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F uv.i -n,l I.. ,.114 <br />In addition, advances in computing power allow the analyses to more accurately <br />compute the factors -of- safety. One of the current capabilities permits the software to <br />utilize a more accurate value for pore water pressure at the base of each slice for a <br />sloping piezometrrc surface. Another advance permits the critical failure surface to be <br />`optimized' by making incremental changes to each segment along the critical failure <br />surface determined from the initial iterations. The result is often not a perfectly circular <br />failure surface; however, the intent is to more accurately define the critical failure surface <br />for a given slope. <br />3.2 Soil Parameters <br />Soil parameters used in the stability analyses were selected based upon <br />information in the referenced documents. As indicated in the 2008 report by Buckhorn, <br />drained conditions were assumed based upon the data obtained from the vibrating wire <br />piezometers. In general, the piezometers near the toe of gob pile #2 are showing a steady <br />decrease in pore pressures. In contrast, the piezometers near the top of the gob pile have <br />shown some increases in pore water pressures since installation. These increases are <br />consistent with the loading associated with addition of gob at the top of the pile. Over <br />time, these piezometers may show some short term increases as the height of gob pile is <br />increased; however, the general trend of the pore pressures in these piezometers is <br />anticipated to show a downward trend as the gob pile dewaters. <br />Based upon the information in the referenced reports, HBET does not believe that <br />the elevated pore pressures recorded in the piezometers higher in the gob pile (VWP -06 <br />and VWP -08) are sufficient to demonstrate undrained conditions in the gob pile. As a <br />result, HBET utilized drained conditions for the analyses. However, the pore pressures <br />should continue to be monitored over time as a sharp increase in the pore pressures over a <br />short time could indicate that undrained conditions are developing which may result in <br />instabilities in the gob pile. <br />The parameters used in the analyses are discussed in the following sections. <br />Compacted Gob <br />The referenced reports indicate that wet unit weights of between 104.6 pcf and <br />110 pcf have been used in various analyses over the last several years. Based upon <br />preliminary sensitivity analyses conducted by HBET, a wet density of 104.6 pcf provides <br />the lowest factor -of- safety. <br />The referenced reports also indicate that angles of internal friction (�') of between <br />33.50 and 34° have been used in various analyses for drained conditions. However, the <br />majority of the analyses used a value of 34 °. <br />XA2004 ALL PROJECTS'00I89 • JE Storer Associates Ine`00789 -0003 Bowie.Mii*000 - Geo'001E9.0003 R0321 LIRI doe 3 <br />