2022-09-29_PERMIT FILE - C1980007 (5)

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Subsidence Evaluation for the Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 30 831-032.923 Wright Water Engineers, Inc. December 2021 bulges in the lower part than were present in the early 1960s (based on image data from July 1963 aerial photographs). • Based on a stereographic review of July 2004 vertical aerial photographs, renewed activity occurred locally in western part of the landslide areas north and south of Dry Fork during wet periods in the 1980s (1984 to 1987) and the mid-1990s (1994-1996). The Dry Fork road was taken out one half-mile west of the Minnesota Reservoir dam by this renewed movement in 1987 (Map 1). b. Landslides were identified on the Dry Fork mining area in the Final Environmental Impact Statement prepared for the U.S. Department of Agriculture – Forest Service in August 2005. The report identified three active landslide areas in the southeast corner of the area. These are located outside the area of influence from mining in longwall panels E4 and E5. The erosional escarpment at the headwaters of Deep Creek is also outside of the area of influence mining. Two small landslides (slope failures) were identified by a Tetra Tech geologist during a site inspection on September 20, 2007. These are located just above the Deep Creek Ditch. 3. Sunset Trail mining area: a. As shown on Map 1, there are several identified landslides within the Lick Creek drainage and others near the mouth of South Prong. Most of the mapped features will be outside the influence of mining, while the remaining few will be monitored throughout mining. 10.1.1 Effects of Subsidence and Mine-induced Seismic Activity on Landslides Some of the most important information regarding mine subsidence and mine-induced seismicity was obtained from observations of active landslides on Jumbo Mountain above B-seam longwall panels 8 and 9, which were mined during the mid-1990s. Landslide movement occurred during unusually wet periods before mining, during mining, and after mining and subsidence was complete. The landslides located north and south of Minnesota Reservoir are similar to those on Jumbo Mountain. Both occur in surficial material (rocks, gravel, sand, silt, clay, and soil) and local outcrops of bedrock that have slumped and flowed downhill during periods of increased saturation. Cracks, bulges, and depressions or troughs, and springs were locally observed in both landslide areas. It is important to note that no earth tremors (seismic activity) were felt by Mr. Dunrud and other field observers in all the annual traverses and observations made above the longwall mining areas in the Jumbo Mountain, Apache Rocks, Box Canyon, Southern Panels, and Sunset Trail mining areas during the past 25 years (1996-2020 inclusive). For example, no tremors were felt during the annual traverse above B-seam longwall panel 13 in 1999, when the mining face was located directly beneath one of the subsidence observation points. This point was located approximately 1,200 feet vertically above the active mining face, and 2,800 feet north of Minnesota Reservoir. In contrast to room-and-pillar mining, longwall mining is a uniform extraction procedure that basically involves 1) the uniform cutting of a coal face, 2) the caving of the roof behind the moving