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• 4.2 UU Triaxial Shear Testing <br />Selection of shear strength parameters is a key constituent of the stability evaluation. The fine- <br />grained nature of the gob indicates that it may behave in an undrained manner during <br />construction. Excess pore pressures have also been recorded by vibrating wire piezometers <br />installed in Gob Pile No. 2. For discussion of the vibrating wire piezometer installation and <br />readings, please refer to Buckhorn (2006). For these reasons, unconsolidated-undrained (UU) <br />triaxial shear tests were conducted onthin-walled (Shelby) tube samples obtained from placed <br />gob at Gob Pile No. 2. <br />A total of eight UU triaxial shear tests have been conducted on compacted gob material to date. <br />Two of the samples (SH-04 and SH-OS) were tested as part of a stability evaluation of Gob Pile <br />No. 2. Six of the samples (ST-07 through ST-11, and ST-14) were obtained from three <br />boreholes drilled at Gob Pile No. 2 in December 2006. The samples were tested at confining <br />pressures approximately equal to the vertical effective stress of the gob at the time of sampling. <br />Table 3 summarizes the undrained shear strengths from the testing program, Laboratory test data <br />are presented in Appendix D.1. <br />Table 3. Undrained Shear Strengths from UU Triaxial Testing <br /> Sample Undrained <br />Sample Borehole Depth Shear Strength, <br />ID ID (ft) Su (pst7 <br />SH-04 BGOS-08 18.5-20.5 1,250 <br />SH-05 BGO5-09 12-13.5 1,655 <br />ST-07 BG06-10 15-16.5 1,480 <br />ST-08 BG06-10 30-31.5 1,315 <br />ST-09 BG06-10 45-46.5 1,440 <br />ST-10 BG06-11 20-21 2,245 <br />ST-11 BG06-11 30-31.5 2,165 <br />ST-14 BG06-12 40-41.5 1,075 <br />Mean = 1,578 psf <br />Standard Deviation = 423.2 <br />Coefficient of Variation = 26.8°f° <br />The undrained shear strength of the eight samples ranges from 1,075 psf to 2,245 psf, with a <br />mean undrained shear strength of 1,578 psf. These values are consistent with rough correlations <br />to the consistency of the placed gob and the standard penetration N-values. The coefficient of <br />variation (COV) for the data is equal to the standard deviation divided by the mean value. The <br />COV is a measure of the data scatter. The COV for the UU triaxial shear data is approximately <br />27%. This falls within the reported range of 13 to 40% for undrained sheaz strength according to <br />Kulhawy (1992), Harr (1987), and Lacasse and Nadim (1997). <br />Bowie Gob Pile No. 4 stability report <br />Project NOS-126-GEO <br />Page 9 of 15 <br />