1915 W MUKILTEO BLVD 2018-01-02 MF Import

Geotechnical Engineering Evaluation and Slide Repairs 1915 Mukilteo Boulevard July 26, 2006 NGA File No. 743206 Page 5 SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the '2003 International Building Code (IBC) for seismic site classification for this project. Since medium dease to dense sand and silty sand deposits 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 medium dense to very dense sand interpreted to underlie the site has a low potential for liquefaction or amplification of ground motion. The dense granular 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. Erosion Hazard The erosion hazard criteria used for determination of affected areas includes soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative cove; 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 loam, 25 to 70 percent slopes. This unit is listed as having a high erosion hazard for areas where the soils are exposed. It is our opinion that the erosion hazard for the exposed slope face should be high, however, in areas where vegetation can be maintained, the erosion hazard for the site soils should be low to moderate. Landslide Hazard/Slope Stability The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. A steep north -facing slope with a gradient of up to approximately 40 degrees (84 percent) with a height of approximately 100 feet is located below the existing building area. We observed evidence of significant land movement and sloughing along the top of the slope. Soil subsidence and tension cracks were noted along the top of the slope and sloughing soil was noted from a near vertical, 10- NELSON GEOTECHNICAL ASSOCIATES, INC.