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A=COiM Henderson 3 Dam Buttress Design January 26, 2018 Page 6 Loadina Conditions Slope stability analyses were performed for the post -earthquake loading condition. The post - earthquake loading condition corresponds to the end of shaking resulting from the MDE. This loading condition was evaluated at each study section with buttress for each stage. The final height (end of deposition with crest at elevation 8,900 feet) analysis is an extreme condition that would last for a relatively short time. Pore pressure conditions at each of the modeled sections will improve with time since dissipation of excess pore pressures in the foundation and tailings materials will continue. Materia! Pmerfies Materials included in the slope stability models are coarse tailings, fine tailings, and upper and lower foundations. The properties for these materials are presented earlier in Table 1. Post -earthquake properties were assigned to the saturated tailings and foundation material, while drained properties were used for unsaturated material, Results The results of 2D limit equilibrium factor of safety analysis for Sections 2 and 4a with designed buttress are presented below, followed by the 3D factor of safety and dynamic deformation analysis results. An iterative process was performed on both study sections to reach the target factor of safety of 1.1 by changing the buttress size at the stepback toe and to the original dam toe. Figures 4 and 5 show the stability figures of Section 2 and 4a with the Stage 4 buttress in place, respectively, for the end of deposition condition when the crest is at elevation 8,900 feet. Similar to the final buttress for the dam with crest elevation of 8,900 ft, the buttress was designed for the first stage of construction when the average crest height reaches the elevation of 8,883 ft. Stability figures of study Sections 2 and 4a are shown on Figures 6 and 7 for the first stage. Three -Dimensional Sloa gta6&Analyses Three-dimensional post -earthquake slope stability analyses were performed with the computer code FLAC3D, Version 5.01 (Itasca 2011), a 3D explicit finite difference program. FLAC3D has been thoroughly verified against closed -form solutions, physical models, and field-testing. Factor of safety analysis can be completed in FLAC3D using the strength reduction method. In this technique, the soil shear strength is reduced in stages until slope failure occurs. The factor of safety is defined as the ratio of the soil's actual shear strength to the reduced strength that results in failure, where failure is indicated by an inability to solve for an equilibrium stress state. A feature of this technique is that the critical failure mechanism is found automatically, rather than having to use the pre -determined shear surfaces with search routines adopted by limit equilibrium slope stability software. As the shear strength is lowered, the failure mode emerges implicitly without having to guess at trial failure configurations in advance. Hence, failures are not restricted to occur along predefined shapes such as circles, ellipsoids of revolution, or log -spirals, and failure mechanisms can also develop in the form of broad shear zones, rather than being forced on a distinct plane (Dawson and Roth 2005). The displacement, velocity and shear strain patterns displayed on the FLAC3D output for the factor of safety routine can be used to illustrate the shape of the failure mode, but the deformation values reported in the output are not used since they are forced to reach an unstable condition. Mi0CSWROJECTB MR16Ml4M MEN]_ 9 MN -3 DAA B MMSS DEVON LETTER REPORT. REV B. FPULDOC%