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_1 Sanborn Creek Subsidence Page 5 September 2, 1997 J MINING EN~IIRONMENT The planned location of the thirteen B Seam longwall panels is shown on Figure 1. Five depth contours are shown; 2500-f t, - 2000-ft, 1500-ft, 1000-ft and 500-ft. Longwall mining will proceed 1 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. 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 1 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. 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 1 recorded in the borehole logs (Table 1) actually predicts an 18° angle of draw. The use of a 25° angle of draw conservatively extends the predicted area of longwall mining induced subsidence effects to a greater distance beyond the surface projection of the 1 mining boundaries, approximately 99~ greater, than predicted by the 18° lithology based angle of draw. The geometric conditions of each of the planned Sanborn Creek 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 at the center of the face-support recovery room. Table 2 demonstrates that the maximum depth of the individual panels is at the starter room and, except for Panel #1, the minimum depth at the 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. 1 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. 5