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1 <br />' San Luis Mine Phase Q, Raise 2 Design Repon <br />' time, thereby lowering the level of saturation within the tailings. Surface runoff will also be <br />directed off the surface of the tailings as quickly as possible through implementation of the <br />reclamation plan. <br />' The phreatic surfaces incorporated into the stability analyses are shown on Figures 10 and 11 which <br />' depict the idealized embankment cross sections adopted for the analyses. <br />4.4 General Procedure <br />For evaluating stability of Raise 2, separate cases have been considered; stability of the upstream <br />slope prior to deposition of tailings behind the raise and stability of the downstream slopes at the <br />maximum height embattlattent section when tailings have been deposited to within 5 ft of the crest <br />of the raise (the maximum level currently anticipated). These two cases are considered the most <br />' critical as stability of the upstream face will increase as the tailings level rises and stability of the <br />downstream face will decrease as the tailings level rises to its maximum. Figures 10 and 11 depict <br />' the embankment geometry used for upstream and downstream slope stability evaluations, <br />respectively. <br />' Stability was analyzed using limiting equilibrium principles and atwo-dimensional idealized cross- <br />section of the embankment, tailings, and foundation. The stability analyses were conducted through <br />_ the use of the computer program PC STABL SM developed at Purdue University in West Lafayette, <br />' Indiana. Rotational or circular failure surfaces utilized the Modified Bishop method for satisfying <br />moment equilibrium. Block or sliding wedge failure surfaces utilized the simplified Janbu method <br />' for satisfying force equilibrium and utilized Janbu's empirical coefficient. Active and passive <br />portions of the wedge surface were determined according to Rankine Theory. <br />The STABL program incorporates a search routine to locate those failure surfaces with the least <br />factor of safety within user defined search limits and can analyze up to 1000 individual failure <br />' surfaces in a single run. Successive program runs are made to determine the most critical potential <br />failure surface. Computer printouts of the stability analyses are contained in Appendix F. <br />Stability analyses of the downstream face considered only failure surfaces which involved the entire <br />crest of the embankment raise thereby having the potential to result in a release of tailings if they <br />' were to occur. Potential downstream failure surfaces involving less of the raise crest which may <br />be slightly less stable would not compromise physical containment of the solid or liquid components <br />of the tailings. Analyses were conducted for potential failures which involve the full height of the <br />' embanlanent and those extending only for the height of the raise(s). <br />u Y 4-5 ro~ect o. <br />