202 ALDER ST Geotech Report 2025-07-30 (2)

Proposed View Ridge Subsurface Exploration, Geologic Hazards, Infiltration Potential, Elementary School Replacement and Preliminary Geotechnical Engineering Report Everett, Washington Project and Site Conditions structural fill applications if the moisture condition is maintained within about 2 percent of optimum for compaction purposes. Published Geologic Map We reviewed published geologic mapping for the project (Distribution and Description of the Geologic Units in the Everett Quadrangle, Washington, by James P. Minard, 1981). This map indicates that the site is expected to be underlain at shallow depths by Vashon lodgement till. Advance outwash is mapped at lower elevations to the east and west. 4.2 Hydrology Ground water was encountered in exploration boring EB-1, where we constructed a well to allow later measurement of ground water levels. At the time this report was prepared, the measured depth to ground water was approximately 39 feet. We also encountered shallow ground water at the location of EB-5, which was interpreted to represent irrigation water. At the time of drilling, the grass field surrounding EB-5 had been heavily watered, with surficial puddles and soft turf. At this site, perched ground water is possible. Perched ground water occurs where infiltration of surface water is impeded by lower permeability soil layers such as silt stringers in native advance outwash sediments. Such perched ground water is common and is typically addressed on construction sites by use of interceptor swales or other similar methods. 4.3 Infiltration Potential/Detention Considerations The site was observed to be underlain by advance outwash sediments that extended beyond the full depth explored of 45 feet in EB-1, our deepest exploration for the project. Ground water at EB-1 was measured to be at a depth of approximately 39 feet. Advance outwash sediments are often used as a storm water infiltration receptor, and the sediments we observed at the site appear to be a good candidate for use as an infiltration receptor. Advance outwash sediments often contain silty stringers that can limit infiltration potential, and therefore infiltration structures that use an advance outwash receptor often use pit drains or trench drains to penetrate stratified sediments and improve infiltration rates. Pit drains and infiltration trenches are excavations that are backfilled with drain rock and allow infiltration water to access deeper strata and facilitate lateral flow from the infiltration facilities. At this site, a detailed investigation of infiltration potential is recommended, and would include several steps. We recommend installing a ground water level logging device (data logger) in MW-1 to observe ground water levels through the coming winter. We recommend completion of a visual reconnaissance of the slope to the east of the site to determine if emergent ground September 22, 2010 ASSOCIATED EARTH SCIENCES, INC. DWG/tb-KE100255A2-Projects1201002551KEIWP Page 5