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 extent. Vashon recessional outwash sediments are suitable to support foundation loads when in a medium dense condition and prepared as recommended in this report. Vashon Lodgement Till All of our explorations typically encountered moist to very moist, medium dense grading to dense, silty to very silty sand with variable gravel and occasional cobbles ranging to sandy silt with trace to some gravel, interpreted as Vashon lodgement till. Lodgement till encountered in our exploration pits extended beyond the maximum depth explored of 17 feet below existing ground surface. Lodgement till was deposited at the base of an active ice sheet and was subsequently compacted by the weight of the overlying glacial ice. Lodgement till typically possesses high-strength and low-compressibility attributes that are favorable for support of foundations, floor slabs, and paving with proper preparation. Lodgement till is silty and moisture-sensitive. In the presence of moisture contents above the optimum moisture content for compaction purposes, lodgement till can be easily disturbed by vehicles and earthwork equipment. Careful management of moisture-sensitive soils, as recommended in this report, will be needed to reduce the potential for disturbance of wet lodgement till soils and costs associated with repairing disturbed soils. Reuse of lodgement till in structural fill applications will require drying to within 1 to 2 percent of optimum moisture content during dry site and weather conditions. Vashon lodgement till is generally considered unsuitable for infiltration of stormwater runoff due to its high fine-grained content and low permeability. 4.2 Hydrology Groundwater Shallow groundwater at the project site is expected to be present in three zones: a perched groundwater interval in the weathered soil horizon above glacial till (interflow), the thin, recessional outwash deposits, and within sandier portions of the lodgement till. It should be noted that the depth to groundwater and duration or quantity of seepage can vary in response to changes in weather, season, changes in land use, and other factors. • Interflow: Shallow groundwater seepage encountered in the weathered till sediments in exploration EP-3 was interpreted as perched groundwater consistent with interflow. Interflow groundwater occurs when rain or surface water infiltrates through upper, looser, and more-permeable soils, such as topsoil and the weathered native till sediments, and becomes trapped on top of or in the upper portions of the less-permeable soils, such as the unweathered glacial till. • Perched water in the recessional outwash: Groundwater was observed perched on top of the lodgement till, near the bottom of the recessional outwash sediments in areas of March 29,2019 ASSOCIATED EARTH SCIENCES,INC. TG/ms-180567E001-2-Pro/ects‘20180567\KE\WP Page 5