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4.2.2 Unconsolidated-Undrained (UU) Triaxial Testing on Foundation Soils <br />'~ UU triaxial tests were conducted on three thin-walled samples obtained during drilling of the <br />foundation soils at Gob Pile No. 3. For samples (ST-I and ST-4), UU tests were conducted on <br />three points at three confining pressures (10, 35, and 75 psi). For sample ST-5, only enough <br />sample was available for two points at confining pressures of 20 and 50 psi. The tests were <br />conducted at the natural moisture content of the soils (i.e., samples were not saturated). <br />Therefore, an increase in the undrained shear strength is generally observed with increasing <br />confining pressures. The confining pressures were selected for the tests to represent effective <br />stresses ranging from the existing vertical stress to the anticipated stress increase due to gob <br />placement. Results of the UU testing of foundation soils at Gob Pile No. 3 are presented in <br />Table 5. As discussed in Section 5.2, total stress shear strength parameters (Mohr-Coulomb ~ <br />and c) were developed for the foundation soils based on the UU triaxial tests of the unsaturated <br />samples. <br />Table 5. Undrained Shear Strengths of Foundation Soils <br />From UU Triaxial Testing <br /> Sample Confining Undrained <br />Sample Borehole Depth Pressure Shear Strength, <br />ID ID (ft) (psi) Su (psf) <br /> 10 1,230 <br />ST-1 BG06-02 10-12 35 6,517 <br /> 75 11,725 <br /> 10 1,402 <br />ST-4 BG06-06 10-12 35 5,188 <br /> 75 12,650 <br />ST-5 BGO6-06 25-27 20 1,866 <br /> 50 1,703 <br />4.2.3 Consolidated-Undrained (CU) Triaxial Testing on Gob <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 (SH-1 and SH-2) taken from placed gob. <br />Samples BS-3 and SH-1 were tested at three different consolidation pressures (20 psi, 40 psi, and <br />80 psi). Only two consolidation pressures (40 psi and 80 psi) were tested for sample SH-2 as not <br />enough sample was recovered for the third point. These consolidation pressures were selected to <br />represent the range of potential overburden stresses anticipated for the ultimate gob stockpile <br />configuration. <br />The results from the CU triaxial shear data were used to estimate effective stress strength <br />parameters (Mohr-Coulomb ~' and c') for long-term stability analyses. Table 6 presents the <br />effective stress parameters for gob determined from the CU triaxial tests. Laboratory data from <br />the testing are presented in Appendix D of this report. For more discussion of these tests, please <br />refer to Buckhorn Geotech (2006). <br />Bowie Gob Pile No. 3 stability report <br />Project N06-481-002 <br />Page 10 of I7 <br />