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

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longwall panels E6 and E7. However, based on observations made by Dunrud above the Somerset Mine in the Bear Creek area, subsidence cracks are not expected to occur in the Dry Fork alluvium where the overburden depth ranges from 375 to 800 feet. No cracks, and no change in stream flow, were observed in the Bear Creek alluvium (estimated to be 10 to 15 feet thick) when coal was extracted by room-and-pillar methods at depths ranging from 220 to 300 feet beneath Bear Creek (Bureau of Land Management, et al., 2002). The probable reason for the lack of cracking in alluvium is that the fine sand- to clay-sized material and overlying soil yields without cracking or bulging as it deforms as a discrete unit, or as discrete units, in the subsidence process. This same reasoning also applies to the colluvium in the area. Although subsidence cracks were locally observed in colluvium less than one foot to a few feet thick, no cracks were observed in colluvium more than about ten feet thick. No cracks have been observed in alluvium above mined longwall panels in the Apache Rocks and Box Canyon mining areas. It is important to continue ongoing surface-water monitoring in the Dry Fork, Lick Creek, and Deep Creek drainages in order to compare the historic information derived from annual subsidence observations in the West Elk Mine area with field observations in selected areas of the South of Divide and Dry Fork mining areas. Subsidence depressions, slope changes, and strain are projected to occur above longwall panels E2 through E8 in Dry Fork and its tributaries and in Lick Creek when the panels are mined. Subsidence depressions, slope changes, and strain aare also projected to occur above longwall panels E3 and E4 in Deep Creek when the panels are mined. The maximum subsidence amount, slope change (tilt), and strain are projected to occur above solid coal barriers and mined longwall panel boundaries, such as above the west ends of longwall panels E2 and E3, where the shallowest overburden occurs. Depressions, ranging in depth from 8.4 to 11.2 feet, are projected in this area. Maximum changes in slope (tilt), ranging from 1.3 to 6.4 percent, are also projected for this area. The maximum horizontal tensile and compressive strain is projected to range from 0.7 to 4.2 percent. Subsidence depressions and slope changes will be less above the gate road pillars than above solid coal barriers, because they are projected to yield during mining by as much as 4 feet (Table 2). No subsidence depressions or changes in stream gradient were observed in Deep Creek, located about 1,050 feet above mined longwall panel 17 during the annual traverse in July 2004 (Dunrud 2004a). There was no observable change in stream gradient or in stream flow. The depression and change in gradient were apparently sufficiently gradual, so as to not be perceived by Dunrud during his traverse along the trail by the stream. 11.3.1 Potential for Hydraulic Connection Between Mine Workings and Surface Near the southwest corner of longwall panel E2, the Dry Fork channel encounters a short reach where the E Seam overburden is less than 400 feet, with a minimum of 375 feet. A prudent concern is whether mining induced subsidence could establish a hydraulic connection between the Dry Fork stream channel and the mine workings. To address this scenario, the maximum projected height of fracturing and the maximum depth of surface cracks were considered. As discussed in Section 5.2, the effective height of fracturing in the South of Divide and Dry Fork mining areas is estimated to range from 9t to 18t, or a maximum fracture height of 252 feet for a mining height of 14 feet. However, Peng (1992) states that the upper one-third of the fractured zone has only minor fractures with little potential for water conductivity. Therefore, the height of the fractured zone capable of transmitting water would be two-thirds of the 18t, or 168 feet. The maximum height of the caved zone is projected to be 5t, or 70 feet, for the South of Divide and Dry Fork mining areas. When added to the effective fracture zone height of 168 feet, the combined heights of the caved and fracture zones capable of transmitting water is projected to be a maximum of 238 feet. Tetra Tech - 090717/P 26