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Geotechnical Engineering Evaluation Miller Property Slope Stabilization Everett, Washington SLOPE STABILITY ANALYSES NGA File No. 1333322 July 1, 2022 Page 9 The failed site slopes were analyzed for stability along Cross -Section C-C' (Figure 5) through the planned repair area using the computer program Slope/W, by Geo-Slope International. Slope/W is a two-dimensional, limit equilibrium slope stability program that generates random potential failure surfaces or specific failure surfaces and determines their corresponding factors of safety with respect to failure. By generating a large number of random surfaces, a critical failure surface with the minimum factor of safety can be identified. The slope stability analyses were performed using information gathered from field and borehole data interpretations from the recent explorations. Soil properties were assigned to the soil layers to reasonably reflect their engineering characteristics based on our experience and behavior under current failure conditions. The following table outlines the material properties for those used in Slope Stability Analyses. Table 2. Material properties interpreted from existing failure conditions. Material Type y (pcf) c' (psf) cp' Landslide Debris/Undocumented Fill 138 -- 20' Advance Outwash 128 -- 36' Transitional Beds 133 150 35' Structural Fill 135 -- 38' Stability analyses were performed within the central portion of the landslide area along the cross-section. Stability analyses were performed for non -seismic conditions for the existing conditions, and non -seismic and seismic conditions for proposed stabilization options A and B, respectively. A peak ground acceleration of 0.22g was used in the seismic analyses. The results of the analyses are presented in Figures 13 through 17. Our slope numerical modeling of the deep-seated stability condition within the existing slope face indicate that this area is unstable under existing static conditions with factors of safety below 1.0 but would exhibit lower factors of safety under seismic conditions with a factor of safety of less than 0.5. We have also analyzed slope stability with the recommended surficial improvements and pile wall in place within the landslide area using the design parameters discussed in the conclusions section of this report. These analyses reflect the two wall configurations where: Option A results in a retaining wall aligned with the existing southern fence line and Option B results in a retaining wall aligned with the existing landslide headscarp. With the use of the subsurface wall, these analyses indicated stable global slope conditions under both static and seismic loading. These analyses also achieved minimum effective factors of safety of 1.2 for seismic loading conditions. NELSON GEOTECHNICAL ASSOCIATES, INC.