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<br />Subsidence (2.05.6(6)) <br />The mine plan for the Roadside Mine has been revised to allow for protective <br />pillars beneath Rapid Creek and Cottonwood Creek within the permit area. <br />Pillars designed to safely carry maximum loads will be retained within an <br />area defined by a 25-degree angle-of-draw. This is a conservative measure <br />since the maximum angle-of-draw determined from actual subsidence measure- <br />ments made over room-and-pillar panels at the Roadside Mine is 21 degrees. <br />Information to verify this determination is contained in Appendix F. <br />Most of the pillars that will be developed beneath the creeks are main-entry <br />support pillars which are typically 80 by 80 feet in dimension. Pillars of <br />this size have a factor of safety against failure of 2.92 under maximum load- <br />ing at 1,500 feet which is the maximum overburden depth below the creeks. <br />Retreat pillars located beneath the creeks in room-and-pillar panels will <br />be retained at a minimum width necessary to insure a safety factor of at <br />least 1.31. This safety factor was selected based on a large scale study of <br />actual pillars which either collapsed or remained stable in underground coal <br />mines (Salamon and Munro, 1967). <br />The pillars designated to protect the Rapid Creek and Cottonwood Creek areas <br />are designed to carry full tributary area loading from the overburden. This <br />is an additional conservative measure which assumes the pillar load will <br />not be transferred by arching to larger and stronger barrier pillars and <br />chain pillars adjacent to the pillar in question. <br />The load carrying capacity of the pillars was estimated using physical pro- <br />perties of the Cameo 'B' seam coal derived from samples taken in the Road- <br />