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4.2 Triaxial Compression Testing <br />' Selection of shear strength parameters is a key constituent of the stability evaluation. Both <br />Unconsolidated-Undrained (UU) and Consolidated-Undrained (CU) triaxial compression tests <br />were conducted on samples of remolded and thin-walled samples of gob and on thin-walled <br />samples of foundation soils at the Gob Pile No. 3 footprint. These test results were used to <br />develop total and effective stress strength parameters for the gob and the foundation soils. <br />Results for UU triaxial tests are presented in Appendix C and results for CU triaxial tests are <br />presented in Appendix D. <br />4.2.1 Unconsolidated-Undrained (UU) Triaxial Testing on Gob <br />The fine-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 Geotech (2006). For these reasons, unconsolidated-Undrained <br />(UU) triaxial shear tests were conducted on thin-walled (Shelby) tube samples obtained from <br />placed 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-I 1, 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 4 summarizes the results of Undrained shear strength testing for compacted gob material. <br />Table 4. Undrained Shear Strengths of Gob from UU Triaxial Testing <br /> Sample Undrained <br />Sample Borehole Depth Shear Strength, <br />ID ID (ft) Su (psf) <br />SH-04 BG05-OB 18.5-20.5 1,250 <br />SH-O5 BG05-09 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% <br /> <br />Bowie Gob Pile No. 3 stability report <br />Project tt06-481-002 <br />Page 9 of 17 <br />