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increasing with depth). The undrained shear strength was capped at a maximum value of 2,000 <br />psf for analyses assuming an increase in Su with depth. <br />Short-term stability analyses were conducted for overall stockpile batters of 5H:1 V (existing), <br />4H:1 V, and 3H: I V. As indicated in Table 8, the critical short-term factor of safety was 1.5 to <br />1.6 for a 5H:1 V slope; 1.3 to 1.4 for a 4H:1 V slope; and approximately 1.0 for a slope of 3H:1 V. <br />All critical failure surfaces for the short-term analyses were located within the coal waste (rather <br />than through the foundation materials. Based on these results, we recommend a maximum <br />overall batter of the coal waste stockpile of 4H:1 V. If it can be demonstrated that pore pressures <br />are dissipating, the recommended slope angle may be increased. Monitoring of piezometers at <br />the gob pile indicate that pore pressures are slightly increasing at some locations. We <br />recommend continued monitoring of piezometers. <br />Table 8. Summary of Short-Term (Undrained) Analyses <br />Notes: <br />1. Overall angle for slopes above lower bench (i.e., above bottom 50 vertical feet) <br />2. Factor of safety according to Spencer's procedure <br />3. d = depth in feet <br />4. Su assumed to increase with depth to a maximum value of 2,000 psf <br />An analysis was conducted to evaluate the sensitivity of the factor of safety to the undrained <br />shear strength, Su. These results are presented in Figure 2. It should be noted that the variations <br />in Su impact the factor of safety significantly. Given the variability in the measured Su, this <br />sensitivity analysis confirms our recommendation that the overall slope of the gob stockpile not <br />exceed 4H:1 V. <br />Bowie No 2 Gob Pile Stability Dec2005 <br />Project #05- 126 -GEO <br />Page 15 of 18 <br />Strength <br />Factor of <br />Slope' <br />Parameters <br />Description <br />Safety2 <br />5H:1V <br />Su= 1650psf <br />Large scale failure through coal waste <br />1.51 <br />Su =53d , <br />1.64 <br />4H:1 V <br />Su =1650 sf <br />Large scale failure through coal waste <br />1.28 <br />Su =53d4 <br />1.39 <br />3H:1V <br />Su =1650 sf <br />Large scale failure through coal waste <br />0.96 <br />Su =53dP4 <br />1.02 <br />Notes: <br />1. Overall angle for slopes above lower bench (i.e., above bottom 50 vertical feet) <br />2. Factor of safety according to Spencer's procedure <br />3. d = depth in feet <br />4. Su assumed to increase with depth to a maximum value of 2,000 psf <br />An analysis was conducted to evaluate the sensitivity of the factor of safety to the undrained <br />shear strength, Su. These results are presented in Figure 2. It should be noted that the variations <br />in Su impact the factor of safety significantly. Given the variability in the measured Su, this <br />sensitivity analysis confirms our recommendation that the overall slope of the gob stockpile not <br />exceed 4H:1 V. <br />Bowie No 2 Gob Pile Stability Dec2005 <br />Project #05- 126 -GEO <br />Page 15 of 18 <br />