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 34 11.0 EFFECTS OF SUBSIDENCE AND MINE-INDUCED SEISMIC ACTIVITY ON MAN-MADE STRUCTURES AND RENEWABLE RESOURCES Man-made structures and renewable resources in the Southern Panels, Apache Rocks West, and Sunset Trail mining areas basically consist of 1) a reservoir (Minnesota Reservoir), 2) stock watering ponds, 3) streams (primarily Dry Fork and the upper part of Lick Creek) and Deep Creek Ditch, 4) roads, and 5) local cabins. Minnesota Reservoir, the ponds, and the Deep Creek Ditch diversion to Dry Fork serve the dual purpose of being both man-made structures and containment structures for the valuable water resources in the area. Based on annual subsidence observations in the Jumbo Mountain, Apache Rocks, Box Canyon, and South of Divide mining areas since 1996, the following information is considered appropriate for the Southern Panels, Apache Rocks West, and Sunset Trail mining areas. 11.1 Minnesota Reservoir Minnesota Reservoir,which provides storage water primarily for irrigation,is located between two landslides—one beginning at the north shore and the other beginning at the south shore. As explained in Section 10.1.1 (above), landslide movement on Jumbo Mountain occurred during unusually wet periods before mining began, during mining, and after mining and subsidence was complete. The conclusions were that landslide movement occurs in response to ground saturation and is not noticeably affected by subsidence and seismic activity produced by longwall mining beneath, or near, landslide areas. Both the landslides on Jumbo Mountain and those north and south of Minnesota Reservoir occur in surficial material (loose rock, gravel, sand, silt, clay, and soil) and local bedrock outcrops. Dunrud therefore expects that the mining of longwall panel E9 will not noticeably affect the large landslide south of Minnesota Reservoir. Mining of the longwall mining panels in the Southern Panels, Apache Rocks West, and Sunset Trail mining areas, as currently planned (Map 1) will not affect Minnesota Reservoir. The reservoir is located outside the area of mining influence of both the B- and E-seam panels, using an extremely conservative 45-degree angle of draw. Monitoring data is presented in the annual Fall Subsidence Reports to verify and demonstrate the accuracy of the predictions. 11.2 Stock Watering Ponds and U.S. Forest Service Water Resources The stock watering ponds in the Southern Panels mining area are located in debris flows or colluvium derived from the debris flows (Dunrud 1989). Some of these ponds are also classified as U.S. Forest Service water resources. The debris flows consist of a heterogeneous mixture of clay derived from the Wasatch Formation and boulders and gravels derived primarily from the Mount Gunnison intrusive (granodiorites and quartz monzanites). Based on observations made during geologic mapping in the area, these debris flows are even less likely to be affected by longwall mining than the alluvium. The debris flows have a very low permeability and, because the clay matrix is armored by the interstitial gravel and boulders, are resistant to erosion(the Deep Creek Ditch locally flows in this material at steep gradients). Based on the above-mentioned observations,no effects are expected when ponds in the Southern Panels,Apache Rocks West, and 831-032.912 Wright Water Engineers, Inc. December 2020