1530 51ST PL SW 2016-01-01 MF Import

Gcotechnical Enginecring Evaluauon l.cttcr Mosman Basement Gvcrett, Wnshington Hydrologic Conditions NGA File No. 586013 December 17, 2013 Pagc 4 Groundwater seepage was not encountered in our explorations. If groundwater seepage occurs on this sitc, we would interpret the water to bc perched watcr. Pen:hcd wa:er occurs when surface water infiltrates through Iess dense, more pertneable soils such as the topsoil, undocumented fill, or granular glacial outwash soils and accumulates on top of a relatively low permeability material. Perched water docs not represent a regional groundwater "table" within thc upper soil horizons. Pcrched water tcnds to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to decreasc during drier times of tl�e year and increase during wetter periods. SENSITIVE ARE4 EVALUATIO� Seis-�ic Hazard We reviewed the 2009 Intemational Building Code (IBC) for scismic site classification for this project. Since medium dense to dcnsc, glacial soils were encountered underlying thc sitc at depth, the sitc conditions bcst fit thc IBC description for Site Class D. Hazards associated with seismic activity include liqucfaction potential and amplification of grcund motion. Liquefaction is caused by a risc in porc pressures in a loose, finc sand deposit beneath thc groundwater tablc. It is our opinion that the medium dense or better glacial deposits interpreted to underlie the site hs��e a low potential for liquc(action or amplification of ground motion. Erosion Hazard The critcria used for determination of crosion ha��rd areas include soil type, slope gradicnt, vegetation cover, and groundwater condiiions. The erosion sensitivity is rcla�ed to vegetative cover and the specific surface soil typcs, which arc related ro thc underlying gcologic soil units. The Soil Survev of Snohomish Countv, Washinn.ton, by the Soil Conscrvation Service (SCS) was reviewed to detenninc thc crosian hazard of thc on-site soils. The site surCacc soils were classitcd using the SCS classification system as Everett gravelly sar.dy loams, 0 to S percent slopes. These soils are listed as having a slight hazard of water erosion. The native soils should havc a low hazard for crosion in areas th��t are not disturbed and where the vegetatiou cover is not removed. NELSON GEOTECHNICAL ASSOCIATES, INC.