202 ALDER ST Geotech Report 2018-01-02 MF Import

1 , Prvposed View RiQge Subsurface Erploration, Geologic Hazards, lnfilrrarion Potential, Elementarv School Reptacemen� and Preliminary Georechnical Engineering Reporr Everett. Washington Projeu and Site Conditions , structural fill applications if the moisture condition is maintained within about 2 percent of optimum for compaction purposes. 1 Pu6lished Geologic Map , We reviewed published geologic mapping for the project (Distribulion and Description of the Geologic Units in the Everett Quadrangle, Washington, by James P. Minard, 1981). This map 1 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 HydrologK 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 prepazed, 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 infilvation of surface water is unpeded by lower permeabiliry soil layers such as ' sitt stringers in native advance outwash sedimenis. Such perched ground water is common and is typically addressed on construction sites by use of interceptor swales or other similaz , methocls. 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 infihration receptor. Advance outwash sediments ' ohen contain silry stringers that can limit infiltration potential, and therefore infiltration structures that use ao advance outwash receptor often use pit drains or trench drains to ' penetrate stratified sediments and vnprove infiltration rates. Pit drains and infiltration trenches are excavations Lhat are backfilled with drain rocN and allow infiltration water to access deeper strata and facilitate lateral flow from the infilvation facilities. ' At this site, a detailed investigation of infiltration potential is recommended, and would indude several steps. We recommend installing a ground water level logging device (data logger) in ' MW-1 to observc ground water levels through the coming winter. We recommend completior. of a visual reconnaissance of the slope to the east of the site to determine if emergent ground 1 Septeniber 22, 2010 ASSOCIATED EARTH SClENCES, INC. 9wGnD�KE/W155AI�ProjCCn120iCVd551KEIWP Page 5 1