2021-02-05_PERMIT FILE - C1980007A (5)

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Subsidence Evaluation for the Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 29 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 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. Mr. C. Richard Dunrud (retired U.S. Geological Survey coal mine subsidence expert and author of USGS Map C-115) 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). Map 1 is a composite showing the landslides in the Southern Panels mining area. 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 West and Southern Panels 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. 1. Apache Rocks West mining area: a. Landslide located just north of Minnesota Reservoir in SW'/4 of Section 29. Mining of the B-seam longwall panels 1312, 1313, and B 13-A did not have any observable effect on the landslide. b. Landslide area located above mined longwall panels E14 and E15 in NW'/4 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 B-seam longwall panel 17 in the NW'/4 of Section 35. This landslide also appeared to be unaffected by mining below when observed in July 2003. 2. Southern Panels mining area(Map 1): a. An extensive landslide is located south of Minnesota Reservoir north of the projected E-seam longwall panel E9, in the N'/2 of Section 32, Township 13 South, Range 90 West (Dunrud 1989). Overburden depth to the E-seam ranges from 500 to 600 feet. The average slope of the slide is roughly 20 percent. The landslide contains more cracks and scarps in the upper part and more depressions and bulges in the lower part 831-032.912 Wright Water Engineers, Inc. December 2020