421 ROCKEFELLER AVE 2023-05-31 - Laserfiche WebLink

• s 4'4 a . COBALT GEOTECHNICAL INVESTIGATION GEOSCIENCES EVERETT,WASHINGTON December 24,2019 6.o Geologic Hazards 6.1 Erosion Hazard The Natural Resources Conservation Services(NRCS)maps for Snohomish County indicate that the site is underlain by Alderwood-Urban land complex (2 to 8 percent slopes). These soils would have a slight to very severe erosion potential in a disturbed state,depending on the slope magnitude. It is our opinion that soil erosion potential at this project site can be reduced through landscaping and surface water runoff control. Typically erosion of exposed soils will be most noticeable during periods of rainfall and may be controlled by the use of normal temporary erosion control measures, such as silt fences, hay bales,mulching, control ditches and diversion trenches. The typical wet weather season,with regard to site grading,is from October 3ist to April 1st. Erosion control measures should be in place before the onset of wet weather. 6.2 Seismic Hazard The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the 2015 International Building Code (2015 IBC). A Site Class D applies to an overall profile consisting of dense to very dense soils within the upper too feet. We referenced the U.S. Geological Survey(USGS) Earthquake Hazards Program Website to obtain values for Ss,Si,Fa, and F. The USGS website includes the most updated published data on seismic conditions. The site specific seismic design parameters and adjusted maximum spectral response acceleration parameters are as follows: PGA (Peak Ground Acceleration,in percent of g) Ss 125.7o%of g S, 47.90%of g FA 1.00 Fv 1.521 Additional seismic considerations include liquefaction potential and amplification of ground motions by soft/loose soil deposits. The liquefaction potential is.highest for loose sand with a high groundwater table. The relatively dense soil deposits that underlie the site have a low liquefaction potential. 7.o DISCUSSION 7.1.1 General The site is underlain by variable thicknesses of fill underlain by weathered and unweathered glacial till. The proposed residential buildings may be supported on shallow foundation systems bearing on medium dense or firmer native soils and/or structural fill placed on suitable native soils. Local overexcavation of fill be necessary below proposed foundation elements. The depth of overexcavation will vary from 3.5 to 3 PO Box 82243 Kenmore,WA 98028 cobaltgeoPgmail.com 2o6-331-4097