3535 RIVERFRONT BLVD Geotech Report 2023-02-22

September 24, 2018 HWA Project No. 2015-061-21 I/ I Soils Deposited or Placed Before HWA's Explorations Historic Fill: Approximately 5 to 10 feet of undocumented historic fill was encountered in the land borings throughout the 3-Acre Park site. It varied from sandy silt with woody debris Ito gravelly sand and sandy gravel, and was loose to medium dense. As noted above and discussed in the recommendations below, additional fill was placed on site during 2010 and I in the summer of 2011, after our exploration borings. Organic Silt and Peat: Organic silt with peaty pockets was encountered in our land borings below the fill, to a depth of approximately 15 to 18 feet, and varied in thickness from 5 to I10 feet. It appeared to be a native alluvial deposit, with disturbance apparent in the upper portion due to former site fill/grading activities. 11 Alluvium: Native silty sand alluvium was encountered in all the borings below the organic silt, and from the river bottom in boring B-107 to the full depth explored(up to 49 feet). This material consists of river deposits and increases in density with depth. IDense Alluvium: The alluvium became dense, as defined by an SPT blow count of 30 blp or greater. The soils were generally similar in composition to the less dense materials about Ithem; however, layers of stiff to hard silts and lean clays were also observed. 3.4 GROUND WATER CONDITIONS IGround water was encountered in each boring drilled across the project site. The information obtained from the subsurface explorations identifies two distinct ground water tables at the I proposed park site. The upper ground water table consists of water perched on top of the peat and organic site deposits. This perched ground water table is expected to vary seasonally with the lowest water levels in August and September and the highest in February through April. The lower ground water table at the site is associated with the alluvial sand deposit encountered below the peat and organic silt. This sand layer is hydraulically connected to the adjacent I Snohomish River and undergoes tidal variations in the water level throughout the day and varies with the level of the Snohomish River throughout the year. The peat and organic silt deposit underlying the 3-Acre Park site generally represents a confining layer for the alluvial sand Ideposit and its associated ground water. A datalogging ground water pressure transducer was installed in Boring BH-105 to monitor variations in ground water level within the alluvial sand deposit. Continuous data was collected from August 1st of 2010 to October 30th of 2010. As Ishown on Figure 4, the ground water level in the alluvial sand varies tidally from a minimum of elevation +1 feet to a maximum of elevation +10 feet. The intersection of 36th Street and Eclipse Mill Road is underlain by numerous underground utilities. Some of these utilities extend under the 3-Acre Park site. The trench backfill I associated with each of these utilities possesses a higher permeability than the surrounding native soils. These trenches act as conduits for conveyance of ground water at the site. Several utilities near the 3-Acre Park site are deep and therefore, break through the peat and organic silt that I Final Geotechnical Report-3-Acre Park.docx 5 HWA GeoSciences Inc. I