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• maximum gob pile slopes and ultimate heights resulting in a minimum factor of safety of 1.50 <br />with respect to stability for both total and effective stress analyses. All analyses included a 15- <br />footwide horizontal bench located at a distance of 50 vertical feet from the downslope toe of <br />gob. The gob slopes above and below the bench were vazied, as well at the total height of the <br />gob pile for the critical Section A-A'. Once optimum slopes and ultimate heights were <br />determined for Section A-A', the stability was verified using the same geometries for <br />Sections B-B' and C-C'. It is important to note that due to the sloping native ground surface, the <br />elevation of the gob pile toe at Section B-B' is higher than the toe corresponding to Section A- <br />A'. Therefore, the gob pile height at Section B-B' will correspond to a greater ultimate height <br />measured at Section A-A'. This is reflected in Table 9. <br />The foundation soils were modeled as a single homogeneous material type. No bedrock was <br />incorporated into the model since bedrock was not definitely encountered to depths of 90 feet in <br />the geotechnical investigation. A groundwater profile was developed for the stability model <br />based on measurements of standpipe piezometers in December 2006 and January 2007. As <br />indicated in Section 3.1, groundwater measurements at this time range from approximately 19 to <br />58 feet below the ground surface. <br />Due the short groundwater monitoring period, critical analyses were verified considering a high <br />groundwater table. Analyses considering a high groundwater table were each gob geometry with <br />a minimum factor of safety of 1.50 for the critical Section A-A'. For these analyses, the <br />groundwater table was modeled approximately 5 feet below the ground surface. <br />• 5.2 Shear Strength Parameters <br />Total and effective stress strength pazameters were estimated for compacted gob and foundation <br />soils based on triaxiai compression test results. The strength pazameters aze discussed below for <br />gob and foundation soils. Table 8 presents the parameters used in the analyses. <br />5.2.1 Gob <br />Shear strength parameters for the coal waste material were determined based on UU and CU <br />triaxial sheaz tests as summarized in Tables 4 and 6. Total stress strength parameters for the gob <br />were based on 8 UU triaxial shear tests, and effective stress strength pazameters for the gob were <br />based on 3 CU triaxial sheaz tests with pore pressure measurement. For the stability analyses, <br />the mean undrained shear strength of the 8 UU triaxial tests (1,578 psf) was used. A <br />conservative estimate of effective strength pazameters for the gob was used in the analyses based <br />on three CU triaxial tests. On-going sheaz strength testing of compacted gob is recommended to <br />assess the variability in sheaz strength due to variation in gob composition, moisture content, and <br />placement. <br />The moist unit weight of the coal waste used in the analyses was 112.5 pcf. This is an average <br />moist unit weight of 114 nucleaz density tests performed to date in compacted gob at Gob Pile <br />No. 2. <br />LJ <br />Bowie Gob Pile No. 3 stability report <br />Project g06~81-002 <br />Page 12 of 17 <br />