4328 HOYT AVE Geotech Report 2022-10-25

Geotechnical Engineering Evaluation—REVISED NGA File No. 1148519 Staniford Residence Remodel June 23,2020 Everett,Washington Page 4 Hydrogeologic Conditions Slight groundwater seepage was encountered in Test Pits 1, 2, and 3 at approximately 7.0 to 10.8 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 a relatively low permeability material. 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 dense to 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 Acceleration Spectral Acceleration Site Coefficients Design Spectral at 0.2 sec. (g) at 1.0 sec. (g) Response SS Si Parameters Fa Fv SDS SD1 D 1.355 0.515 1.000 1.500 0.903 0.515 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 glacial deposits interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion due to the native sand soils medium dense or better condition. NELSON GEOTECHNICAL ASSOCIATES,INC.