5228 EAST DR Geotech Report 2022-10-25

COBALT GEOTECHNICAL INVESTIGATION C e o S C I E N'C R S EVERETT,•WASHINGTON December 5,2019 iii. The increased buffer is necessary to protect public health,safety and welfare based upon findings and recommendations of the geological assessment. The steep slope areas appear stable at this time with no evidence of historic or recent landslide activity. At all times, the steep slopes should be remain fully vegetated. Stormwater runoff should not be directed toward or over slope areas. 6.2 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),Alderwood gravelly sandy loam(15 to`3o percent slopes), and Alderwood-Everett gravelly sandy loams (25 to 7o 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 31st to April ist. Erosion control measures should be in place before the onset of wet weather. 6.3 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 10o 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 139.6o%ofg Si >•53,0094 of g Fa 1.00 Fv 1.50 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 underliethetsite have a low liquefaction potential. 5 PO Box 82243 Kenmore,WA 98028 cobaltgeo Cagmaihcont 206-331-1097