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4.3 CU Triaxial Shear Testing <br />Three consolidated-undrained (CU) triaxial shear tests with pore pressure measurement were <br />conducted on gob as part of the stability evaluation for Gob Pile No. 2. One sample (BS-3) was <br />remolded and two samples were from Shelby tubes taken from placed gob. The results from the <br />CU triaxial shear data were used to estimate effective stress strength parameters (Mohr-Coulomb <br />~' and c') for long-term stability analyses. Table 4 presents the effective stress parameters <br />determined from the CU triaxial tests. Laboratory data from the testing are presented in <br />Appendix D.2 of this report. For more discussion of these tests, please refer to Buckhorn (2006). <br />Table 4. Effective Stress Shear Strength Parameters <br />From CU Triaxial Testing <br />Sample Borehole ~' Cohesion, <br />ID ID (degrees) c'(psf) <br />BS-3 N/A 31 330 <br />SH-1 BG05-05 34 60 <br />SH-2 BGOS-06 33 60 <br />5.0 STABILITY EVALUATIONS <br />Slope stability of Gob Pile No. 4 was evaluated using two-dimensional limiting equilibrium <br />stability analyses with the computer software SLIDE developed by Rocscience (2004). Critical <br />block and circular failure surfaces were determined using total and effective stress analyses. The <br />simplified Bishop and Spencer procedures were used. Only static loading conditions were <br />evaluated. <br />Selection of the factor of safety for stability analyses depends primarily on the uncertainty of the <br />analysis conditions and the consequence of failure. The greater the uncertainty or consequence <br />of failure, the larger the required factor of safety. We understand that a minimum factor of <br />safety of I.50 is required for both short-term and long-term stability. Results from the stability <br />evaluations are discussed herein. Output from critical stability analyses is presented in <br />Appendix E. <br />5.1 Cross Sections and Geometry <br />Both total and effective stress analyses were conducted for long term and short term stability <br />evaluations. Stability of the Gob Pile No. 4 was evaluated for the maximum section (through the <br />approximate central portion of the stockpile) as indicated on Drawing 2 in Appendix A. The <br />maximum section is considered to represent the likely worst case scenario with respect to slope <br />geometry due to the maximum height of waste. <br />As previously indicated, native soils are currently being excavated from the proposed Gob Pile <br />No. 4 footprint for cover material for Gob Pile No. 2 and to be blended with gob at Gob Pile <br />No. 2. It is anticipated that excavation of native soils will continue during gob placement at Pile <br />No. 4 for the same purposes. Furthermore, excavation at the waste pile footprint provides <br />Bowie Gob Pile No. 4 stability report <br />Project #t05-12fi-GEO <br />Page ]0 o(15 <br />