215 DORN AVE Geotech Report 2022-10-25

Geotechnical Engineering Evaluation NGA File No. 1031018 Multi-Family Residential Development April 19,2018 Everett,Washington Page 3 At the surface of all explorations,test pits encountered a 0.4-to 2.8-foot-thick layer of dark brown, silty, fine to medium sand with roots,gravel,and anthropogenic debris,including concrete rubble and PVC pipes, which we interpreted as undocumented fill.Notably,Test Pit One and Infiltration Pit One in the southern portion of the property encountered the thickest layer of undocumented fill at 2.8 and 2.0 feet,respectively. In all explorations except Infiltration Pit One,the undocumented fill was underlain by tan-brown to gray, silty fine to medium sand with gravel and varying amounts of iron oxidation staining. This layer was encountered in an increasingly dense condition,ranging in depth between 1.8 and 3.5 feet.We interpreted this soil to be weathered,native,glacial till.Underlying the weathered soil in Test Pits One and Infiltration Pit Two,explorations encountered the same material,only gray, dense, and without oxidation, interpreted to be unweathered glacial till soils. The aforementioned explorations terminated within the native glacial soils at depths between 2.5 and 7.3 feet below the surface. Infiltration Pit One encountered tan-orange silty fine to medium sand with gravel and iron oxidation staining in a medium dense condition directly underlying undocumented fill soils.Underlying the weathered,native soil,the exploration encountered tan,gravelly,fine to coarse sand with silt to a depth of 3.5 feet below the adjacent surface elevation.We interpreted this soil to be derived from glacial recessional outwash,or from subglacial meltwater streams.Underlying the anomalous soil,the exploration encountered the gray, silty, fine to medium sand with gravel in a dense condition found elsewhere on site,which was interpreted to be the native,mapped, glacial till. Infiltration Pit One was completed at a depth of 3.7 feet below the surface within the native glacial soil. Hydrogeologic Conditions No groundwater seepage was observed in any of our soil explorations. If groundwater were to be observed within the soils on this site,we would interpret it to be perched water and not a regional groundwater table. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of a relatively low permeability material. Perched water does not represent a regional groundwater"table"within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to decrease during drier times of the year and increase during wetter periods. SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2015 International Building Code (IBC) for seismic site classification for this project. Since dense to very dense soils are interpreted to underlie the site at depth, the site best fits the IBC description for Site Class D. NELSON GEOTECHNICAL ASSOCIATES, INC. 86