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0 <br />Figure 2. Phreatic surfaces used for the stability analyses. The lower phreatic surface is based on pore <br />pressure measurements from the installed vibrating wire piezometers at CWDA No. 2 while • <br />the upper phreatic surface is an assumed increased surface in order to evaluate the model <br />sensitivity to pore pressure increases. <br />5.4 Stability Evaluations <br />Slope stability of CWDA No. 4 was evaluated using two- dimensional limiting equilibrium stability <br />analyses with the computer software SLIDE developed by Rocscience (2004). Critical block and <br />circular failure surfaces were determined using effective stress analyses. Spencer's procedure <br />was used. In accordance with DRMS (2005), only static loading conditions were evaluated. <br />Search methods employed by SLIDE examine tens of thousands of potential failure surfaces for <br />each analysis. The factors of safety for critical circular and non - circular (block -type) failure <br />surfaces are presented in Table 4. Output from critical stability analyses is presented in <br />Appendix D. Critical failures surfaces for the analyses were found to be shallow failures at the <br />lower portion of the refuse bank where slopes are steepest (2.5H: 1V). The assumed increased <br />phreatic surface did not significantly affect the factors of safety. All critical factors of safety <br />exceeded minimum requirements as specified by the Regulations of the Colorado Mined Land <br />Reclamation Board far Coal Mining (DRMS, 2005). <br />l J <br />Bowie No 2 Refuse Pie No 4 2009 Revised stnbl ity.doc <br />Pmlect tt06i81 -003 <br />Page 10 of 12 <br />