10627 19TH AVE SE COOLEY SMILES DENTISTRY 2019-03-18

Geotechnical Engineering Evaluation NGA File No. 1008917 Cooley Smiles Commercial Development October 3,2017 Everett,Washington Page 3 In all Test Pits and Infiltration Pits except for Test Pit 1,we encountered approximately 0.2 to 0.9 feet of surficial topsoil at the ground surface. Test Pit 1, on the northern property, encountered loose surficial fill composed of coarse sand, gravel, and silty sand with gravel to a depth of 3.0 feet, where we also encountered a 1.25-inch PVC drainpipe. Underlying the topsoil in the other explorations, the test pits encountered between 0.5 and 1.9 feet of tan-brown silty fine to medium sand with gravel in a loose to medium dense condition. Underlying the tan silt and sand across the site, explorations encountered between 0.4 and 1.6 feet of gray to light gray silty fine to medium sand with trace fine gravel and iron oxidation staining in a medium dense condition. We interpreted this material to be weathered glacial till. Underlying the weathered glacial till across the site, and beneath the undocumented fill in Test Pit 1, the explorations encountered gray silty fine to medium sand with trace gravel in a dense to very dense condition that we interpreted as native unweathered glacial till soils. All of our test pit explorations and infiltration pits were terminated within the native unweathered glacial till soils at depths in the range of 3.2 to 7.0 feet below the existing ground surface. Hydrogeologic Conditions Groundwater seepage was not encountered in any of our explorations. If groundwater were to be encountered, we would interpret it to be perched water. Perched water occurs when surface water infiltrates through less dense,more permeable soils and accumulates on top of relatively low permeability materials. 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 competent glacial till soils are inferred to underlie the site at depth, the site conditions best fit the IBC description for Site Class D. Table 1 below provides seismic design parameters for the site that are in conformance with the 2015 IBC, which specifies a design earthquake having a 2%probability of occurrence in 50 years (return interval of 2,475 years),and the 2008 USGS seismic hazard maps. NELSON GEOTECHNICAL ASSOCIATES,INC.