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h(urch 2006 <br />• STABILITY ANALYSIS <br />Pil 9aekfi!lS~ufiility *IIW <br />1"he results of the field and laboratory testing were used to assign material properties for the stability <br />analysis of the pit backfilling at the IIW Mine. The average, corrected dry density of the spoil material <br />was used for modeling because it best represents the in-rim material. Although, both the higher[ and <br />lowest in-situ dry density values were used in two models to compare their effect on the stability of <br />the slope and also establish if the density is a controlling factor of the model. Both conceptual and <br />postmining topographic models were created to produce a variety of distinct scenarios to determine <br />the controlling parameters of the stability analysis. An idealized cross section of the pits was <br />developed which assumes a 0.3H:1V highwall slope. The highwall of the conceptual models was <br />constructed as being 50, 100, and 200 feet tall, in order to generate a vaziety of slope scenarios. The <br />postmining topographic model has a vertical relief of 475 feet, The floor of the pit was conservatively <br />estimated to be horizontal. In reality the pit floors are generally sloped back towards the highwall and <br />will provide some buttressing to the slope, <br />The spoils material was modeled with a slope of 2.SH:1V or the equivalent grade of 40% for the <br />conceptual models, and in their natural state for the postmining topographic model (average slope = <br />23.1% grade). The stability analysis was performed under both unsaturated and partially saturated <br />conditions in order to determine what water level would result in a Factor of Safety of 1.30. The <br />Wadge Coal under/overburden phreadc surfaces were used for the ground water conditions on the <br />postmining topographic model. Due to a concern in the range of unit weight values obtained through <br />laboratory testing, a unit weight sensitivity analysis was also performed on the postmining topographic <br />model using the lowest (94.2 pcf) and highest (115.6, pcf) values. <br />• The results of the sensitivity analysis are shown in Table 2, IIW Mine Prt Barkfill Stability Analytrr. <br />Printouts of the stability runs are shown in Appendix B, IIW Mine Stability Runs. <br />CONCLUSION <br />The steepest remedial slope at the backfilled pits IIW Mine was measured to be 50 vertical feet at a <br />slope of 2.SH:1V (40% grade). Consequently, the preliminary stability analyses were modeled using <br />this slope. When placed at a 2.SH:1 V slope that is 50 vertical feet tall, the spoils material for the IIW <br />Mine has a Factor of Safety (F.S.) of 2.38 for unsaturated conditions and a F.S. of 1.31 for a phreatic <br />surface that is approximately five feet below ground Level near the slope crest and dayGghting at the <br />slope toe. To be conservative and determine the effect of slope height, 100 and 200 vertical foot <br />models were also created. The 100-foot slope produced a F.S. of 2.17 for unsaturated conditions and <br />a F.S. of 1.34 with a phreadc surface 18 feet below ground level near the slope crest and day6ghting at <br />the toe slope. The 200-foot slope produced a F.S. of 2.04 for unsaturated conditions and a F.S. of <br />1.32 with a phreadc surface 40 feet below ground level near the slope crest and daylighdng at the toe <br />slope. A summary of the stability analysis is shown in Table 3, IIW Mine Pit Backfrll.Stability Analytic. <br />See Appendix B, IIW Mine Stability Runt for specific runs, locations of phreadc surfaces, and their <br />respective F.S. <br />After the conceptual models were run, the largest slope (475 vertical feet) containing the steepest <br />section (150 vertical feet, avg. slope = 20°, avg. grade = 36.4%) of the postmining topography was <br />modeled to investigate the effects of a composite slope. The upper 150 vertical feet of the section <br />contains the steepest slopes, below the steep slopes the terrain gradually levels off. Using information <br />from estimated Wadge Coal under/overburden phteatic surfaces with the actual postmining <br />• topography produces the same F.S. of 2.34 for unsaturated and partially saturated conditions. The <br />phreadc surface ranges from 250 below the ground surface at the head of the slope to daylighting at <br />the toe of the slope. The unit weight sensitivity models produced F.S. values that vary from 2.33 <br />d1[I'/FI * 1475 Pirse Groue Roud, .Suite 109 * Steamboat Spnngt, CO 80487 <br />U'.VI'llmnlllUll4i lLr.'kl4I ~m64'i,/II U'Jlm.ll~... <br />Il.'RIYf rM1' <br />