Coal Age Magazine - For nearly 100 years, Coal Age has been the magazine that readers can trust for guidance and insight on this important industry.
Issue link: https://coal.epubxp.com/i/95257
ground control continued Remnant pillars, gob areas and interburden thickness all influence the seam interaction at Four West. The under-mining in the Pittsburgh No. 8 seam was a mix of room- and-pillar and longwall operations. The remnant pillars in the Pittsburgh No. 8 seam shown in Figure 3 indicate that the remnant pillars are irregularly shaped and appear intermittently. The width of the remnant pillars varies from 30-200 ft (9.14-60.96 m). The interburden thickness across the mine varies from 70-130 ft (21.34-39.62 m). Statistical analysis of the borehole data indi- cates that 80-90 ft (24.38-27.43 m) interbur- den thickness is predominant across the mine. As an adverse influence, remnant pil- lar incurs stress concentrations that propa- gate through the interburden, causing roof failure, floor heave and rib sloughage. Analysis of Interaction Stress To analyze and better understand the interaction mechanism, a computer numerical model was established based on the stratigraphic column. The interbur- den between the Sewickley and Pittsburgh No. 8 seams is 120 ft (36.58 m). The width of the remnant pillar is 100 ft (30.48 m) with both sides mined out with an average mining height of 7 ft (2.13 m). The gob width is 300 ft (91.44 m) in this model. The vertical stress distribution across the mod- el is shown in Figure 4. Three vertical monitoring lines were established to examine the vertical stresses from the sur- face to the Pittsburgh No. 8 seam floor at the gob center, remnant pillar edge and remnant pillar center. It can be seen from Figure 4 that stress concentrations occur around the remnant pillar and propagate to the roof of Sewickley seam. Above the gob and close to the Sewickley seam, a tension zone occurs, with the strata above the gob under a much lower stress. The vertical stress along the vertical gob center line shown in Figure 5 indicates that the Sewickley seam that "over hangs" the remnant pillar(s) is under compression while the Sewickley seam roof 100-150 ft (30.48- 45.72 m) away from the remnant pillar edge experiences near zero compression. As indi- cated in Figure 4, the Sewickley seam roof above the center area of both gobs is domi- nated by tension. The vertical stresses along the three ver- tical monitoring lines further verify that, comparing with the theoretical original ver- tical stress (γH, H is overburden depth), the vertical stresses along the gob center line are much lower and along the remnant pil- lar center line much higher due to the stress superimposition from both side gobs (Figure 6). From the subsidence point of view, the Sewickley seam roof and floor at Mine A are within the fracture zone developed by the under-mining (24-60 x h, h is the mining height, 7 ft (2.13 m) in this case). Essentially, the roof and floor strata expe- rience a mining disturbance twice. After Figure 2: Surface topography (units: ft). November 2012 www.coalage.com 39