2020-07-20_REVISION - M1977342 (5)

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A=COM Mr. Ron Hickman Henderson Mill July 15, 2020 Page 2 4:1(horizontal to vertical). Steeper benches are constructed as part of normal operations as the impoundment steps upstream. These benches serve as access routes during operation until the following upstream step is constructed. A schematic showing the design slope and benches is provided in Figure 1. These benches are regularly graded as part of erosion and storm water control measures and are considered standard maintenance of the impoundment. The overall slope of 4:1 provides embankment stability and this slope is maintained during any bench regrading activities. DRMS Comment 2 Please provide additional discussion and stability results for the 3 Dam Phase II buttress design under static conditions to ensure a minimum factor of safety of 1.5 is met as stated under Section 30. AECOM Response 2 Steady-state drained loading conditions represents the long-term stability of the dam. Stability analyses were performed on the design section, Sections 2 and 4A, with the designed Phase II buttress and crest elevation of 8,887 ft under steady-state (static) loading conditions using the drained shear strengths shown on Figures 2 and 3. The limit equilibrium computer program UTEXAS4 (Wright, 2008) and Spencer's method of slices were used for the stability analyses. Spencer's method satisfies all conditions of static equilibrium, including horizontal and vertical force imbalance and moment imbalance. Search routines available within UTEXAS4 were used for circular and noncircular trial shear surfaces to locate critical shear surfaces. The results of the two-dimensional (213) limit equilibrium steady state drained factor of safety for Section 2 is 2.5 and for Section 4A is 2.1. Figures 2 and 3 show resulting failure surfaces for Section 2 and 4A, respectively. DRMS Comment 3 It is stated under the cover page for the proposed TR-32 application that a geotechnical investigation was scheduled for June 2020 which could potentially result in minor modifications to the geometry of the buttress. Please provide additional details of the June 2020 investigation (if occurred) and whether this investigation will have an impact on the currently proposed Phase II 3 Dam buttress design. AECOM Response 3 A Cone Penetration Test (CPT) field investigation consisting of 14 CPTs with pore pressure dissipation tests was completed in the original 3 Dam buttress area June 2 to 5, 2020. The CPTS ranged in depth from 10 to 20 ft to the natural ground. Results from the CPT program C\USERS2ANA KARIMI\DOCUMENTS\HENDERSON STABILrrY\DRMS PH 11 RESPONSE FINAL DOCX