1209 MADRONA AVE 2016-10-05 - Laserfiche WebLink

• • Geotechnical Engineering Evaluation NGA File No. 870113 Packer Property March 11,2013 Retaining Walls and Steep Slope Evaluation Page 6 Everett,Washington is dependent upon the amount of rainfall. We would expect the amount of groundwater to slightly decrease during drier times of the year and increase during wetter periods. SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2009 International Building Code (IBC) for seismic site classification for this project. Since medium dense/stiff to dense/hard silty sand and silt sols were encountered underlying the site, 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 by soft deposits. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. The dense/hard silty sand and silt soils interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. The competent silty sand and silt soils interpreted to form the core of the site slope are considered stable with respect to deep-seated slope failures. However,the overlying loose surficial materials on the slopes have the potential for shallow sloughing failures during seismic events. Such events should not affect the proposed slope and wall configuration provided the recommendations in this report are followed. Erosion Hazard The criteria used for determination of erosion hazard 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-Everett gravelly sandy loams, 25 to 70 percent slopes. This unit is listed as having a moderate erosion hazard. The on-site soils should have a low hazard for erosion where the vegetation is not disturbed. NELSON GEOTECHNICAL ASSOCIATES, INC.