3400 RIVERFRONT BLVD Geotech Report 2022-10-25

S S • • Snohomish River likely deposited this alluvium. The site is mapped as having a moderate to high • liquefaction potential (DNR, 2015). There are no mapped landslides at the site (DNR, 2015). • 4.2 SURFACE CONDITIONS The site is approximately 16 acres in size and is located on the southeast side of Everett, • Washington. The site is bound by the Snohomish River on the east, a Burlington Northern Santa Fe railroad line on the west, an office building and vegetation on the north, and vacant land on the south. The site is relatively flat to gently sloping. Polygon recently placed fill at the site to raise grades to approximately elevation 20 to 24 feet. Prior to this fill being placed, site S elevations generally ranged from approximately 14 to 20 feet. The lowest elevations were in a • drainage swale located west of Eclipse Mill Road, although this drainage swale has since been S filled by Polygon. Soft organic soil was originally present at the bottom of the drainage swale, but most of the organic soil was over-excavated and then replaced with a geotextile and quarry • spalls. The location of the drainage swale is shown on Figure 2. • The site is currently vacant and Polygon is in the process of placing a surcharge across the site at . locations that were not previously surcharged. The approximate locations of previous • surcharges are shown on Figure 3. Once the underlying soil has been sufficiently consolidated, the surcharge will be removed. The ground surface beneath the surcharge generally consists of • fill that was placed by Polygon or previous site owners. The fill placed by Polygon in 2015 was placed as structural fill and earlier fill placed by previous owners was generally undocumented. The ground surface at the site consists primarily of bare soil that is free of vegetation. The fill • slopes at the edges of the property appear to range from approximately 1%H:1 V to 2H:1V. 41 S 4.3 SUBSURFACE CONDITIONS • We performed a limited subsurface exploration program at the site that consisted of advancing three soil borings (B-1 through B-3) at the approximate locations shown on Figure 2. • Descriptions of the field explorations and laboratory test results and logs of the borings are • provided in Appendix A of this report. • Multiple geotechnical explorations have previously been performed at the site by other firms. We reviewed the information collected during previous explorations and used it to evaluate the site and develop our recommendations. Figures, logs (boring and test pit), and laboratory test results from previous explorations are presented in Appendix B of this report. • Based on the data collected at the site, the subsurface soil consists of recent and historical fill • that overlies alluvial deposits. Each of these deposits is described below. • 411 4.3.1 Fill • The fill deposits consist of undocumented fill placed during industrial activity over the last 100 years and structural fill placed by Polygon in 2015. The fill generally extends from the • existing ground surface (elevation 20 to 24 feet)to an elevation of 0 to 10 feet(a depth of • approximately 15 to 20 feet BGS). The undocumented fill placed by previous site owners i consists of a mixture of different soil types, including sand, sand with silt, silty sand, silt, silt with sand, sandy silt, organic silt, and gravel. The soil is generally gray to brown and moist to wet. The sand is generally loose to medium dense and the silt is soft to medium stiff. Debris was • • • G EO DESIGN? 4 Polygon-128-01:091615 • S