4626 HARBOR LN 2018-01-02 MF Import

Geotechnicai Engineering Evaluation Fox Residence Underpinning and Retaining Wall Design Everett, Washington NGA File No.853512 August 3,20'2 Page 3 7. We present a brief summary of the subsurface conditions in the following paragraphs. For a detailed description of the subsurface conditions, the hand auger logs should be reviewed. I land Auger 1 was advanced along the downhill edge of the concrete walkway. At the surface of Hand Auger 1, we encountered 5.0 feet of loose, brown -gray silty fine to medium sand. We interpreted this material as undocumented fill. Below the fill, we encountered medium dense, light brown -gray, fine to medium sand with silt and iron -oxide staining. We interpreted this material to be native soil. Hand Auger I was tenninated in the fine sand with silt at a depth of 6.5 feet below the existing ground surface. Hand Augers 2 and 3 were advanced within the garage slab. Below the concrete in Hand Augers 2 and 3, we encountered loose to medium dense, brown -gray, light brown, and light orange -brown, fine to medium sand with silt with trace organics. We interpreted this material to be undocumented fill. "ILc hand augers were refused on gravel within the fill at depths of 5.0 and 3.3 feet below the existing ground surface, respectively. Hydrologic Conditions Groundwater seepage was not encountered in any of the hand agers. If groundwater is encountered on this site, we would interpret the water to be perched groundwater. Perched water occurs when surface water infiltrates through less dense, more permeable soils such as the topsoil and undocumented fill, and accumulates on top of a relatively low pert ability material. 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 We reviewed the 2009 International Building Code (IBC) for seismic site classification for this project. Since medium dense to dense native soils were encountered at depth in our explorations, the site conditions best fit the 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 glacial material interpreted to underlie the site has a low potential for liquefaction or amplification of ground motion; however, the NELSON GEOTECHVICAL ASSOCIATES, INC.