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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 aad colluvium in Deep Creek above four mined <br />longwall panels during periodic field studies in the Apache Rocks and Box Canyon flblining areas <br />(2002 through 2004). Ta addition, no cracks were observed in the upper reaches of Sylvester <br />Gulch above throe 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 thiclmess 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 />Creels Therefore, it is less likely that mining-iodated subsidence cracks will occur in the <br />• alluvium aad colluvium of the South of Divide mining area, particularly where the 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 area). 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 Bear Creek only 220 to 300 feet above partially extractedroom-and-pillaz 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 more uniform langwall mining method (compared to <br />mom-and-pillar mining) will make surface cracking and loss of water even less likely than the <br />mom-and-pillar mining method in Dry Fork and its tnbutaries_ Furthermore, the minimum <br />overburden depth is,greater in the shallowest overburden in the Dry Fork area than it is in the <br />Bear Creek area. <br /> <br />831-032.621 Wright Water Engineers, Inc. Page 1 <br />November 2004 <br />