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U8/18.'05 18:33 $1 303 U85 2080 <br />~, . <br />August 18, 1995 <br />COf.Dl:R CCS <br />• <br />-2- <br />953-2900.007 <br />minimum strength parameters that the relocated tailings would have to exhibit in order to <br />achieve a Factor of Safety (FOS) of 1.0. Therefore, the minimum strength determined in <br />this analysis for [he tailings could then be used as a criteria for lift tllicluless. <br />The foundation material was assigned a density of 125 pcf, a friction angle of 40 degrees <br />and a cohesion of 1000 psf, in order to model a failure through the tailings material. The <br />tailings wcre given a density of 80 pcf. The tailings were also modeled under the <br />conservative assumption that the tailings would have oNy cohesion and no friction angle, <br />representing the worst case strength parameters. This situation is often referred to as the <br />"~ = 0" concept, and used to model situations when a saturated material is subjected to a <br />load and not allowed to consolidate or dissipate pore water pressures. <br />The current tailings relocation configuration consis[s of a berm constructed from waste <br />rock and local native material to an elevation of approximately 9585. The function of the <br />berm was to provide additional stability support to the tailings material and act as a <br />buttress. Based on an actual survey of the berm, the downstream slope of the berm was <br />constructed at a slope of 1.3H:1 V, and the upstream slope was estimated to be the same <br />contguration. The waste rock was conservatively modeled with a density of 125 pcf, no <br />cohesion and a friction angle of 36 degrees. The relocated tailings were modeled with no <br />friction angle and a cohesion of 300 psi. As the contractor is currently placing tailings <br />material at the upper end of Upper Arequa Gulch, the surface of the relocated tailings was <br />assumed to slope upgradient at a 5 percent slope. This is illusu~ated in Figure 1. <br />The stability of the facility was then analyzed and calculated to be 1.4, with the critical <br />failure occurring in the waste rock berm. <br />An additional scenario was modeled, in which a second waste rock berm was constructed <br />at a 4H:1V slope, in an upstream fashion (illustrated in Figure 2) to elevation 9610. The <br />berm was constructed to provide a buttress to the tailings material, and the FOS increased <br />to 1.2. The failure in this case was also through the berm material. <br />The final configuration, when waste ruck is placed in the "V" between the relocated <br />tailings and Highway 67 Realignment Fill, increases the FOS greatly. <br />The tailings material was modeled as conservatively as possible, with a density of SO pcf, <br />no friction angle, and a cohesion of 300 psf. Even with these extremely low values, FOS's <br />greater than 1 were calculated. This was due to the construction of a waste rock berm at <br />the downstream end of the relocated tailings, which provides the additional stability <br />support required. <br />Golder recommends that the waste rock berm continue to be constructed along the <br />downstream slope of the relocated tailings, as the berm serves to increase the overall <br />stabiliq~ of the relocated tailings. The stability analysis performed by Golder has <br />demonstrated that the stability of the relocated tailings are essentially independent of the <br />tailings lift thickness, since the very conservative strength pazameters used in the analysis <br />r~003 <br />Golder Aseociatee <br />