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1 <br />t <br />' <br />' <br />�l <br />� <br />' <br />' <br />1 <br />' <br />' <br />' <br />' <br />' <br />' <br />' <br />1 <br />' <br />' <br />Gustafson Residence Retaining Wall <br />1215 Madrona Avenue <br />Everett, Washington <br />SENSITIVE AREA EVALUATION <br />Seismic Hazard <br />NGA File No 924115 <br />June 12, 2015 <br />Page 5 <br />We reviewed tl�e 2012 International Building Code (IBC) for seismic site classification for this project. <br />Since inedium dense/stiff to dense/hard sand and silt soils were encountered underlying the site, the site <br />conditions best fit the IBC description for Site Class D. <br />Hazards associated with seismic activity include liquefaction potential and amplification of ground <br />motion by soft deposits. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit <br />beneath the groundwater table. The dense/hard silty sand and silt soils interpreted to underlie the site <br />have a low potential for liquefaction or ampli�cation of ground motion. <br />The competent sand and silt soils interpreted to form the core of the site slope are considered stable with <br />respect to deep-seated slope failures. However, the overlying loose surficial materials on the slopes have <br />the potential for shallow sloughing failures during seismic events. Surficial failures within this site could <br />be reduced in intensity and frequency by implementing the recommendations provided in this letter. <br />Erosion Hazard <br />The criteria used for determination of erosion liazard areas include soil type, slope gradient, vegetation <br />cover, and groundwater conditions. The erosion sensitivity is related to vegetative cover and the specific <br />surface soil types, which are related to the underlying geologic soil units. The Soil Survey of Snohomish <br />Countv Area, Washin tg on, by the Soil Conservation Service (SCS) was reviewed to determine the erosion <br />liazard of the on-site soils. The site surface soils were classified using the SCS classification systein as <br />Alderwood-Everett gravelly sandy loams, 25 to 70 percent slopes. This unit is listed as having a moderate <br />erosion hazard. The on-site soils should have a low hazard for erosion where the vegetation is not <br />disturbed, and restoration inethods are iinplemented and properly maintained. Exposed areas on the steep <br />slopes will continue to have a high erosion potential. <br />Landslide Hazard <br />The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater <br />conditions. Steep north-facing slopes with gradients in the range of approximately 30 to 43 degrees (58 to <br />NELSON GEOTECHNICAL ASSOCIATES, INC. <br />