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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 coal waste material and thin - walled "Shelby tube" <br />samples obtained during drilling. <br />4.2.1 Unconsolidated - Undrained (UU) Triaxial Testing <br />The Unconsolidated - Undrained (UU) triaxial test is conducted on the soil sample without <br />consolidating the sample or allowing drainage to occur during shearing. The UU test is used to <br />determine total stress shear strength parameters, usually expressed as the undrained shear <br />strength, Su. This test represents short-term conditions. "Short-term conditions" are essentially <br />the time required for drainage to occur. Depending on the hydraulic conductivity of the soil and <br />the length of the drainage path, drainage may occur during construction (for relatively permeable <br />soils such as sands and gravels) or many years after completion (for soils with low hydraulic <br />conductivity such as clayey soils). Undrained shear strengths are used in total stress analyses for <br />situations where excess pore pressures generated during fill placement dissipate slowly in <br />comparison with the rate of fill placement. <br />A total of five UU triaxial tests were conducted on samples taken from the coal waste stockpile <br />at Bowie. Two tests were conducted on remolded samples of gob material (BS -1 and BS -4) and <br />three tests were conducted on thin - walled Shelby tube samples (SH -04, SH -05, and SH -07). The <br />samples were saturated under back pressure and then sheared without drainage under a confining <br />pressure of 20 psi. Stress - strain curves for the tests are presented in Appendix E and undrained <br />shear strengths are summarized in Table 4. The undrained shear strengths vary considerably <br />(from 180 psf to 2,630 psf). This range of values is likely an indication of the variability of soil <br />density and composition, but also is attributed to sample disturbance and inherent inaccuracies <br />associated with the UU test. The undrained shear strength for SH -07 was particularly low. The <br />laboratory indicated the sample was very loose and, therefore, disturbed during extrusion. The <br />average undrained shear strength of four tests, not considering SH -07, was 1,656 psf. <br />Table 4. Undrained Shear Strengths from UU Triaxial Testing <br />4.2.2 Consolidated - Undrained (CU) Triaxial Testing <br />Bowie No 2 Gob Pile Stability Dec2005 <br />Project #05- 126 -GEO <br />Page 9 of 18 <br />Undrained <br />Sample <br />Borehole <br />Sample Shear Strength, <br />ID <br />ID <br />Type Su (psf) <br />BS -1 <br />N/A <br />Remolded 650 <br />BS -4 <br />N/A <br />Remolded 3,050 <br />SH -04 <br />BG05 -08 <br />Shelby Tube 1,250 <br />SH -05 <br />BG05 -09 <br />Shelby Tube 1,655 <br />SH -07' <br />BG05 -11 <br />Shelby Tube 180 <br />Average = 1,656 psf <br />Notes: <br />1. Laboratory noted sample to be disturbed and very loose <br />2. Su for SH -07 not considered in the calculation, due to sample disturbance <br />4.2.2 Consolidated - Undrained (CU) Triaxial Testing <br />Bowie No 2 Gob Pile Stability Dec2005 <br />Project #05- 126 -GEO <br />Page 9 of 18 <br />