10725 19TH AVE SE Geotech Report 2023-04-07

Geotechnical Engineering Evaluation NGA File No. 1014317 Zerzura Townhome Development November 9,2017 Everett,Washington Page 3 At the surface of each test pit exploration we generally encountered approximately 1.0 to 1.5 feet of grass and topsoil underlain by loose, light brown to orange-brown, silty fine to medium sand with gravel, iron- oxide weathering, roots, charcoal, organics, and other debris extending to depths ranging from 2.5 to 3.5 feet below the existing ground surface, which we interpreted as undocumented fill soils. Below the fill soils in each of our test pits, we generally encountered orange-brown to gray, well-cemented silty fine to medium sand with varying amounts of gravel and iron-oxide staining, which we interpreted as native glacial till soils. Test Pits 1 through 3 and Infiltration Pit 1 were terminated within the native glacial till soils at respective depths of 8.5,9.0,6.0,and 5.0 feet below the existing ground surface. Hydrogeologic Conditions Groundwater seepage was not encountered in any of our explorations. If groundwater were to be encountered, we would interpret it to be perched water. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of relatively low permeability materials. 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 2015 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 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 S, S 1 Parameters Fa F,, SDS SDI D 1.417 0.545 1.000 1.500 0.945 0.545 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. NELSON GEOTECHNICAL ASSOCIATES,INC.