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Geotechnical Engineering Evaluation NGA File No. 959616 Watson Residence Addition July 6,2016 Everett,Washington Page 4 dense, more permeable soils and accumulates on top of a less permeable soil. 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 groundwater to decrease during drier times of the year and increase during wetter periods. SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2012 International Building Code (IBC) for seismic site classification for this project. Since medium dense or better native glacial soils were encountered underlying the site at depth, the site conditions best fit the IBC description for Site Class D. Hazards associated with seismic activity include liquefaction potential and amplification of ground motion. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. It is our opinion that the medium dense or better native glacial deposits interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. The medium dense to very dense granular glacial soils interpreted to form the core of the site slope are considered stable with respect to deep-seated slope failures. However, the loose surficial materials and undocumented fill along the slope have the potential for shallow sloughing failures during seismic events. Such events should not affect the planned addition provided they are designed with the setback from the slope as described in the Structure Setback subsection of this letter. Erosion Hazard The criteria used for determination of the erosion hazard for affected areas include soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative cover and the specific surface soil types,which are related to the underlying geologic soil units. The Soil Survey of Snohomish County Area, Washington by the Soil Conservation Service (SCS), was reviewed to determine the erosion hazard of the on-site soils. The site surface soils were classified using the SCS classification system as Alderwood-Urban land complex, 2 to 8 percent slopes and Alderwood-Everett gravelly sandy loams, 25 to 70 percent slopes. The erosion hazard for these materials is listed as slight and moderate to high, respectively. It is our opinion that the erosion hazard for soils on the property, including the slope, should be low in areas where vegetation is not disturbed. NELSON GEOTECHNICAL ASSOCIATES, INC.