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spoil ridge is created in order to grade the disturbed areas in a diverse manner such that surface <br />irregularities are created to minimize erosion, improve range and wildlife habitat, increase infiltration and <br />improve soil moisture holding characteristics for the revegetation process. Creating an acceptable <br />poslmining land form is the primary consideration that determines the number of spoil ridges that may exist <br />in the field. Once the spoil ridges are reduces to the point that equipment can be safely operated across <br />the slope of the rough graded area, final grading will be done on the contour. <br />Final interior spoil grading will produce a diverse topography with slopes constructed at as shallow <br />a grade as the conditions allow. Final highwalls will be generally reduced to slopes of 3H:1V or <br />flatter. In some cases backfill material is placed against undisturbed virgin ground at the top of a <br />final highwall where the final slopes may exceed 3H:1V. In these isolated areas, above the final <br />highwall, the natural slope was originally steeper than 3H:1V. <br />As mining has progressed to the end of mine life, it has become evident that the spoil material <br />cannot be left in the most optimum locations to achieve 3H:1V slopes in all situations. This <br />situation is especially evident in the Sage Creek/Wolf Creek mining areas associated with the pit <br />end highwalls along the ridgetops. SCC recognized this problem and initiated a stability study to <br />determine minimum factors of safety for varying highwall heights and slopes (under saturated and <br />unsaturated conditions). Twenty-two moisture-density tests were conducted within the Wolf Creek, <br />Sage Creek, North Wadge and South Wadge pits. The spoil material was modeled with a slope of <br />3H:1V. The stability analysis was pertormed under both unsaturated and saturated conditions in <br />order to determine the best and worst case scenarios. When placed at a 3H:1 V slope and 100 feet of <br />height, the spoil material has a Factor of Safety of 2.55 for unsaturated conditions and 2.30 for <br />saturated conditions. A sensitivity analysis was done to determine factors of safety under different <br />conditions. The highwall was changed to 50 feet and 200 feet to see the effect of fill depth on <br />stability. In addition, analyses were performed with the spoil at a slope of 2.SH:1 V with a highwall of <br />200 feet in order to determine the "worst case" scenario. The results of the sensitivity analysis <br />shows that the spoil material has a Factor of Safety greater than the required value of 1.3 even <br />under the saturated conditions with pit slope heights up to 200 feet and slopes of 2.SH:1V. The <br />stability report and data are included as Attachment 20-A, Pit Backfill Stability Report. SCC <br />proposes to continue to construct final slopes at as moderate a slope as conditions allow and to <br />remain in compliance with Rule 4.14.2(1)(b). As shown on Exhibit 20-2, some areas will be Steeper <br />than the originally approved 3H:1V slope, but will not exceed the angle of repose, All backfilled <br />slopes in disturbed areas will be placed to achieve a static factor of safety of at least 1.3. <br />~, <br />L.J <br />PR-05 3 Revised 01/06 <br />