Laserfiche WebLink
4.27 OPERATIONS ON STEEP SLOPES <br />Colowyo will conduct no surface coal mining or reclamation on "steep slopes" as defined by <br />grades in excess of 20 degrees by Rule 4.27; therefore, this part is not generally applicable to <br />this permit application. <br />As discussed in Section 4.14.2, final grading will be accomplished such that overall grades <br />will not exceed lv:3h. Rule 4.27 requires that a showing be made which demonstrates a <br />minimum static factor of safety of 1.3 for all portions of the reclaimed land. <br />The following analysis is provided for that demonstration: <br />As a general observation, such a demonstration can easily by made when postmining grades <br />do not exceed lv:3h (approximately equivalent to 18.4 degrees). For example, assuming a <br />cohesionless dumped spoil slope with a 3H:1 V slope composed of 125 lbs/sq. ft. in-place <br />density and an internal friction angle (phi) of 35 degrees, the safety factor F for this "infinite <br />slope" problem simplifies to: <br />F = tan (35 degrees) /tan(18.4 degrees) = 2.1 <br />This factor is well above the required safety factor of 1.3. This analysis assumes that no <br />phreatic surface has developed, i.e. no groundwater is present. For the purposes of this <br />analysis, this is a valid assumption. According to the U.S. Army Corp. of Engineers Manual <br />entitled "Engineering Design, Slope Stability, October, 2003" (EM 1110-2-1902), in the case <br />of cohesionless soils, "the critical mechanism is shallow sliding, which can be analyzed as <br />the infinite slope failure mechanism." In this case, a graphical solution from the manual can <br />be used to verify the equation above. <br />The calculated factor of safety shown above is for a shallow surface failure, and that surface <br />is controlling. Adeeper-seated, larger failure surface would have an even higher factor of <br />safety. It is also generally recognized that such a 2-dimensional analysis is conservative. <br />This is because it does not account for additional soil strength that occurs when 3- <br />dimensional effects are considered. <br />In addition, each of the spoil pile designs (Streeter Fill, West Pit Fill, and Section 16 Fill) <br />contain further information regarding other stability analyses that have been performed. <br />These include additional information regarding material properties, hydrologic assumptions, <br />and laboratory testing results that have been performed as components of the stability <br />analyses. See Section 2.05.3 and Exhibit 19 for more information. <br /> <br />4.27-1 Revision Date: 4/11/06 <br />Revision No.: TR-63 <br />