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"D" Seam Longwall Subsidence Page 9 August 17, 1998 MINING ENVIRONMENT The planned location of the sixteen "D" Seam longwall panels is shown on Figure 1 and Plate 11. The "D" Seam outcrop and five overburden depth contours are shown; 2500-ft, 2000-ft, 1500-f t, 1000-ft and 500-ft. Longwall mining will proceed from the maximum depth starter rooms at the north end of all planned panels, #1 - #16, toward the "D" Seam outcrop to the final shield recovery rooms at the south end of the panels. The recovery rooms will be isolated from the "D" Seam Main Entry by minimum 200-foot thick barrier pillars, as shown on Plate 11. There are no planned connections between the "D" Seam workings and any existing or planned Sanborn Creek Mine workings. The drill hole interburden lithology data provided was broken down into the proportions of sandstone, shale and coal and was presented in Table 2. 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 the southern ends of all of the planned "D" Seam longwall panels. Sandstone is favorable because of its generally higher strength which more rapidly distributes the stress concentrated above the barrier pillars in the underlying mine workings (Chanda, 1989). Figure 2 indicates that the weighted average 588 sandstone recorded in the borehole logs (Table 2) actually predicts an 17.5° mean angle of draw (Abel & Lee, 1984). The use of a 35° angle of draw conservatively extends the predicted area of longwall mining induced subsidence effects to a greater distance beyond the surface projection of the mining boundaries, approximately SSB greater, than predicted by the 17.5° lithology based angle of draw. The geometric conditions of each of the planned Sanborn Creek Mine longwall panels are presented in Table 3. Specifically, the individual. planned longwall panel dimensions, the approximate depth of various locations along the boundaries of the panel and the estimated thickness of the "D" Seam along the boundaries of the panel. The depth and seam thickness are indicated at the individual subsidence prediction locations, i.e. the depth at the center of the starter rooms, where the individual 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 rooms. Table 3 demonstrates that the maximum depth of the individual panels is at the starter rooms, the north ends, and the minimum depth at the face-support recovery rooms, the south ends. 9