4110 CLIFF DR 2022-01-26 - Laserfiche WebLink

Geotechnical Engineering Evaluation NGA File No. 1021318 Cliff Drive Short Plat Residence Development February 2, 2018 Everett, Washington Page 3 At the surface of Test Pits 1 through 4 we encountered approximately 1.0 to 3.0 feet of dark brown to orange -brown, silty fine to medium sand with varying amounts of gravel, roots, and organics, which we generally interpreted as topsoil and/or undocumented fill soils. Underlying the fill and topsoil we encountered light brown to gray, fine to medium sand with varying amounts of silt and gravel along along with iron -oxide weathering, which we interpreted as native glacial outwash soils. Test Pit 1 through 4 were terminated within native advanced outwash soils at respective depths of 7.0, 8.0, 10.0, and 8.0 feet below the existing ground surface. Hydrogeologic Conditions Moderate to heavy groundwater seepage was encountered within Test Pit 4 at approximately 8.0 feet below the existing ground surface. We interpret the water observed to be perched groundwater. Perched water occurs when surface water infiltrates. 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 The 2015 International Building Code (IBC) seismic design section provides a basis for seismic design of structures. Since medium dense or better glacial outwash soils were generally encountered underlying 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 2015 IBC, which specifies a design earthquake having a 2% probability of occurrence in 50 years (return interval of 2,475 years), and the 2008 USGS seismic hazard maps. Table 1— 2015 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 Fa F, SDs SDI r _7 D 1.399 0.531 1.000 1.500 0.933 0.531 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 NELSON GEOTECHN/CAL ASSOCIATES, INC.