2202 100TH ST SW Geotech Report 2022-10-25

Subsurface Exploration,Geologic Hazard,and Stephens Everett Property Preliminary Geotechnical Engineering Report Everett, Washington Project and Site Conditions 4.1 Stratigraphv Forest Duff/Topsoil A surficial layer of forest duff/topsoil was encountered at the location of each of the explorations. This organic layer was approximately 4 to 9 inches thick where present. Due to their high organic content, these materials are not considered suitable for foundation, roadway, or slab-on-grade floor support, or for use in a structural fill. Fill While not observed in our March 2019 explorations, fill soils may be present in unexplored areas of the site, such as in existing utility trench areas and at previously graded landscaped areas. Existing fill soils are likely variable in density and composition and not suitable for structural support. Excavated existing fill material may be suitable for reuse in structural fill applications if such reuse is specifically allowed by project plans and specifications, if excessively organic and any other deleterious materials are removed, and if moisture content is adjusted to allow compaction to the specified level and to a firm and unyielding condition. Existing fill is not considered suitable for infiltration of stormwater runoff due to its high variability. Vashon Recessional Outwash Explorations EP-1, EP-4, EP-5, EP-6, and EP-7 encountered sediments generally consisting of moist to wet, loose to medium dense, light brown to brownish gray, fine to medium sand with variable amounts of silt and gravel interpreted as Vashon recessional outwash sediments. These sediments extended to a maximum depth of 8 feet below the existing ground surface. Recessional outwash sediments were deposited from meltwater streams as the glacier retreated, and were not overridden by glaciers; thus, they are typically loose to medium dense. In several of our explorations, we encountered perched groundwater within the recessional outwash sediments onsite and as such, excavated recessional outwash sediments are expected to be above optimum moisture content for compaction purposes. Portions of the recessional outwash onsite contains a significant fine-grained fraction and is sensitive to excess moisture during placement in structural fill applications. If allowed by project plans and specifications, reuse of excavated recessional outwash sediments in structural fill applications will require aeration and drying during favorable dry site and weather conditions to allow compaction to a firm and unyielding condition at the specified level of compaction. The silty and saturated portions of these sediments can be easily disturbed by vehicles and earthwork equipment and careful management will be needed to reduce the potential for disturbance of these soils and costs associated with repairing disturbed soils. Recessional outwash can be a suitable receptor for stormwater infiltration, provided it is unsaturated and of sufficient vertical and lateral March 29,2019 ASSOCIATED EARTH SCIENCES,INC. TG/ms-180567E001-2-Projects Page 4