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The Consolidated - Undrained (CU) triaxial test is conducted on the soil sample allowing for <br />consolidation of the sample at a predetermined pressure prior to shearing. The samples are <br />saturated under back pressure and then isotropically consolidated. Once the sample has <br />consolidated, the sample is sheared at a constant rate without allowing drainage. The pore <br />pressures are measured and recorded during shearing, allowing for both total stress and effective <br />stress analysis. <br />A total of three CU triaxial shear tests were conducted on samples taken from the coal waste <br />stockpile. One test was conducted on a remolded sample of gob material (BS -3) and two tests <br />were conducted on thin - walled Shelby tube samples (SH -1 and SH -2). Samples BH -3 and SH -1 <br />were tested at three different consolidation pressures (20 psi, 40 psi, and 80 psi). Only two <br />consolidation pressures (40 psi and 80 psi) were tested for sample SH -2 as not enough sample <br />was recovered for the third point. These consolidation pressures were selected to represent the <br />range of potential overburden stresses anticipated for the ultimate gob stockpile configuration. <br />Undrained shear strength parameters were also determined from the CU triaxial tests by <br />evaluating the relation between the undrained shear strength, Su, and the effective overburden <br />stress, 6',,. The effective overburden stress can be related to the depth of coal waste. As <br />undrained shear strengths are defined by cohesion only (� = 0), this relationship essentially <br />represents a linear increase in undrained shear strength with depth. This assumes that some <br />consolidation has taken place prior to failure. For the stability evaluations, the undrained shear <br />strength increased according to the relationships presented in Table 5 to a maximum value of <br />2,000 psf. <br />Table 5. Relation between Su (psf) and Depth, d (ft) <br />From CU Triaxial Testing <br />Sample Borehole <br />ID ID Relationship',2 <br />BS -3 N/A Su (psf) = 1000psf + (39 psf /ft)(d ft) <br />SH -1 BG05 -05 Su (psf) = (53.1 psf /ft)(d ft) <br />SH -2 BG05 -06 Su (psf) = (53.9 psf /ft)(d ft) <br />Notes: <br />1. d = depth in feet <br />2. For stability evaluations, Su increases with depth to a maximum of 2,000 psf <br />Drained strength parameters were evaluated from the test data by plotting Mohr circles and stress <br />paths. As discussed in Section 6.2, drained effective stress analyses are intended for "long- term" <br />stability. These analyses assume that there are no excess pore pressures. If the soil is not <br />drained, pore pressures must be incorporated into the effective stress analysis. The proper pore <br />pressures are difficult to estimate and are subject to change with time and changes in loading. <br />Failure was selected at the maximum pore pressure measured during the CU triaxial test to <br />determine the effective stress strength parameters. The effective strength parameters, �' (angle <br />of internal friction) and c' (cohesion), were fairly consistent for the three samples tested. The <br />remolded sample BS -3 was found to have a slightly lower �' and slightly higher c'. Triaxial <br />shear test data are included in Appendix E. The effective shear strength parameters are presented <br />in Table 6. <br />Bowie No 2 Gob Pile Stability Dec2005 <br />Project #05- 126 -GEO <br />Page 10 of 18 <br />