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RULE 4 PERFORMANCE STANDARDS <br />Section 19 has also been removed. Finally, the revised PMT associated with TR -81 as depicted on Map 41A <br />also brings post- mining surface hydrology closer to the pre- mining condition. <br />All requirements set forth in Section 4.27 of the Regulations will be followed during operation and <br />reclamation. Drainage plans are shown in Exhibit 7, Item 20, Erosion and Sediment Control Structures. The <br />post- mining topography is shown on Map 19B. The post- mining watershed drainage areas will be the same <br />as the pre- mining drainage areas. <br />Highwalls will be completely backfilled with spoil material in a manner which results in a static safety factor <br />of at least 1.3. No land above the highwalls will be disturbed except as shown on Map 23A, Mine Plan. The <br />highwall will be blended into the backfilled material to result in a natural and gradual slope change. <br />As discussed in Section 4.14.2, final grading will be accomplished such that overall grades will not exceed <br />1 v: 3h. Rule 4.27 requires that a showing be made which demonstrates a minimum static factor of safety of <br />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 do not exceed <br />lv:3h (approximately equivalent to 18.4 degrees). For example, assuming a cohesionless dumped spoil slope <br />with a 311:1V slope composed of 125 lbs /sq. ft. in -place density and an internal friction angle (phi) of 35 <br />degrees, the safety factor F for this "infinite 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 phreatic surface has <br />developed, i.e. no groundwater is present. For the purposes of this analysis, this is a valid assumption. <br />According to the U.S. Army Corp. of Engineers Manual entitled "Engineering Design, Slope Stability, <br />October, 2003" (EM 1110 -2- 1902), in the case of cohesionless soils, "the critical mechanism is shallow <br />sliding, which can be analyzed as the infinite slope failure mechanism." In this case, a graphical solution <br />from the manual can be used to verify the equation above. <br />The calculated factor of safety shown above is for a shallow surface failure, and that surface is controlling. A <br />deeper- seated, larger failure surface would have an even higher factor of safety. It is also generally <br />recognized that such a 2- dimensional analysis is conservative. This is because it does not account for <br />additional soil strength that occurs when 3- dimensional effects are considered. <br />In addition, each of the spoil pile designs (West Taylor and East Taylor Fills) contain further information <br />regarding other stability analyses that have been performed. These include additional information regarding <br />material properties, hydrologic assumptions, and laboratory testing results that have been performed as <br />components of the stability analyses. See Section 2.05.3 and Exhibit 19 for more information. <br />4.28 FACILITIES NOT LOCATED AT THE MINESITE <br />This section is not applicable to the permit revision. All facilities used by Colowyo in their current operations <br />will continue to be used for the South Taylor mining operations. <br />4.29 IN SITU PROCESSING <br />This section is not applicable. <br />South Taylor /Lower Wilson — Rule 4, Page 23 Revision Date: 10/31/13 <br />Revision No.: MR -129 <br />