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Geotechnical Engineering Evaluation NGA File No. 1322621 Clark Garage Slope February 4,2022 Everett,Washington Page 3 Explorations: We visited the site on January 4, 2022 to explore the subsurface conditions along the steep slope and the proposed residence addition. The explorations consisted of three hand augers extending to depths of up to 5.0 feet below existing ground surface. A geologist from Nelson Geotechnical Associates, Inc. (NGA) completed the explorations, and examined the soils and geologic conditions encountered. All of our explorations encountered approximately 1.0 to 1.5 feet of orange-brown organic-rich silty sand with gravel and roots that we interpreted as surficial topsoil and/or undocumented fill. Underlying the undocumented fill and/or surficial topsoil in all of our explorations, we encountered medium dense to dense gray, silty fine to coarse sand with varying amounts of gravel and cobbles that we interpreted to be native glacial till soils. All of our explorations were terminated within native glacial soils at depths in the range of 3.0 to 5.0 feet below the existing ground surface. Hydrogeologic Conditions Groundwater seepage was encountered within our explorations in hand auger 1 and 2 at depths of 1.5 and 1.0 feet below the existing ground surface, respectfully. We interpreted this water to be perched water. Perched water conditions may occur on the site during periods of wet weather. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of underlying, less permeable soils. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched water to decrease during drier times of the year and increase during wetter periods. SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2018 International Building Code (IBC) for seismic site classification for this project. Since very dense soils are interpreted to underlie the site at depth,the site best fits the IBC description for Site Class D. Table 1 below provides seismic design parameters for the site that are in conformance with the 2018 IBC, which specifies a design earthquake having a two percent probability of occurrence in 50 years(return interval of 2,475 years), and the 2008 USGS seismic hazard maps. Table 1—2018 IBC Seismic Design Parameters Site Class Spectral Spectral Site Coefficients Design Spectral Acceleration at 0.2 Acceleration at 1.0 Response Parameters sec. (g) sec. (g) S, S1 Fa F„ SDS Sol D 1.420 0.543 1.000 1.500 0.946 0.543 NELSON GEOTECHNICAL ASSOCIATES, INC.