesota Reservoir-outside any mining influence-have naturally <br />831-032.610 Wright Water Engineers, Inc. Page 34 <br />Seplember2003 <br />