3400 RIVERFRONT BLVD Geotech Report 2022-10-25

• • • • observed throughout the undocumented fill that includes asphalt, concrete, rebar, brick, quarry • spalls, wood shavings, dimensional lumber, logs, trees, cobbles, and scrap metal. Organics are frequently mixed in with the soil. Groundwater seepage was typically observed in this layer. • • The structural fill placed by Polygon generally consists of silty sand with gravel, silty sand with •. minor gravel, silty sand, and sand with silt. The structural fill was placed and compacted in lifts • up to 12 inches thick. GeoDesign regularly observed the placement and compaction of the structural fill and performed moisture-density testing to verify that adequate compaction was • achieved. The results of our testing and observations were documented in daily field reports. In our opinion, the structural fill was compacted to a dense condition and to 95 percent of the • material's maximum dry density, as determined by ASTM D 1557. The thickness of the structural fill was generally 2 to 4 feet, but was up to 10 feet thick in the drainage swale area• west of Eclipse Mill Road. • • According to the URS report for the site, there are two heavily reinforced concrete slabs • measuring up to 60 feet long, 40 feet wide, and 12 inches high that are present above a City 72-inch sewer pipeline. The purpose of these slabs is to protect the underlying pipeline. These • slabs are believed to be located at an elevation of approximately 10 feet. The URS report • recommends leaving these slabs in place unless it is necessary to remove them. The locations of • these slabs are shown on the URS 72-inch pipeline profile drawing in Appendix B. • It is possible that foundations from previous industrial facilities were buried in place by the • undocumented fill. Some test pits encountered piles, which may be foundation elements from • earlier structures or from an existing pile-supported City sewer line. • 4.3.2 Alluvial Deposits • Beneath the fill the explorations encountered a layer of silt and organic silt that generally extends • from elevation 0 to 10 feet to elevation -2 to -14 feet (typically 10 to 20 feet thick). The material • is generally gray and brown, soft to medium stiff, wet, and contains occasional sand and • scattered wood and organics. Large pieces of wood such as stumps were occasionally observed near the top of this layer. Laboratory testing indicates that moisture contents in this layer were • between 30 and 60 percent at the time of the explorations. This layer is generally non-plastic to • low plasticity, but there are some locations along the western side of the site where this layer is • of medium plasticity. There is also a 10-to 20-foot-thick layer of hard silt along the western side • of the property that underlies the soft to medium stiff silt. • Beneath the silt and organic silt is a layer of sand that extends to at least elevation -48 feet, the • maximum depth of the borings. The upper 10 to 20 feet of sand is generally loose to medium • dense. Deeper sand is generally dense to very dense. The transition to dense and very dense sand typically occurs between elevations -20 and -30 feet. The sand is generally gray, wet, fine- • to coarse-grained, and the fines content decreases with depth. • • 4.3.3 Groundwater • The explorations performed at the site typically encountered groundwater between elevations of 2 and 13 feet, and the most frequent groundwater observations were between elevations of 5 • and 8 feet. Given the close proximity of the site to the Snohomish River, the elevation of • • • G EODESIGN? 5 Polygon-128-01:091615