2008-09-29_PERMIT FILE - C1981019A (15)

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RULE 4 PERFORMANCE STANDARDS 4.27 Operations on Steep Slopes Colowyo will conduct no surface coal mining or reclamation on "steep slopes" as defined by grades in excess of 20 degrees by Rule 4.27; therefore, this part is not generally applicable to this permit application. As discussed in Section 4.14.2, final grading will be accomplished such that overall grades will not exceed lv:3h. Rule 4.27 requires that a showing be made which demonstrates a minimum static factor of safety of 1.3 for all portions of the reclaimed land. The following analysis is provided for that demonstration: As a general observation, such a demonstration can easily by made when postmining grades do not exceed lv:3h (approximately equivalent to 18.4 degrees). For example, assuming a cohesionless dumped spoil slope with a 3H:1 V slope composed of 125 lbs/sq. ft. in-place density and an internal friction angle (phi) of 35 degrees, the safety factor F for this "infinite slope" problem simplifies to: F = tan (35 degrees) /tan(18.4 degrees) = 2.1 This factor is well above the required safety factor of 1.3. This analysis assumes that no phreatic surface has developed, i.e. no groundwater is present. For the purposes of this analysis, this is a valid assumption. According to the U.S. Army Corp. of Engineers Manual entitled "Engineering Design, Slope Stability, October, 2003" (EM 1110-2-1902), in the case of cohesionless soils, "the critical mechanism is shallow sliding, which can be analyzed as the infinite slope failure mechanism." In this case, a graphical solution from the manual can be used to verify the equation above. The calculated factor of safety shown above is for a shallow surface failure, and that surface is controlling. A deeper-seated, larger failure surface would have an even higher factor of safety. It is also generally recognized that such a 2-dimensional analysis is conservative. This is because it does not account for additional soil strength that occurs when 3-dimensional effects are considered. In addition, each of the spoil pile designs (Streeter Fill, West Pit Fill, and Section 16 Fill) contain further information regarding other stability analyses that have been performed. These include additional information regarding material properties, hydrologic assumptions, and laboratory testing results that have been performed as components of the stability analyses. See Section 2.05.3 and Exhibit 19 for more information. Rule 4 Performance Standards 4.27-1 Revision Date: 6/23/08 Revision No.: MR-91