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Sanborn Creek Subsidence Paqe 5 September 2, 1997 r ~ MINING ENVIRONMENT -• The planned location of the thirteen B Seam longwall panels is shown on Figure 1. Five depth contours are shown; 2500-ft, 2000-ft, 1500-ft, 1000-ft and 500-ft. Longwall mining will proceed ~. from the maximum depth of Panels #2 - #13 toward the shallower existing Sanborn Creek Mine workings. The long axis of Panel #1 ~- will roughly parallel to the strike of the B Seam and increases erratically in depth by about 260-ft from east to west because of the ridges that protrude into the North Fork valley between Bardine ._ Canyon and "C" Gulch. C The drill hole interburden lithology data provided was broken down into the proportions of sandstone, shale and coal and is presented in Table 1. The lithologic distribution in the - interburden is variable. However, sandstone is the predominant lithology present in the interburden. Sandstone is the most ~~ favorable lithology for multiple seam mining, which is the case for portions of all of the planned Sanborn Creek Mine longwall panels. L Sandstone is favorable because of its generally higher strength which more rapidly distributes the stress concentrated under the barrier pillars in the overlying mine workings (Chanda, 1989). ~- Figure 2 indicates that the weighted average 56~ sandstone recorded in the borehole logs (Table 1) actually predicts an 18° r~ angle of draw. The use of a 25° angle of draw conservatively LL extends the predicted area of longwall mining induced subsidence effects to a greater distance beyond the surface projection of the mining boundaries, approximately 99$ greater, than predicted by the - 18° lithology based angle of draw. r The geometric conditions of each of the planned Sanborn Creek IL Mine longwall panels are presented in Table 2. Specifically, the individual planned longwall panel dimensions and the approximate ~ depth of various locations along the boundaries of the panel, i.e. the depth at the center of the starter room, where the panel `-' ribsides are one-half the panel width from the ends of the panel, where the depth contours cross the panel boundaries and the depth L, at the center of the face-support recovery room. Table 2 L demonstrates that the maximum depth of the individual panels is at the starter room and, except for Panel #1, the minimum depth at the L face-support recovery room. The southeast corner of Panel #7 is approximately 90-ft lower than the center of the recovery room because of the side-hill slope on the west side of Hawksnest Creek. L Similarly, the west end of the Panel #10 recovery room is slightly shallower than the center and the east end of the Panel #11 recovery room is slightly shallower than the center because of the ~ side-hill slopes of "A" Gulch. L~ 5