2008-03-21_PERMIT FILE - C1980007A (8)

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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 South of Divide mining area basically consist of 1) a reservoir (Minnesota Reservoir), 2) stock watering ponds, 3) streams (primarily Dry Fork, the upper part of Lick Creek, and the Upper Part of Deep Creek), 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, and Box Canyon mining areas during the last ten years, the following information is considered appropriate for the South of Divide mining area. 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 (2006, p. 23) therefore expects that the mining of longwall panel E9 will not noticeably affect the large landslide south of Minnesota Reservoir. Mining of the nine longwall mining panels in the South of Divide mining area, as currently planned (Map 1) will not affect Minnesota Reservoir. The reservoir is located outside the area of mining influence, using the most conservative 45-degree angle of draw. In order to verify and demonstrate the accuracy of predictions, based on the results of past subsidence observations in the Apache Rocks and Box Canyon mining areas, it is recommended that three procedures be implemented to monitor this structure prior to mining. The following procedures will be implemented in order to account for seasonal precipitation changes. 1. Conduct periodic aerial photo surveys of the landslides located north and south of the reservoir, using the July 2004 photos as a baseline reference. 2. Install, and periodically measure, survey monuments strategically located on the dam and on the north and south edges of the reservoir to monitor any movement prior to, during, and after mining in the area. 3. Install seismometers in strategic places, such as in the dam area and toe areas of the landslides, in order to detect vertical and horizontal ground motion prior to, during, and after mining, particularly during the mining of longwall panel E9. 11.2 Stock Watering Ponds and U.S. Forest Service Water Resources The stock watering ponds in the South of Divide and Dry Fork mining areas are located in debris flows or colluvium derived from the debris flows (Dunrud 1989). A total of 26 ponds have been mapped in the South of Divide mining area. Ten of these ponds are also classified as U.S. Forest Service water resources. Eight stock ponds are located in the Dry Fork mining area. 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 Tetra Tech - 090717/P 24