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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 16 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />SD1 = Design Spectral Response Acceleration for 1-second period =2/3 • SM1 <br />S0 = Design Spectral Response at T = 0 seconds, which is assumed to equal the design PGA <br />Fa = Short Period Site Coefficients <br />Fv = Long Period Site Coefficients <br />T0 = 0.2•SD1/SDS <br />Ts = SD1/SDS <br />TL = Long Period Transition period <br />4.2.1 Near-Fault Effects <br />Near-fault effects should be considered for structures within 6 miles of a known active fault. The <br />Southern Whidbey Island fault trace is located approximately 5.2 miles aways from the project <br />site. For seismic design using 2021 IBC and ASCE 7-16 methods, near-fault ground motions are <br />to be considered in four ways: (1) the large amplitude of the ground motions given the proximity <br />to the fault, (2) potential for ground rupture, (3) forward directivity, and (4) basin effects. <br />The first impact of large amplitude ground motions that could occur due to rupture of the <br />Southern Whidbey Island Fault are accounted for in the seismic design coefficients provided, <br />which are based on the national hazard maps where the influence of the Fault Zone is already <br />included. Given the short period nature of the existing structures, HWA does not believe that <br />forward directivity or basin effects are applicable for this project. If it is determined that the <br />structural period of the existing structures is approximately 1 second or more, this assumption <br />would need to be revisited. With regards to basin effects, given the relative distance of the <br />mapped Southern Whidbey Island fault to the project site, which is greater than 5 miles, it is <br />unlikely that a rupture within the fault would extend far enough to propagate to the ground <br />surface at the project site. <br />4.2.2 Liquefaction <br />Liquefaction is a temporary loss of soil shear strength due to earthquake shaking. Loose, <br />saturated, and cohesionless soils are highly susceptible to earthquake-induced liquefaction. <br />Recent experience and research have shown that certain silts and low-plasticity clays are also <br />susceptible. Primary factors controlling the development of liquefaction include the intensity and <br />duration of strong ground motions, the characteristics of subsurface soils, in-situ stress <br />conditions and the depth to groundwater. To evaluate the liquefaction susceptibility of the soils <br />along the project alignment, the simplified procedure originally developed by Seed and Idriss <br />(1971), updated by Youd et al. (2001) and by Idriss and Boulanger (2004, 2006, 2008), was used. <br />HWA’s analyses indicate that the saturated loose sands encountered below the groundwater table <br />in all the borings will liquefy during the design earthquake. The depth to which liquefaction is <br />anticipated to occur varies along the site, as shown in the two geologic cross sections (Figures <br />4A and 4B). Given the number and geometry of the existing structural foundation piles, HWA