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 4 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. Table 1—2015 IBC Seismic Design Parameters Site Class Spectral Spectral Acceleration Site Coefficients Design Spectral Acceleration at 0.2 at 1.0 sec.(g) Response sec.(g) S 7 Parameters Ss Fa Fv SDS SDl D 1.364 0.484 1.000 1.816 0.909 0.586 The spectral response accelerations were obtained from the USGS Earthquake Hazards Program Interpolated Probabilistic Ground Motion website(2008 data)for the project latitude and longitude. The site is located within the South Whidbey Island Fault Zone (SWIFZ): an active, shallow region of seismicity within central Puget Sound stretching from the Strait of Juan de Fuca to North Bend.Information published in 2013 by the Washington State Department of Natural Resources suggests the SWIFZ last ruptured less than 2,700 years ago,and that the fault zone can produce a M7.5 earthquake. 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 very dense glacial'deposits interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. 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) classifies the site as Alderwood-Urban land complex, 2 to 8 percent slopes. Based on our experience in the area and our observations in the field, it is our opinion that the site would have a slight to moderate erosion hazard for areas where the soils are exposed. It is our opinion that the erosion hazard for site soils should be low in areas where vegetation is not disturbed. NELSON GEOTECHNICAL ASSOCIATES, INC. 87