600 54TH ST SW I Geotech Report 2025-01-29

GEOTECHNICAL INVESTIGATION EVERETT,WASHINGTON March 2,2018 A majority of the slopes within building lots 7 through io(and detention tract)appear to be oversteepened due to previous grading activities. We did not observe evidence of sloughing,erosion,or landslide activity on any of these slope areas. The slopes appear stable at this time,but could experience sloughing in the future if they are not re-graded to create more stable configurations. Below variable thicknesses of fill,the site is underlain by dense to very dense native soils which are resistant to global instability. With properly considered grading and earthwork,the proposed development will increase slope stability within the building lots. This can be accomplished by removing oversteepened and/or filled slopes and replacing them with properly controlled fills placed on benched and keyed native soils. 6.2 Erosion Hazard The Natural Resources Conservation Services(NRCS)maps for Snohomish County indicate that the site is underlain by Alderwood gravelly sandy loam(15-3o percent slopes),Everett very gravelly sandy loam(15- 30 percent slopes),Alderwood-Everett gravelly sandy loams(25-7o percent slopes),and Alderwood- Urban land complex(8-15 percent slopes). In general,all of these soils have a"Moderate"erosion potential when sloped less than about 25 percent and a"Severe"erosion potential in slope magnitudes greater than about 25 percent. 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. 6.3 Seismic Hazard The overall subsurface profile below undocumented fill 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 141.90%of g Si 53.90%of g FA 1.00 Fv 1.50 5