PERMFILE45400 - Laserfiche WebLink

Home Browse Search

"D" Seam Longwall Subsidence Page 39 August 17, 1998 will be carried by the rubble. Whatever part of the overburden • load above the panels is carried by the collapsed roof rock will reduce the gateroad pillar crushing subsidence developing at the ground surface over the center of the adjacent panels. It would probably be more realistic, but less conservative, to assume that only one-half of the subsidence related to gateroad pillar crushing would be transferred to the centerlines of the outside panels in the groups of three panels. It is unlikely that subsidence resulting from gateroad pillar failure will result in significant vertical ground surface subsidence across the planned 280-foot wide barrier pillars between panel groups. The minimum width/height ratio of the 17-foot high barrier pillar between panel groups between Panel #3 and Panel #9 is more than sixteen. The maximum width/height ratio of the 10.9-foot high barrier pillar between Panel #12 and Panel #13 is nearly twenty-seven. The extremely flat barrier pillars are extremely strong. The average gateroad pillar stresses and vertical surface subsidence predicted when the gateroad pillars fail were calculated with Abel and Lee's method. Their method, shown graphically on Figure 5 was modified for an overburden density of 150 PCF instead of their original 144 PCF. 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, initially shedding its load to the wider gateroad pillar. The magnitude of vertical gateroad pillar stress should be sufficient to crush the gateroad pillars when the second adjacent panel is mined past. Figure 6 presents the predicted surface subsidence resulting from the planned crushing of the 8.5-foot high gateroad pillars over the full range of overburden contour depths; 500 feet, 1000 feet, 1500 feet, 2000 feet and 2500 feet. Subsidence predicted from gateroad pillar crushing was calculated using the method indicated on Figure 5. The only adjustment applied was to change the overlying rock density constant, from 0.0226 Mpa per meter of depth, equivalent to 149 PCF, to 0.0235 Mpa meter of depth, equivalent to 150 PCF. The following example for the gateroad pillar between Panel #1 and Panel #2 at 1500-ft (457m) of depth explains the method. o.ozssies~t = 119.6 MPa (17340 si) L max = 1-0.91 p Lma.,~ w = 119.6 ( Sp ) = 20.3 Subsidence($) = 8.469 + 11.95 In (20.3] = 99.4$ Subsidence(ft) _ °ioo (8.5) = 3.76-ft (Table 7) • 39