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• 2004 Geologic Hazard Field Observations <br />South of Divide Mining Area <br />1.0 SUMMARY OF MAJOR FINDINGS <br />No cracks were observed in the alluvium and colluvium in Deep Creek above four mined <br />longwall panels during periodic field studies in the Apache Rocks and Box Canyon Mining areas <br />(2002 through 2004). In addition, no cracks were observed in the upper reaches of Sylvester <br />Gulch above three mined longwall panels. The overburden depth to the B Seam ranges from <br />about 800 to 1,600 feet in these two drainages. The estimated thickness of the alluvium and <br />colluvium ranges from 25 to 150 feet thick in the two drainages. <br />The alluvium and colluvium in Dry Fork and its tributaries, which also has an estimated <br />thickness range of 25 to 150 feet, contains a higher percentage of clay than the alluvium in Deep <br />Creek. Therefore, it is less likely that mining-induced subsidence cracks will occur in the <br />• alluvium and colluvium of the South of Divide mining azea, particulazty where t[1e overburden <br />depth to the E Seam is equal to, or greater than, 800 feet. <br />The occurrence of subsidence cracks is also considered unlikely above longwall mining panels <br />where the E Seam overburden depth is between 350 and 800 feet (350 to 400 feet is the <br />minimum overburden depth to the E Seam for the northwestern part of the South of Divide <br />mining azea). This statement is based on observations made by while conducting geologic <br />mapping for the U.S. Geological Survey (USGS), where no cracks were observed in the alluvium <br />in Beaz Creek only 220 to 300 feet above partially extracted room-and-pillar mine workings. <br />No changes were observed in flow downstream from the site and, to the author's knowledge, no <br />loss in stream flow was reported. The mote uniform longwall mining method (compared to <br />room-and-pillaz mining) will make surface cracking and loss of water even less likely than the <br />room-and-pillaz mining method in Dry Fork and its tributaries. Furthermore, the minimum <br />overburden depth is greater in the shallowest overburden in the Dry Fork azea than it is in the <br />Beaz Creek azea. <br />n <br />U <br />831-032.621 Wright Water Engineers, Inc. Page 1 <br />November 2004 <br />