2001-12-11_PERMIT FILE - C1980007A (2)

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April 1989 SELECTION OF MINING AREAS FOR DESIGN After careful study of geologic maps and sections provided by ARCO, it became apparent that a realistic average geometric description for each of the three .seams could not be obtained in terms of depth and interburden thickness. Variations in topography, seam interburden, attd itt the occurrence of the seams themselves caused the development of a five area plan for tl~e property. Geometric parameters within each area could then be averaged for room- and-pillar attd Longwall designs. ALthottgh this approach increased significantly the design effort, it produced a more realistic design than that based on only one average geometric characterization for each seam. Figure 3.4.A.3 shows the location of the five areas and Table 3.4.A.6 summarizes the depth of cover and interburden thick- nesses. SEAM MINING SEQUEPICE Within these five areas there is both two-and three-seam mining in which there is a choice of an ascending or descending sequence. In general, from a ground control point of view, it is better to mine the • upper seams first to protect the lower seams from mine-induced ground movements. Theoretically, in this descending sequence, overburden movements, including those in caved areas, should be allowed to settle before commencing lower seam mining. Abutment stresses on pillars, however, cannot be avoided and are transmitted to lower seams. The magnitude of these stresses depends on the interburden thickness- the thinner the interburden, the higher the transmitted stresses. Titis mechatii.sm occurs both on a descending or ascending sequence. Other factors, aside from ground control, affect the mining sequence such as resource recovery, surface subsidence impacts, coal quality, etc. A preliminary evaluation was made in this study based on a comparison of three factors directly related to the geotechnical evaluation: resource recovery, ground control, and surface impact. Table 3.4.A.7 summarized the results for ascending and descending mining sequences in Area III. Both fall and partial caving was assumed with t}tree-seam mining. partial caving includes the ttse of arching l.o min.imize ground movements in adjacent seams and is applicable for the room-and-pillar layouts analyzed. An ascending mining sequence was chosen for analysis because of an expected seven percent improvement in the B seam resource recovery and better ground conditions. Tltis advantage is reinforced by the better coal quality of the B seam. In one of the areas, however, a descending sequence was used because of the large interburden thickness between the F and B seams and the absence of the E seam. This is the area where mining • is planned in the 5 year permit area. Table 3.4.A.8 shows the mining segttettce used in the design for the five areas. l