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Please provide additional discussion and stability results for the 3 Dam Phase II buttress design <br /> under static conditions to ensure a minimum factor ofsafety of 1.5 is met as stated under Section <br /> 30. <br /> Henderson/AECOM: <br /> Steady-state drained loading conditions represents the long-term stability of the dam. Stability <br /> analyses were performed on the design section, Sections 2 and 4A, with the designed Phase II <br /> buttress and crest elevation of 8,887 ft under steady-state (static) loading conditions using the <br /> drained shear strengths shown on Figures 2 and 3. The limit equilibrium computer program <br /> UTEXAS4 (Wright, 2008) and Spencer's method ofslices were used for the stability analyses. <br /> Spencer's method satisfies all conditions ofstatic equilibrium, including horizontal and vertical <br /> force imbalance and moment imbalance. Search routines available within UTEXAS4 were used <br /> for circular and noncircular trial shear surfaces to locate critical shear surfaces. The results of <br /> the two-dimensional(2D) limit equilibrium steady state drained factor ofsafety for Section 2 is <br /> 2.5 and for Section 4A is 2.1. Figures 2 and 3 show resulting failure surfaces for Section 2 and <br /> 4A, respectively. <br /> Division: <br /> With the provided factors of safety being 2.5 (Section 2)and 2.1 (Section 4A)under static <br /> conditions for the 3 Dam,the minimum requirements of Section 30 have been met. The <br /> Division's Comment 2 has been satisfied with no additional comments. <br /> Division Comment 3: <br /> It is stated under the cover page for the proposed TR-32 application that a geotechnical <br /> investigation was scheduled for June 2020 which could potentially result in minor modifications <br /> to the geometry of the buttress. Please provide additional details of the June 2020 investigation <br /> (if occurred) and whether this investigation will have an impact on the currently proposed Phase <br /> II 3 Dam buttress design. <br /> Henderson/AECOM: <br /> A Cone Penetration Test(CPT)field investigation consisting of 14 CPTs with pore pressure <br /> dissipation tests was completed in the original 3 Dam buttress area June 2 to 5, 2020. The CPTS <br /> ranged in depth from 10 to 20 ft to the natural ground. Results from the CPT program were used <br /> to refine the fine tailings extent to and to confirm pore pressure conditions. The CPT <br /> investigation showed that the fine tailings material located on the northern side of the original 3 <br /> Dam buttress are isolated and confirmed pore pressure conditions.An updated 2D post- <br /> earthquake loading stability analysis was completed on design section 4A with the updated fine <br /> extents. This analysis was completed using the same procedures described in the 3 Dam, Phase II <br /> Buttress Design letter report dated June 1, 2020. Figure 4 presents the updated slope stability <br /> results for post-earthquake loading conditions which meets the stability criteria. As a result, the <br /> proposed regrade or slope flattening of the northern original 3 Dam buttress is no longer <br /> required. Remainder of the design remains consistent with the 3 Dam Phase II submission. <br /> Henderson Mine Geotechnical Review Memo—TR-32 July 14,2020 <br />