5801 S 2ND AVE B 2024-11-07 - Laserfiche WebLink

Geotechnical Engineering Evaluation Lyashenko Accessory Dwelling Unit Development Everett, Washington NGA File No. 1165620 April 10, 2020 Page 3 At the surface of each exploration, we generally encountered 4.0 to 7.0 feet of dark brown to brown, silty fine to medium sand with varying amounts of gravel, roots, and organics which we interpreted as undocumented fill and/or topsoil. Underlying the fill soils and topsoil within Infiltration Pit 1, we encountered approximately one foot of gray to gray -orange silt with fine to medium sand interbedded with silty fine to medium sand and gravel which we interpreted as undocumented fill. Underlying the topsoil/undocumented fill in each of our explorations, we generally encountered gray to gray -brown fine to medium sand with silt, silt lenses, gravel and iron oxide staining which we interpreted as native advance outwash soils. All of our test pits were terminated within the native advance outwash soils at depths in the range of 5.0 to 10.0 feet below the existing ground surface. Hydrogeologic Conditions We encountered slight groundwater seepage within each of our explorations in a range of depths from 3.5 to 9.5 feet which we interpreted as perched groundwater. 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 of 2,475 years), and the 2008 USGS seismic hazard maps. NELSON GEOTECHNICAL ASSOCIATES, INC.