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GEOTECHNICAL INVESTIGATION <br />EVERETT, WASHINGTON <br />October 28, 2019 <br />6.0 Geologic Hazards <br />6.1 Erosion Hazard <br />COBALT <br />G E 0 S C I E N C E S <br />The Natural Resources Conservation Services (NRCS) maps for Snohomish County indicate that the site is <br />underlain by Alderwood-Urban land complex (2 to 8 percent slopes). These soils would have a slight to <br />severe erosion potential in a disturbed state, depending on the slope magnitude. <br />It is our opinion that soil erosion potential at this project site can be reduced through landscaping and <br />surface water runoff control. Typically erosion of exposed soils will be most noticeable during periods of <br />rainfall and may be controlled by the use of normal temporary erosion control measures, such as silt <br />fences, hay bales, mulching, control ditches and diversion trenches. The typical wet weather season, with <br />regard to site grading, is from October 31st to April 1st. Erosion control measures should be in place before <br />the onset of wet weather. <br />6.2 Seismic Hazard <br />The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the 2015 <br />International Building Code (2015 IBC). A Site Class D applies to an overall profile consisting of dense to <br />very dense soils within the upper too feet. <br />We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to obtain values <br />for Ss, S,, Fa, and F,,. The USGS website includes the most updated published data on seismic conditions. <br />The site specific seismic design parameters and adjusted maximum spectral response acceleration <br />parameters are as follows: <br />PGA (Peak Ground Acceleration, in percent of g) <br />Ss <br />138.00% of g <br />S, <br />52.6o% of g <br />Fa <br />1.00 <br />Fv <br />1.50 <br />Additional seismic considerations include liquefaction potential and amplification of ground motions by <br />soft/loose soil deposits. The liquefaction potential is highest for loose sand with a high groundwater table. <br />The relatively dense soil deposits that underlie the site have a low liquefaction potential. <br />7.o DISCUSSION <br />7-1.1 General <br />The site is underlain by weathered and unweathered glacial till. The proposed residential buildings may <br />be supported on shallow foundation systems bearing on medium dense or firmer native soils and/or <br />structural fill placed on suitable native soils. Local overexcavation of fill and/or loose soils may be <br />necessary below proposed foundation elements. <br />3 <br />PO Box 82243 <br />Kenmore, WA 98028 <br />cobaltgeoOgmail.com <br />2o6-331-1097 <br />