9702 19TH AVE SE Geotech Report 2024-03-28

Pacific Ridge — DRH, LLC ES-7829.01 August 26, 2021 Page 5 Slope Stability Analysis ESNW completed a quantitative slope stability analysis of the existing site and proposed grading conditions using Morgenstern-Price methodology with GeoStudio 2021.3 Slope/W software. Soil strength parameters included in the analysis are representative of the soil conditions observed at the site, and are relatively conservative with respect to the recommended design values provided in the referenced geotechnical properties and design documents; the cohesion value for the cemented glacial till was reduced for the static condition in the analysis. The slope stability analysis was completed through representative critical cross-sections extending through and beyond the proposed project area for both the critical slip surface and the proposed development slip surface; the location of the cross-section is displayed on Plate 2 (Subsurface Exploration Plan). The results of our analyses, as well as additional modeling parameters, are provided in Appendix C. In accordance with the EMC, a peak horizontal ground acceleration (PGA) value of 0.584g was used for site-specific modeling. The pseudostatic coefficient used in the stability analyses was equivalent to one-half of the PGA, or 0.292g, which is conservative for the height of the slope modeled. The critical slip surface for the slope as well as slip surfaces which extend to the proposed development area were analyzed. The static and seismic factors of safety (FS) for critical slip surface and proposed development slip surface are provided below. Critical Slip Surface Static FS Seismic FS A-A' Existing Condition 1.1 0.6 A-A' Proposed Condition 1.1 0.6 Proposed Development Slip Surface Static FS Seismic FS A-A' Existing Condition 3.0 1.2 A-A' Proposed Condition 3.0 1 .2 The results of the analysis indicate the proposed project will not adversely affect the stability of the slope in accordance with Section 19.37.080 of the EMC; additionally, FS values for the proposed development slip surface are greater than 1.1 for seismic conditions and 1.5 for static conditions in post-construction configurations. Based on our analysis, the proposed project layout is feasible from a geotechnical standpoint. In our opinion, there will be no adverse impact to the slope provided the recommendations in this report are incorporated into the design and construction of the project. Seismic/Liquefaction Hazard Areas With respect to seismic/liquefaction hazard areas, section 19.37.080 of the EMC defines seismic/liquefaction hazard areas as areas mapped as seismic/liquefaction hazards per the Dames and Moore Methodology for the Inventory, and areas mapped as high and moderate to high liquefaction susceptibility on the Liquefaction Susceptibility Map of Snohomish County, Washington. Earth Solutions NW, LLC