2011-02-14_PERMIT FILE - C1980007A (3)

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Subsidence Evaluation for the Exhibit 60E South of Divide and Dry Fork Mining Areas Page 32 mining. There is an area of the Deep Creek Ditch that has a hard rock bottom. This area is limited to the lower gradient areas above the first landslide in the Dry Fork mining area (Refer to Map 1). The ditch at that location lies over potentially extended E -seam longwall panels E5 and E6. Should these panels be extended under this area, there is a possibility that cracks may develop at this location. It is recommended that this area be monitored and appropriate action taken to seal cracks if they occur. As discussed in Section 5.3.2, no cracks were observed in the alluvium and colluvium of Sylvester Gulch and Deep Creek during periodic field observations in the Apache Rocks and Box Canyon mining areas. The near - surface alluvial material consists of primarily sand, silt, clay, and soil that ranges in estimated thickness from 25 to 150 feet. In the two areas mentioned, the drainages were located above rigid pillars and panel boundaries where the overburden depth ranges from 800 to 1,050 feet. This is expected to also be the case in the upper region of Deep Creek within the Dry Fork mining area. The alluvium and colluvium in Dry Fork and Lick Creek, which also has an estimated thickness range of 25 to 75 feet, contains more clay than does the Deep Creek alluvium. Therefore, it is even less likely that cracks will occur in colluvium and alluvium in the stream valleys of the South of Divide mining area despite the shallow overburden. This should also be the case for the stream valleys in the Dry Fork mining area, where the overburden is 800 to 1,800 feet thick. In the South of Divide mining area, the overburden depth to the E -seam ranges from approximately 375 feet above the western edge of longwall panel E2 to 1,300 feet above the maximum extended E -seam 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 E -seam longwall panels E2 through E8 in Dry Fork and its tributaries in Deep Creek and its tributaries and in Lick 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 E -seam . longwall panels E2 and E3, where the shallowest overburden occurs. Depressions, ranging in 831 - 032.810 Wright Water Engineers, Inc.