3505 SHORE AVE 2016-01-01 MF Import

Gcotcchnical Engincaiag Letter 3505 S6ore Avcnue Dcck Additioa Everett,Washington May 26,2010 NGA Filc No.827]]0 Page 4 • intilvates through less dense,moro petmcable soils and accumulaus on top of relatively]ow pumeability matcrials. Thc more permeable soils consist of the loou topsoil and the sand soils. Lower pecmeability ;il[soil may be interbedded in tt.e sand dcposit, which could pench tLe in6lttating weter. Perched water dces not repmunt a regional groundwater"table" witbin lhe upper soil horiwns. Perched waur tends to vary spatielly and is dependent upon the amount of�ainfalL We would expect ffie amount of ptrched groundwata to decrease during drier rimes of the ycar and inc�ease during wetter periods. SENSITIVE AREA EVALUATION Seismlc Hazard We reviewed the 2006 Intemadonal Building Code (BC) for seismic site classi5cation for this project. Since medium dense to dense sand and stiPl'silt deposits were encounured undedying the site, the site conditions best fit the IBC description for Site Cless D. Hazards associated with scismic activity include liquefaction potential and emplificaflon of ground motion by soR deposits. Liquefaction is caused by a rise in pore pressures in a]oose, fine sand deposit beneath the groundwater table. T6e medium dense to dense sand interpreted to underlie the site has a low potential for liquefaction or amplification of�round motion. The dense granular soils and very stiff silt intetproted to frnm lhe core of the site slope are considerod stable with respcct to dcep-seated slope failures. However, the ovedying loose surficial materials on the slopes have the potential for shallow sloughing failures duting seismic events. Such evnnts should not affect the planned stnictures provided the foundations ane desig�ed with the rocommendcd utback from the face ofthe slope as described in the Structure Setback subsection oft6is letter. Erosion Hazard Thc erosion ha7ard criteria used for detemilnation of affected areas includes soil typc, slope �adient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative cover and the specific surface soil types,which are nlated to tlie underlying gcologic soil units. The Soil Survcv of Snohomish Countv Arca. Wastuneton by the Soil Conservation Service (SCS) was reviewed to determine the erosion hazard of the on-site soils. T6e site surface soils were classified using the SCS classification system as Alderwood-Everett gravelly sandy loam, 25 to 70 percent slopes. This unit is NELSON GEOTECHNICAL ASSOClATES, INC. / � i S ��