APPCOR13326 - Laserfiche WebLink

Home Browse Search

Memo to Tony Waldron Bowie No. 2 Geotechnical Adequacy page 5 The preparer's SEH method application is correct for the case chosen. However, the case chosen is neither the most geotechnically sensitive nor the highest projected subsidence case for the Bowie No. 2 mine plan. As an exmple I performed an SEH method projection of subsidence above proposed Panel 36. Panel 36 is of particular interest because it occupies a situation roughly analogous to that of the 1st West and 2nd West panels of the Bowie #1, which experienced signficant subsidence cracking due to an hypoithesized outcrop barrier cantilever failure. Panel 36 is oriented with its long axis parallel to the steep outcrop slope gradient. As such its driection of retreat will be into the slope, which is the preferred orientation beneath a steep slope. The panel is 440 feet wide and approximately 2,100 feet long. Its overburden ranges from slightly less than 300 feet at its southern end to a high of approximately 815 feet about 200 feet south of its northern extremity. As such Panel 36 underlies the steep slope outcrop area of concern for possible poor quality coal burn effects and near surface chimney collpase potential. It also underlies the historicaly and potentially unstable surface of the steep southern permit area. I completed a very rudimentary analysis of the projected maximum vertical subsidence for two points along the long axial centerline of Panel 36. The first point was chosen to be 0.7 times the panel width from the southern end of the panel. The SEH generally projects maximum subsidence above asub-critical width panel to be achieved at a distance from the panel end equal or greater than 0.7 times the overburden depth. This condition is first occurs in Panel 36 at a point 280 feet from the southern end of the panel, which coincidentally has an overburden depth of approximately (122 meters) 400 feet (0.7 x 400' = 280'-. This point would be susceptible to the development of the maximum projected vertical subsidence after the face had passed a location 1.4 times the overburden depth from the southern end of the panel. Using figures 3 and 4 of the SEH to project the maximum projected subsidence at this point results in a Subsidence Factor, Sm,,, = 0.73 (figure 3), and a Subsidence Correction Factor, s/S = 0.7 (figure 41. Multiplying these factors times one another and the extracted thickness of 12 feet results in a projected maximum vertical subsidence of 0.73 x 0.7 x 12.0 feet = 6.1 feet. This significantly larger than the maximum projected subsidence of 3.5 feet determined by the preparer of Exhibit 15. Further, it is projected to occur beneath one of the permit area's most geotechnically fragile areas. The second point chosen for a rudimentary SEH subsidence projection is located on the panel long axial centerline approximately 975 feet north of the southern panel extremity. This point, with an overburden depth of approximately 550 feet (168 meters) is close to *.he overall center of Panel 36. Using the same SEH graphical subsidence projection methodology for this point results in a Subsidence Factor, STe% = 0.69 and a Subsience Coreection Factor, s/S = 0.56. Multiplying these factors times one another and the