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Geotechnical Engineering Evaluation NGA File No. 961916 Mason Residence Addition and Deck August 24, 2016 Everett,Washington Page 5 1 'Z SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2015 International Building Code (IBC). Since mostly native medium dense/stiff to dense/hard soils are interpreted to underlie the site at depth,the site conditions best fit the IBC description for Soil 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 competent glacial deposits interpreted to underlie the site at depth have a low potential for liquefaction or amplification of ground motion. The medium dense/stiff or better native 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 on the slope have the potential for shallow sloughing failures during seismic events. Such events should not directly affect the proposed addition and deck provided our recommendations are implemented as described in this report. 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 the 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 surface soils for this site were mapped 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 to moderate. It is It is our opinion that the erosion ha a d for the site soils should be slight to moderate in areas where the site is not disturbed. Landslide Hazard/Slope Stability The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. Steep north- and south-facing slopes with gradients of up to 50 degrees (119 percent) are located below the existing residence and planned development area. We did not observe significant evidence of recent deep-seated slope instability or major erosion or sloughing events on this slope during our site visit. However, we did observe evidence of a past surficial sloughing event within the steep NELSON GEOTECHNICAL ASSOCIATES, INC.