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• Table 4. Strength Parameters Used in the Stability Evaluations <br /> <br />Unit Effective Stress <br />Parameters <br />Material <br />Type Weight <br />(pctf) ~' <br />(deg.) c' <br />(psi <br />Coal Mine Waste 110 34.Ot 0 <br />Native Foundation Soil 125 34.0 0 <br />Notes: <br />1. Sensitivity analyses conducted for ~'= 30.0 and 32.0 degrees <br />5.3 Pore Pressure Conditions <br />Pore pressures used in the effective stress analyses were from the pore pressures measured by <br />the vibrating wire piezometers. The pore pressures within the coal waste bank were <br />conservatively modeled by incorporating a phreatic surface into the limit equilibrium stability <br />model. <br />A phreatic surface was first estimated considering pore pressures measured by VWP-03 (which <br />has recorded the highest pore pressures) and by VWP-05 (located near the toe of the waste <br />bank). This phreatic surface is conservative as it over-estimates pore pressures as measured by <br />piezometer VWP-02. The data from VWP-02 and VWP-03, which are installed in the same <br />borehole but at different elevations, suggest that a phreatic surface is not developing within the <br />• coal waste (pore pressures are greater in the upper piezometer VWP-03 than the lower <br />piezometer VWP-02). <br />A sensitivity analysis was also conducted with respect to the location of the assumed phreatic <br />surface. This was conducted to evaluate the possibility of development of a higher phreatic <br />surface than described above. Figure 3 illustrates the phreatic surface estimated from pore <br />pressure measurements as well as the assumed increased phreatic surface used for the stability <br />analyses. The increased phreatic surface is considered very conservative and is not anticipated <br />to be realized. Nonetheless, we recommend on-going monitoring of the vibrating wire <br />piezometers to measure the pore pressures during and after additional waste placement. This <br />monitoring will allow for verification of the results of this stability evaluation as well as better <br />characterization of the coal mine waste behavior. The effects of this increased phreatic surface <br />are discussed in Section 5.4 while the stability output for these sensitivity analyses are <br />presented in Appendix D. <br />• <br />Bowie No 2 2008 Reevalution of Coal Mine Waste Stability.doc <br />Project #OS-126-GEO <br />Page 1 ] of 15 <br />