2320 CALIFORNIA ST Geotech Report 2025-07-25

1 111 SITE CONDITIONS Surface Conditions The majority of the site for the proposed addition is currently covered with grass and is relatively flat. Existing surface parking is located south of the Administration Building with access roads on the east and west sides of the building. The propose stormwater pond may be located east of the southeast corner of the south parking lot. The locations of surrounding site features with respect to the proposed Administration Building Expansion are shown on the Site Plan, Figure 2. Geology Published geologic information for the project vicinity includes a United States Geological Survey(USGS) map of the Mukilteo and Everett quadrangle(Smith, 1976). Mapped soils in the project vicinity consist of glacially consolidated Vashon till deposits(glacial till). Glacial till commonly consists of a very compact, poorly sorted, non-stratified mixture of clay,silt,sand,gravel and cobbles. Glacial till commonly appears gray or blue on a fresh surface, while weathered glacial till may be brown to yellow in color. Till may include cobbles and large boulders. Glacial till was observed in each of the test pits completed at the site. Subsurface Conditions Based on the test pits completed at the site,the subsurface conditions generally consist of a thin layer of fill overlying glacially consolidated till deposits,as discussed below. At the surface,we observed 3 to 4 inches of sod and topsoil overlying approximately 12 inches of fill. The fill consists of loose to medium dense fine to medium sand. The fill overlies native glacial till deposits consisting of very dense silty fine sand with occasional gravel. The glacial till was observed 12 to 18 inches below the ground surface. Groundwater Conditions Groundwater was not observed during the test pit excavations. Perched groundwater overlying the dense native till soils was observed in previously completed test pits for the Pole Storage Addition project. Perched groundwater may be encountered within the fill and overlying the denser till deposits during the wet season (October through May). CONCLUSIONS AND RECOMMENDATIONS Summary of Key Geotechnical Issues Based on the results of our subsurface exploration program, it is our opinion that the Administration Building 1 Office Expansion project may be constructed satisfactorily as planned with respect to geotechnical elements. The key geotechnical issues for the project include: •• The site is designated as seismic Site Class C per the 2009 International Building Code(IBC 2009). Ai The proposed building addition may be supported on shallow foundations bearing on native undisturbed dense glacial deposits. The foundations may be designed using an allowable soil bearing pressure of 1 8,000 pounds per square foot(psf)for isolated spread footings and continuous footings. Page 2