3535 RIVERFRONT BLVD Geotech Report 2023-02-22

September 24, 2018 HWA Project No. 2015-061-21 subsurface soils conditions across the project site are indicated in Geologic profiles A-A' through D-D' presented on Figures 3A through 3D, respectively. Brief descriptions of the soil units observed in our explorations or known to be placed after our explorations were completed are presented below in order of deposition, beginning with the most recently deposited. The exploration logs in Appendix A provide more detail of subsurface conditions observed at the specific bore hole locations and depths and should be referred to for site-specific information. Soil Placed After HWA Explorations were Completed 2016 Preload Soils: In June 2016, a preload was constructed at the 3-Acre Park site to prepare for the construction of the proposed improvements. The lateral extents of the preload were limited to avoid damage of the 72-inch diameter sanitary sewer main along the west side of the project and the 60-inch diameter CSO line along the southern edge of the site. The preload was constructed to consolidate compressible subsurface soils before the construction of the proposed park improvements. The preload material consisted of imported glacial till fill that was excavated from a construction site on the University of Washington campus by KLB Construction. Construction of the preload embankment was monitored and compaction testing was completed in the lower reaches of the preload to ensure adequate fill density where the restroom and picnic shelters would be founded. Fill Placed in 2011: During the summer of 2011, Tri-State Construction placed several feet of additional fill across the southern portion of the old Eclipse Mill site. This area of fill placement encompassed most the 3-Acre Park site as well as the current intersection of 36th Street and Eclipse Mill Road. This fill consisted of uniformly graded sand that had been dredged from the Snohomish River years earlier. Very little compactive effort was applied to this material during placement. Accordingly, this material is in a relatively loose condition, and side slope armoring has been required at several locations within the dredge sand placement area due to the occurrence of runoff erosion during storm events. Fill Placed in 2010: During the summer of 2010, several feet of Common Borrow was placed across most of the old Eclipse Mill site, including the northern edge of the 3-Acre Park site, to raise grade to approximate elevation 18 feet. This Common Borrow consisted of excess preload material removed from the old landfill site to the south. The Common Borrow was placed by KLB Construction and compacted to 90 percent of Modified Proctor relative density. This material possessed high fines content, was highly moisture sensitive, and was placed at between 1 and 2 percent over optimum moisture content. During the summer of 2011, KLB Construction removed the upper 1.5 feet of fill across the entire Eclipse Mill site, processed it with recycled concrete, replaced it, and compacted it to 95 percent of the Modified Proctor relative density. As part of construction monitoring activities, an HWA representative was periodically present on site to test and document the level of compaction being attained. Final Geotechnical Report-3-Acre Park.docx 4 HWA GeoSciences Inc.