2007-11-08_PERMIT FILE - C1981041 (2)

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• Retreat pillars located beneath the creeks in room-and-pillar panels will be retained at a minimum width necessary to insure a 1.31 minimum safety factor. This safety factor was selected based on a large scale study of actual pillars which either collapsed or remained stable in underground coal mines (Salmon and Munro, 1967). The pillars designated to protect the Rapid Creek and Cottonwood Creek areas are designed to carry full tributary area loading from the overburden. This is an additional conservative measure which assumes the pillar load will not be transferred by arching to larger and stronger barrier and chain pillars adjacent to the pillar in question. The load carrying capacity of the pillars was estimated using physical properties of the Cameo `B' Seam coal derived from samples taken in the Roadside South Mine (Abel, 1982). An angle of internal friction of 38.8° and a compression strength at pillar ribside of 698 psi were used to provide a conservative estimate of coal • strength. A confined core pillar loading analysis adapted from A.H. Wilson was used to estimate the factors of safety for various retreat pillar sizes at various overburden depths. The. results of the pillar design analyses are presented in Figure 20-1 and Table 20-1. Example calculations and information to support these results are provided in Appendix 20-3. The calculation used in this appendix was applied to the Roadside North Mine room pillars to calculate the 5.27 safety factor. Protective pillars 15 feet wide were retained within 100 feet of Rapid Creek in the 3rd East Panel, and have been sufficiently strong to stop the collapse which has occurred, subsequent to mining in the remainder of the panel where maximum- recovery techniques were employed. The comparison of subsidence measurements over the Rapid Creek area to that over the panel to the north is depicted in the subsidence graphs of Exhibit 48, RS-21-020, RS-21-023 and RS- 21-024. 20-4 (Rev. 01/01)