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Subsidence Evaluation for the <br />Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 29 <br />10.0 POTENTIAL IMPACTS OF SUBSIDENCE AND MINE -INDUCED <br />SEISMIC ACTIVITY ON LANDSLIDES AND ROCKFALLS <br />10.1 Landslides <br />The landslides listed below are all naturally occurring features, which become less stable, or <br />become unstable and slide, during periods of increased precipitation. A review of aerial <br />photographs of the Apache Rocks and South of Divide mining areas, which were taken in 1963 <br />and 2004, show that the slides listed below appeared to be more stable in 1963 than they are now. <br />Dunrud observed that many of the existing landslides during periods of high precipitation were <br />less stable, or became unstable and moved in the mid-1980s and mid-1990s. Landslides were <br />identified in the Dry Fork mining area as part of the Final Environmental Impact Statement (2005). <br />Map 1 is a composite showing the landslides in the Southern Panels mining area. <br />The landslides in the southeast side of West Flatiron and on the west side of Deep Creek in the <br />Apache Rocks mining area showed no visual effects when longwall mining occurred beneath the <br />areas. It therefore seems apparent that wet seasons affect landslides more than does longwall <br />mining. During very wet periods, however, landslides that are already unstable may locally be <br />triggered by mine subsidence. <br />A total of twelve landslides, landslide areas, or landslide and rockfall areas, have been mapped in <br />the Apache Rocks West and Southern Panels mining areas during field mapping with the U.S. <br />Geological Survey (USGS) in the 1980s and also during the annual subsidence observations in <br />these areas. <br />1. Apache Rocks West mining area: <br />a. Landslide located just north of Minnesota Reservoir in SW'/4 of Section 29. Mining of <br />the B -seam longwall panels B 12, B 13, and B 13-A did not have any observable effect <br />on the landslide. <br />b. Landslide area located above mined longwall panels E14 and E15 in NW'/4 of Section <br />26. These landslides appeared to be unaffected by longwall mining beneath them. <br />c. Landslide area located on the west side of Deep Creek, located above the southeastern <br />part of mined B -seam longwall panel 17 in the NW'/4 of Section 35. This landslide also <br />appeared to be unaffected by mining below when observed in July 2003. <br />2. Southern Panels mining area (Map 1): <br />a. An extensive landslide is located south of Minnesota Reservoir north of the projected <br />E -seam longwall panel E9, in the N%2 of Section 32, Township 13 South, Range 90 <br />West (Dunrud 1989). Overburden depth to the E -seam ranges from 500 to 600 feet. <br />The average slope of the slide is roughly 20 percent. The landslide contains more <br />cracks and scarps in the upper part and more depressions and bulges in the lower part <br />than were present in the early 1960s (based on image data from July 1963 aerial <br />photographs). <br />831-032.816 Wright Water Engineers, Inc. <br />October 2018 <br />