2012-09-06_PERMIT FILE - C1981019A

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RULE 4 PERFORMANCE STANDARDS With the more recent mine plan that no longer extends as far into the NW /4 of Section 21 or the SEA of Section 20, a revised mine plan has been adopted. This results in an overall smaller pit footprint and reduced overburden and backfill volumes than those originally approved in the South Taylor mine plan in PR -02. To enable development of the box cut, the out -of -pit valley fills included in that PR -02 are still necessary, and it is necessary to extend the toe of the East Taylor Fill by approximately 900 feet and the West Taylor Fill by approximately 600 feet, in order to accommodate the temporary spoil piles needed to establish the box cut. That extension is one of the major items in technical revision TR -81. The above described pull -back of the southeasterly limit of the pit results in the mine plan and PMT as described in Mine Plan Map 23A and Map 45 of TR -81. As is evident on the cross section drawings associated with those maps, the TR -81 PMT more closely resembles pre- mining topography than that previously approved in PR -02. The temporary spoil stockpiles are depicted on Map 45. The need for a permanent valley fill in the headwaters of West Taylor Creek in the SWA of Section 17 and the NE /4 of Section 19 has also been removed. Finally, the revised PMT associated with TR -81 as depicted on Map 4 1 A also brings post- mining surface hydrology closer to the pre- mining condition. All requirements set forth in Section 4.27 of the Regulations will be followed during operation and reclamation. Drainage plans are shown in Exhibit 7, Item 20, Erosion and Sediment Control Structures. The post- mining topography is shown on Map 19B. The post- mining watershed drainage areas will be the same as the pre- mining drainage areas. Highwalls will be completely backfilled with spoil material in a manner which results in a static safety factor of at least 1.3. No land above the highwalls will be disturbed except as shown on Map 23A, Mine Plan. The highwall will be blended into the backfilled material to result in a natural and gradual slope change. As discussed in Section 4.14.2, final grading will be accomplished such that overall grades will not exceed 1v: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. South Taylor /Lower Wilson — Rule 4, Page 31 Revision Date: 5/31/10 Revision No.: TR -81