1502 WALL ST IMAGINE CHILDRENS MUSEUM Geotech Report 2022-10-25

Imagine Children's Museum Building Addition Revised Geotechnical Engineering Report Everett, Washington Project and Site Conditions 4.1 Stratigraphy Vashon Lodgement Till The exploration borings advanced within the proposed building addition footprint encountered medium dense to very dense, mottled gray grading to gray, silty sand with gravel interpreted as Vashon lodgement till. Lodgement till was deposited at the base of an active continental glacier and was compacted by the weight of the overlying glacial ice. Lodgement till is suitable for structural support when properly prepared. Excavated lodgement till materia► is suitable for use in structural fill applications if suitable moisture conditions are achieved, which will require drying during favorable dry weather. At the time of exploration (February 2007), we estimated that most of the lodgement till soils that we observed were at or above optimum moisture content for compaction purposes, and therefore the site soils will likely require drying during favorable weather prior to compaction in structural fill applications. 4.2 Hydrology Groundwater was encountered only in one exploration boring completed for the previous study which was located about 110 feet south of the existing southwest building corner (EB-7/MW-7) on Figure 2. We constructed a well in that boring to allow accurate measurement of static groundwater levels after the completion of the exploration program. The groundwater observed was interpreted to represent isolated seepage from a relatively more-permeable zone within the lodgement till and is not thought to represent a laterally or vertically extensive aquifer. On sites characterized by lodgement till sediments, perched groundwater commonly develops, particularly during wetter times of the year. At this site, perched groundwater may currently be suppressed by extensive paving; however, when existing paving is removed, perched seepage will likely be observed. Perched groundwater occurs where the vertical infiltration of surface water is impeded by low-permeability soil units, and the groundwater tends to flow laterally above the less-permeable strata. Perched groundwater is most common within existing topographic swales and low areas and is most common during the wetter part of the year but can be encountered anywhere on the site at any time of year. 4.3 Environmental Considerations Although the purpose of our exploration borings was to identify engineering properties of shallow subsurface materials, during completion of our 2007 subsurface explorations, our borings encountered soils with noticeable hydrocarbon odors in an exploration boring located approximately 110 feet south of the southeast corner of the existing building (EB-1 on Figure 2). None of the other borings completed as part of our 2007 study encountered soils with unusual odors. March 28,2019;Revised April 16,2019 ASSOCIATED EARTH SCIENCES,INC. SGB/Id-190104E001-3-Projects\20190104\KE\WP Page, �/