1921 W MUKILTEO BLVD 2018-01-02 MF Import

Preliminary Geotechnical Engineering Evaluation 1919 Mukilteo Boulevard October 20, 2005 NGA File No. 718805 Page 4 Hydrologic Conditions Groundwater seepage was not encountered within any of the explorations extending to depth of to 48 feet below the existing ground surface. However, it is our opinion that a perched groundwater condition could develop on this site during wetter times of the year. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of relatively low permeability materials. The more permeable soils consist of the loose topsoil and loose upper portions of the sand soils. Lower permeability medium dense or better silty sand lenses may be present within the sand deposit, which could perch the infiltrating water. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to decrease during drier times of the year and increase during wetter periods. SENSITIVE AREA EVALUATION Seismic Hazard ''i We reviewed the 2003 International Building Code (IBC) for seismic site classification for this project. Since medium dense 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. Such events should not affect the planned structures provided the foundations are designed with the recommended effective setback from the face of the slope as described in the Foundations and Structure Setback subsections of this reFort. 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 cover and NELSON GEOTECHNICAL ASSOCIATES, INC.