11831 AIRPORT RD WHOLE SITE FILE Geotech Report 2022-10-25

earthquake that has a 7 percent probability of exceedance in a 75-year period (approximate 1,000-year return period). TABLE 2.AASHTO SEISMIC DESIGN PARAMETERS 2015 AASHTO Parameter Preliminary Values Site Class D Peak Ground Acceleration 0.39 PGA x Fpga(g)=(1.1)(0.39) 0.43 Short Period Spectral Response Acceleration,Ss 0.88 1-Second Period Spectral Response Acceleration,Si 0.30 Seismic Coefficient, FPGA 1.1 Seismic Coefficient, FA 1.15 Seismic Coefficient, Fv 1.80 Seismic Hazards Based on USGS maps of active faults in the Puget Sound region,the site is located more than 1 mile south and east of the official Southern Whidbey Island Fault Zone. Because the thickness of Quaternary sediments below the site, which are commonly more than 1,000 feet thick in the region, and lack of fault displacement evidence in the project area,the potential for surface fault rupture is considered remote. We evaluated the site conditions for seismic hazards including liquefaction, lateral spreading and seismically induced landsliding. Our evaluation indicates the majority of the sites have a low risk of liquefaction because of the presence of dense glacially consolidated soils below the site and/or because of the deep groundwater table. Because there is a low risk of liquefaction, the site has a low risk of liquefaction-induced ground disturbance including lateral spreading. Our evaluation of seismically induced landsliding indicates that there is also a low risk; however, localized sloughing and slope instability may occur during a large seismic event. Station Retaining Walls We understand that retaining walls are currently planned at nine of the new stations.The walls are typically 3 to 4 feet in height, and will retain both new cut and fill areas. We understand that the wall foundations are typically designed using an allowable bearing pressure of 2,000 pounds per square foot (psf). This allowable bearing pressure is suitable provided the site preparation and fill placement criteria are followed as recommended in the following paragraphs. Anticipated Soil Conditions Variable soil conditions are anticipated along the different walls.Where walls will be situated on existing fill slopes, we anticipate that the fill typically consists of loose to medium dense silty sand. Where the walls will be cut into an existing slope, we anticipate that the soil conditions will vary from recessional sand and gravel to glacial till.A summary of the anticipated soil conditions at each wall is presented below in Table 3. GEOENGINEERS April 14,2017 Page 11 File No.2095-032-00