2008-02-22_APPLICATION CORRESPONDENCE - C2008086 (43)

Home Browse Search

Subsidence. The locations of the maximum tensile and compressive strains with respect to the individual panel centerlines were estimated using Figure 12. NCB Horizontal Strain Profile Graph in Affected Environment/Subsidence. The maximum tensile and compressive strains are important because if they can be conservatively predicted steps can be taken to reinforce critical surface structures or modify the mining plan to reduce the maximum tensile and compressive strains. For example, high pressure natural gas pipelines have been undermined by longwalls by maintaining a smooth pipeline through the period when the trough is forming under the pipeline, while the longwall face advances across or along the pipeline. This has been accomplished by digging up, temporarily supporting the section of the pipeline ahead of the advancing longwall face and reburying the pipeline after the longwall face has advanced well past the elevated section of the pipeline. This procedure prevents the buried pipeline from being pulled apart at an open fracture. Many countries with significant longwall coal mining operations have recommended and(or) established allowable strains for particular surface features. Some of these are included in APPENDIX A. RECOMMENDED LIMITS FOR SUBSIDENCE INDUCED STRAIN AND TILT. 7.2 Maximum Horizontal Strain The maximum horizontal tensile strains are the most serious potential hazard with respect to anticipated subsidence impacts from longwall mining in the proposed Red Cliff Mine lease area. This involves protecting the public from larger open fractures, as shown on Figure 20. Tension Crack Over Starter Room, York Canyon Mine, when longwall mining at shallow depths (<500 feet). There is also the temporary potential for large boulders being dislodged from sandstone cliffs on the canyon walls by smaller tensile strains from deeper active longwall panels, as indicated on Figure 18. Ribside Tension Cracks in Road Fill and Cliff Face, York Canyon Mine. Table 10. Predicted Surface Fracture Widths Based on York Canyon Mine Measurements presents the relationship between predicted tensile strain and the measured width of selected open subsidence fractures above three longwall panels at the York Canyon Mine west of Raton New Mexico. The York Canyon Mine was mining coal in the Mesaverde Group, but the overburden lithology could well differ from that present at the Red Cliff Mine proposed Project Area. The horizontal tensile strain over the barrier pillars between panel groups will probably increase because the strain at the surface over the barrier pillar caused by each adjacent panel is additive. It is possible that the maximum horizontal tensile strain above the larger barrier pillars planned between panel groups could as much as double the tensile strain on the surface over the center of such a barrier pillar. This is possible because it depends on the panels on both sides being subcritical precisely enough to place the maximum tensile strain at the center of the barrier pillar. For example, using Figure 12. NCB Horizontal Strain Profile Graph in Affected Environment/Subsidence, the center of a 1000-foot wide panel at the depth of 2000 feet (Panel Width/Depth Ratio = 0.500) is 600 feet from the center of a 200-foot wide barrier pillar, 0.300 times the 2000-foot depth. The predicted tensile strain over the center of the barrier pillar from the first longwall panel to be completed on one side of the group barrier pillar is 95% of the predicted maximum horizontal tensile strain. If a longwall panel group with the same dimensions and depth is mined on the other side of the barrier pillar is completed it would add 95% of its maximum horizontal tensile strain at the center of the 200-foot barrier pillar, nearly doubling (approximately 1.9 times) the tensile strain at that location, C-32 DBMS 324