1910 W MUKILTEO BLVD Geotech Report 2022-10-25

Geotechnical Engineering Eval410n • NGA File No. 1238721 Spencer Garage Addition February 11,2021 Everett,Washington Page 3 A geologist from NGA was present during the explorations, examined the soils,and geologic conditions encountered, obtained samples of the different soil types,and maintained logs of the test pits.The soils were visually classified in general accordance with'the'Unified Soil Classification System, presented in gThe logs of our'explorations are attached to this report and are presented Figure 4. p p p esented as Figures 5 and 6. We present a brief summary of the subsurface conditions in the following paragraph. For a detailed description of the subsurface conditions,the logs of the test pits should be reviewed. At the surface of Infiltration Pit One and Hand Augers One through Three, we generally encountered approximately 2.0 to 4.0 feet of dark brown to orange-brown, silty sand with varying amounts of roots, gravel, and organics which we interpreted as undocumented fill soils. Underlying the topsoil and/or undocumented fill'soils we generally encountered gray,fine to medium silty sand with varying amounts of iron-oxide staining, and lower amounts of gravel, which we interpreted as native transitional beds. Infiltration Pit One and Hand Augers One through Three terminated at respective depths of 6.6, 5.5,5.0, and 4.5 feet below the existing ground surface. Hydrogeologic Conditions We encountered ground water seepage at approximately 4.0 feet below the existing ground surface in all our explorations which we interpreted as perched water. Perched water occurs when surface water infiltrates: through less dense, more,permeable soils and accumulates on top::of relatively low permeability materials. The more permeable soils consist of the topsoil/weathered soils and undocumented fill. The low permeability soil consists of relatively silty native deposits. 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 groundwater 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 competent glacial till soils are inferred to underlie the site at depth, the site conditions best fit` 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 of2,475 years),and the 2008'USGS seismic hazard maps. NELSON GEOTECHNICAL ASSOCIATES, INC.