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(dC071 <br />Henderson 3 Dam Buttress Design <br />January 26, 2018 <br />Page 5 <br />Initial SQ1§mic Slope Stability Results <br />Slope stability analysis was performed for the 10,000 -year event post -earthquake loading condition. <br />The results of the stability analysis showed that the embankment needs modifications to meet the <br />seismic design criteria when the average 3 Dam crest reaches the elevation 8,883 ft. <br />RECOMMENDED DESIGN <br />Additional stabilization measures are being considered to improve the MDE post -earthquake stability <br />of 3 Dam. Buttressing the stepback and part of the toe of the original 3 Dam was found to be the <br />most practical and effective solution. The layout of the 3 Dam buttress was designed to meet the <br />previously stated design criteria and 2D and 3D stability and dynamic deformation analyses were <br />performed. The stabilization measures proposed satisfy the most critical MDE case when the dam <br />has reached its maximum height and deposition has just ceased. The following sections present the <br />summary of these analyses. The plan, profile and details associated with the 3 Dam buttress <br />construction are shown on attached Drawings. <br />The buttress will be constructed in stages. This will allow the observational approach to be <br />incorporated into the overall design strategy, adjusting prior to each construction stage based on <br />actual site conditions at the time. The four stages are as follows: <br />• Stage 1: average crest elevation at 8,883 ft <br />• Stage 2: average crest elevation at 8,887 ft <br />• Stage 3: average crest elevation at 8,893 ft <br />• Stage 4: average crest elevation at 8,900 ft <br />The buttress design slopes will be based on a 3:1 (horizontal: vertical). There is limited room at the <br />toe at the 3 Dam on the south side to accommodate a flatter slope due to ongoing operations. As <br />Henderson nears closure, changes in the overall slope could be considered with Stage 4 or following <br />deposition. <br />General Design Approach <br />The geotechnical analyses included completing two-dimensional stability analyses for selected 3 Dam <br />design sections and three-dimensional slope stability and dynamic deformation analysis. The two- <br />dimensional analyses were performed on two critical sections at 3 Dam, Sections 2 and 4a. <br />Figures 2 and 3 show the model geometry of Section 2 and 4a, respectively. <br />Two -Dimensional Slope Stability Analyses <br />Slope stability analyses were conducted for selected study Sections 2 and 4a with the designed <br />buttress under post -earthquake loading conditions for crest elevations of 8,883 and 8,900 feet. The <br />buttress was designed for a 2D target factor of safety of 1,1 at the end of each of these cases. <br />Sections 2 and 4a were selected to be representative of south-east and north-west side of the dam <br />respectively for analyses. <br />The limit equilibrium computer program UTEXAS4 (Wright, 2008) and Spencer's method of slices <br />were used for the stability analyses. Spencer's method satisfies all conditions of static equilibrium, <br />including horizontal and vertical force imbalance and moment imbalance. Search routines available <br />within UTEXAS4 were used for circular and noncircular trial shear surfaces to locate critical shear <br />surfaces. In addition to the 2D limit equilibrium analyses performed with UTEXAS, a 3D model was <br />also developed to compare the 3D stability results with the 2D analysis. The details of the 3D model <br />and stability analysis are presented in detail later in this letter. <br />M%WMM0XCT9WRRt=11W MEM ftUnRdWbELN8VMLMRM5 SOH_ 3 OIW WIMMn OEMN LETTER REFORT_REV B_ FNM -W= <br />