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Subsidence and Geologic Field Observations <br />Apache Rocks and Box Canyon Mining Areas <br />July 12-15, 2004 <br />this soft, weathered sandstone, as the weathered material sloughs into them, eventually filling <br />them in response to the forces of erosion and deposition. <br />Most of the major cracks that occurred in the durable sandstone at Apache Rocks, and since the <br />azea was undermined by longwall panel 12 in 2000, appeaz similar to their condition in 2003 <br />(when they were estimated to be 10 to 20 percent wider and 20 to 40 percent longer than they <br />were in 2001). Some of the widest cracks aze longer than they were in 2003, because more <br />colluvium and soil have caved and sloughed into the underlying cracks (which were very likely <br />present, even in 2000). <br />The landslides located on the southwest flank of West Flatiron above mined longwall panels 14 <br />and 15 show no evidence of being affected by the mining. However, the rockfall and landslide <br />area located at the head scarp of the first east drainage of Sylvester Gulch shows renewed <br />rockfall activity. The natural rockfall process in this cliff escarpment, which has the highest <br />cliffs and steepest slopes in the Apache Rocks and Box Canyon mining areas, was likely <br />accelerated by mining above longwall panel 22. <br />Subsidence most likely accelerated the natural rockfall process, rather than any seismic activity <br />caused by mining beneath the thick overburden of West Flatiron (thickness as much as 2,250 <br />feet). Although the rockfall potential is locally very high, the risk to life is low because the <br />location is remote. The natural rockfall activity may also be accelerated in the head of the first <br />east drainage of Sylvester Gulch when longwall panels 21 and 23 aze mined, because the high <br />cliff scarps of this azea will be within their areas of mining influence. <br />The condition of the subsidence cracks, which were first observed above the west end of mined- <br />out longwall panel 14 on August 28, 2002, is a function of the material in which the crack <br />occurs. The cracks in the durable sandstone and siltstone will likely continue to look much like <br />they did in 2003. The other cracks, in aeeas underlain by weathered bedrock and colluvium were <br />reclaimed prior to this observation period. <br />831-032.640 Wright Water Engineers, Inc. Page 2 <br />September 2004 <br />