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Sanborn Creek Subsidence Page 24 September 2, 1997 ] is 8.09-ft. This results primarily from the crushing of the gateroad yield pillars. Mining of Panel #3 makes a smaller contribution to predicted worst-case subsidence of the ground surface over Panel #2. 1 LONGWALL PANEL GROUP SUBSIDENCE PREDICTION The maximum surface subsidence predicted from longwall mining 12-ft of coal in Panel #2 and Panel #3 is more than would result from longwall mining the individual panels. Table 5 presents the calculated worst-case surface subsidence and strains resulting from applying the NCB method using the assumption that a panel 1300-ft wide by 2520-ft long is longwall mined at the location of Panel #2 and Panel #3. Such a panel matches the outer boundaries of Panel #2 and Panel #3 without any intervening gateroad pillars. The maximum predicted vertical subsidence in this case is 8.77-ft compared to an individual panel subsidence prediction of 3.90-ft for Panel #2 by itself and 5.19-ft for Panel #3 by itself. Table 5 presents the maximum subsidence prediction for the same assumption _ applied to all three panels in the first panel group longwall mined as a single 1900-ft wide panel, without any gateroad yield pillars. 1 The maximum predicted vertical subsidence increases to 9.62-ft for Panel #2, Panel #3 and Panel #9 mined as a single longwall panel, without gateroad yield pillars. J The assumption of no gateroad yield pillars is unrealistic, particularly for the B.5-ft high gateroads and 12-ft high panels. Therefore, the room and pillar subsidence prediction method presented by Abel and Lee (1984) was used to estimate the • subsidence resulting from crushing of the intervening gateroad -~ yield pillars. Table 6 presents the results of the predictive analysis. The planned gateroad pillars are 100-ft long on 120-ft centers. One row of pillars is 50-ft wide and the other 30-ft j wide. The critical pillar, i.e. the pillar that will flatten the least under the load transferred to the gateroad pillars, is the wider 50-ft pillar. Table 6 presents the average tributary gateroad pillar stress. The actual pillar stress on the 50-ft wide pillar will be somewhat higher immediately before failure because the narrower 30-ft pillar should crush first, shedding its load to the wider gateroad pillar. The magnitude of vertical gateroad I pillar stress will crush the gateroad pillars when the second adjacent panel is mined past. 24