2011-02-14_PERMIT FILE - C1980007A (3)

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Subsidence Evaluation for the Exhibit 60E South of Divide and Dry Fork Mining Areas Page 24 10.0 POTENTIAL IMPACTS OF SUBSIDENCE AND MINE - INDUCED SEISMIC ACTIVITY ON LANDSLIDES AND ROCKFALLS 10.1 Landslides The landslides listed below are all naturally occurring features, which become less stable, or become unstable and slide, during periods of increased precipitation. A review of aerial photographs of the Apache Rocks, Box Canyon, and South of Divide mining areas, which were taken in 1963 and 2004, show that the slides listed below appeared to be more stable in 1963 than they are now. Dunrud observed that many of the existing landslides during periods of high precipitation were less stable, or became unstable and moved in the mid 1980s and mid 1990s. Landslides were identified in the Dry Fork mining area as part of the Final Environmental Impact Statement (2005). The landslides. in the southeast side of West Flatiron and on the west side of Deep Creek in the Apache Rocks mining area showed no visual effects when longwall mining occurred beneath the areas. It therefore seems apparent that wet seasons affect landslides more than does longwall mining. During very wet periods, however, landslides that are already unstable may locally be triggered by mine subsidence. A total of twelve landslides, landslide areas, or landslide and rockfall areas, have been mapped in • the Apache Rocks, Box Canyon, and South of Divide mining areas during field mapping with the U.S. Geological Survey (USGS) in the 1980s and also during the annual subsidence observations in these areas (all the landslides listed in the Apache Rocks and Box Canyon mining areas occur in Township 13 South, Range 90 West). 1. Apache Rocks mining area a. Landslide located just north of Minnesota Reservoir in SW' /< of Section 29. No mining has occurred in the area. b. Landslide area located above mined longwall panels 14 and 15 in NW' /. of Section 26. These landslides appeared to be unaffected by longwall mining beneath them. c. Landslide area located on the west side of Deep Creek, located above the southeastern part of mined longwall panel 17 in the NW' /. of Section 35. This landslide also appeared to be unaffected by mining below when observed in July 2003. 2. Box Canyon mining area (no mining had occurred when these landslides were last observed): a. Landslide and rockfall area located above mined longwall panel 22 and un -mined longwall panel 23 in the NE' /, of Section 22 and the NW' /+ of Section 23. b. Landslide and rockfall area located above the southeastern part of un -mined longwall panel 21 in the NE' /. of Section 23. i 831 - 032.810 Wright Water Engineers, Inc.