11229 19TH AVE SE SILVER LAKE TOWNHOMES WHOLE SITE Geotech Report 2022-10-25

Geotechnical Report Proposed Development: 11229 - 19th Avenue SE, Everett, WA September 16, 2016 Table 1 —Seismic Design Parameters Spectral Spectral Design Spectral Site Acceleration Acceleration Site Coefficients Response Class at 0.2 sec. [g] at 1.0 sec. [g] Parameters Ss S1 Fd F, Sus Sol D 1.411 0.543 1.0 1.5 0.941 0.543 Liquefaction Potential: Liquefaction is a process that can occur when soils lose shear strength for short periods of time during a seismic event. Ground shaking of sufficient strength and duration results in the loss of grain-to-grain contact and an increase in pore water pressure, causing the soil to behave as a fluid. Soils with a potential for liquefaction are typically cohesionless, predominately silt and sand sized, must be loose, and be below the groundwater table. Based on dense soil conditions at shallow depths, it is our opinion the liquefaction potential of the site is low and design considerations related to soil liquefaction are not necessary for this project. 5.2 FOUNDATIONS Based on the subsurface conditions encountered at the site and our understanding of the planned development, it is our opinion the proposed buildings may be supported on conventional spread and continuous footings bearing on the native medium dense to very dense soils or structural fill placed on competent native soils. Approximately 3 to 41/2 feet of fill was encountered in PG-3 and PG-4. Depending on the elevation of the proposed floor for Buildings A through D, fill may still be present at the footing subgrade elevation at these locations and limited foundation soil over-excavation may be required to expose native competent soils. Any over-excavations should be backfilled with compacted structural fill or lean-mix concrete. The over-excavation, if needed, should extend at least one foot beyond the edges of the footings. Fill was encountered to about 8 feet below the surface in PG-1 at the Building E location. Over- excavation to remove the fill will require relatively deep temporary excavations and will likely extend below the groundwater table. If the over-excavation option is used at this location, we recommend that 2- to 4-inch quarry spalls be used as backfill to about 12 inches above the 16-210 11229 19th Ave SE Rpt Page 5 PanGEO, Inc.