1910 W MUKILTEO BLVD Geotech Report 2022-10-25

Geotechnical Engineering Evalualn • NGA File No.1238721 Spencer Garage Addition February 11,2021 Everett,Washington Page 4 Table 1—2018 IBC Seismic Design Parameters Site Class Spectral Acceleration Spectral Acceleration Site Coefficients `Design Spectral at 0.2 sec. (g) at 1.0,sec.(g) Response SS Si Parameters Fe F„ Sips SD1 D 1.363 0.518 1.000 1.50 0.909 0.518 The spectral response accelerations were obtained from the USGS Earthquake Hazards Program Interpolated Probabilistic Ground Motion website(2008 data)for the project latitude and longitude. Hazards associated with seismic activity include liquefaction potential and amplification of ground motion. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. It is our opinion that the medium dense or better native glacial deposits interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. Erosion Hazard The criteria used for determination of erosion hazard areas include soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to the vegetative cover and the specific surface soil types, which are related to the underlying geologic soil units. The Soil Survey of Snohomish County Area, Washington, by the Natural Resources Conservation Service (NRCS), was reviewed to determine the erosion hazard of the on-site soils.The site surface soils were classified using the NRCS classification system as Indianola loamy sands, 15 to 30 percent slopes. These soils are listed as having a slight erosion hazard. It is our opinion that the erosion hazard should be low to moderate in areas with vegetated,undisturbed soil. Landslide Hazards The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions.Topographically,the site is relatively level, but the extreme southwestern portion of the site includes steep, north-and east-facing slope which extend upward onto adjacent properties to the south and west at measured gradients in the range of approximately,30.degrees (58 percent) as shown on Cross-Sections A-A' in Figure 3. Portions of the site slopes above the proposed development area are mapped and classified as a 'Very High' Landslide Hazard Area according to the City of Everett Municipal Code Chapter 19.37.080.A.1.a.1 due to the slope morphology-inclinations are greater than 15 percent and are underlain by"transition beds" deposits and undocumented fill. We did not observe evidence of significant slope instability NELSON GEOTECHNICAL ASSOCIATES, INC.