3535 RIVERFRONT BLVD Geotech Report 2023-02-22

September 24, 2018 HWA Project No. 2015-061-21 I I was processed and removed from the project site during the summer of 2011. Considerable amounts of fill material were placed to raise grade across the park site between the summer of 2010 and the fall of 2011. IMuch of the present riverbank is lined with timber piles, remnants of former docks and railroad trestles that were once constructed along the western bank of the Snohomish River. The timber I piles that were readily observable, appeared to be gapped somewhat and support what appears to be an old dock structure associated with previous lumber industry activities on the site. The most prominent timber pile section exists in the southeast corner of the site, supporting the bank I area in the vicinity of the old crane. The timber pile wall appears to extend of the order of 100 to 150 feet to the north and then becomes obscured by river bank vegetation and sediments. It is not clearly evident how far the timber pile wall extends relative to the 3-Acre Park limits, but it Iappears to terminate about 80 feet short of the northern boundary. At the location of the existing crane on the riverbank at the proposed 3-Acre Park, a short(about 60 foot) section of bank is Ipresently shored by an old steel sheet-pile wall. 3.2 GENERAL GEOLOGIC CONDITIONS I The 3-Acre Park area is located on the historic floodplain of the Snohomish River estuary. The floodplain has formed since the end of the last continental glaciation by river delta deposition in a glaciated trough. In general, thick alluvial silt and sand with peat and organic silt is present in the valley with depths ranging from less than 40 to over 100 feet. The deposits are underlain by glacially-consolidated soils. Geologic information for the project area was obtained from the Geologic Map of the Everett 7.5-Minute Quadrangle, Snohomish County, Washington (Minard, I1985). According to Minard, near-surface deposits at the project site consist of recent alluvium, deposited by the Snohomish River. Such deposits typically consist of layers of soft and loose Isand, silt, clay, organic soils and peat. The physical properties of the alluvial soils are highly variable spatially and with depth. The alluvium, particularly the peat, is typically weak and highly compressible. The more silty and Isandy facies of this alluvium are potentially liquefiable under severe seismic shaking. The biggest geotechnical challenge with site development over such deposits is potential long-term I settlement and instability of the alluvium due to placement of fill, which may be necessary to raise grades above design flood levels. ' i 3.3 SOIL CONDITIONS As part of HWA's exploration activity, seven borings, designated B-l O 1 through B-107, were I advanced within the project site at the approximate locations indicated on Figure 2. These borings were drilled to depths ranging from 16.5 to 86 feet. Borehole B-107 was drilled in the river bottom from a barge, to a depth of 49 feet below mudline. In general, our land-based I subsurface explorations encountered undocumented fill of varying thicknesses and characteristics over native alluvial soils, with organic-rich deposits commonly forming the top of the alluvium. Our river boring revealed alluvium over its entire depth. Our interpretation of the Final Geotechnical Report-3-Acre Park.docx 3 HWA GeoSciences Inc.