Laserfiche WebLink
• <br />• <br />• <br />Spring 2011 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />Another active rockfall /landslide area visited was that first observed on the west and east sides of <br />West Flatiron during the spring 2007 and fall 2008 field visits, respectively. These areas <br />(Locations 4 and 5 of the fall 2009 subsidence report) are along one of the most narrow parts of <br />West Flatiron ridge top at a site where cracks (most likely gravity- induced) were first observed. <br />Active cracking and slabbing above mined B -seam Longwall Panel 21 both on the west and east <br />side of West Flatiron appears to have lessened over the past several years of observations (Map <br />3). Cumulative extension of the surface cracks, as observed in June 2008, was approximately 2.5 <br />to 3 feet. In October 2008, the MDW pad on which these surface cracks were observed was <br />reclaimed and nearly all evidence of surface cracks obscured except at the sandstone cliff face. <br />Aside from a few remaining MDW pads near the north end of West Flatiron, mining - related <br />activities in this area are complete. Nonetheless, the potential for more cracking and rockfall <br />activity on the west- and east - facing cliffs remains high. <br />E -seam longwall mining in the South of Divide mining area was initiated on the east end of <br />Longwall Panel El in December 2008. As of the time of our spring 2011 field visit (June 20 -21, <br />2011), longwall mining was complete on E -seam Longwall Panel El and nearly complete on E- <br />seam Longwall Panel E2. Subsidence related observations at the surface were most notable at <br />the western end of these panels for two principle reasons: <br />1. This is where the overburden is thinnest, and <br />2. This was the area of current longwall mining activities (i.e., area of expected dynamic <br />subsidence) during two of the last three field visits. <br />Lastly, observations were made of a few cracks in MDW pads above unmined portions of E- <br />seam Longwall Panel E3. As discussed in earlier subsidence reports, evidence suggests that the <br />observed cracks are the result of differential settlement of portions of these pads where fill <br />placement is the greatest. <br />831 - 032.793 <br />September 2011 <br />Wright Water Engineers, Inc. Page 3 <br />