2008-01-16_APPLICATION CORRESPONDENCE - C2008086 (3)

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DRAFT Shortly after the advancing longwall face has opened up enough area to initiate the first major roof cave behind the shields, the wave of surface subsidence accompanying face advance will start. The movement of the longwall face and the ground surface are so closely tied together that when the advance of the face stops the advance of the accompanying wave of surface subsidence advance may stop in less than a shift, but definitely over a weekend. Stopping the advance of a longwall face will, however, potentially increase the loads on the face supports. Sloughing from the coal face can also occur during stoppages. Restarting face advance after holiday periods, etc. can be difficult. Peng (1992, p. 20-22) reports maximum dynamic tilt and horizontal strain decreases with increasing speed of longwall extraction. Peng presents graphical data for the rate of face advance for various longwall faces in a West Virginia coal mine which increased from roughly 10 feet/day to roughly 43 feet/day: Maximum dynamic tilt appears to have decreased an average of approximately 44 percent (Peng, 1992, Fig. 3.6). The scatter of the dynamic tilt data is so large and the contradictory indication of an increasing maximum dynamic tilt for the single most rapid 43 feet/day face advance indicated on Fig. 3.6 that it appears statistically only possible to state that the tilt probably decreased with increasing face advance rate. 2. Maximum dynamic tensile strain decreased by an average of approximately 28 percent (Peng, 1992, Fig. 3.7). The scatter of the dynamic tensile strain data indicated on Fig. 3.7 is less than for the dynamic tilt data and it may be statistically possible to indicate a rough numerical relationship between decreasing tilt and increasing face advance rate. 3. Maximum dynamic compressive strain decreased by an average of approximately 62 percent (Peng, 1992, Fig. 3.8). The scatter of the dynamic compressive strain data indicated on Fig. 3.8 is nearly as large as that for the dynamic tilt data. It appears statistically possible to state that the maximum dynamic compressive strain decreased with increasing face advance rate. 8.0 IMPACTS OF SUBSIDENCE ON STRUCTURALLY SENSITIVE AREAS 8.1 Longwall Mining in Geologic Hazard Areas of Landslides, Rockfalls, and Unstable Slopes These unstable areas occur naturally on steep canyon walls in the Mesaverde Group. Unstable slope features already present can be adversely impacted by longwall mining. • It is therefore important to develop an inventory of baseline data on any landslide, rockfall, and generally unstable areas before mining begins, so that movements due to natural processes can be excluded from any potential mining impacts. • It is also important to have an assessment plan to distinguish between mining-related impacts on existing unstable areas and other activities, such as road construction. The assessment plan should include a subsidence monitoring program which should indicate the maximum angle of draw to the maximum limit of subsidence effects for the Red Cliff Mine Project area. Page 41 of 57