7505 SEAWAY BLVD Geotech Report 2022-10-25

November 11, 2016 HWA Project No. 2014-117-21 4.2 SEISMIC DESIGN CONSIDERATIONS 4.2.1 Seismic Design Parameters Seismic design parameters are provided based on the requirements from the 2012 International Building Code (IBC) (ICC, 2011) for design of the Transit Center. In the code, above-grade structures must be designed for the inertial forces induced by a"Maximum Considered Earthquake" (MCE), which corresponds to an earthquake with a 2%probability of exceedance (PE) in 50 years (approximately 2,475 year return period). The relevant probabilistic spectral response parameters were developed using the United States Geological Survey's website. The IBC accounts for the effects of site-specific subsurface ground conditions on the response of structures in terms of site classes. Site classes are defined by the average density and stiffness of the soil profile underlying the site. The Site Class can be correlated to the average standard penetration resistance (Nsp-r) in the upper 100 feet of the soil profile. Based on our characterization of the subsurface conditions, the subject site classifies as IBC Site Class C. Table 1 presents the design spectral seismic coefficients obtained for this site. The design peak ground acceleration was computed to be 0.385 g. Table 1. Design Seismic Coefficients for IBC 2012 Code Based Evaluation Spectral Spectral Design Spectral Design Spectral Site Site Acceleration Acceleration at Acceleration at Acceleration at Coefficients Class at 0.2 sec. 1.0 sec 0.2sec. 1.0sec. Ss,g Si,g SDs,g SD1,g Fa F,. C 1.443 0.555 0.962 0.481 1.0 1.3 4.2.2 Ground Rupture The site is located in close proximity to the Southern Whidbey Island fault zone (SWIFZ), and is located within one to two kilometers of the northern trace of the fault(Sherrod et al., 2005). Given the location of the fault, rupture along the northern trace of the SWIFZ could propagate up through the surficial soils, resulting in potential ground rupture at the site. In our opinion, the risk of ground rupture is relatively low, but should be given consideration for essential structures. 4.2.3 Soil Liquefaction The potential for soil liquefaction must be considered during the design of any soil-supported structure. Soil liquefaction is a phenomenon where loose, saturated, granular deposits 2014-117 Final Geotechnical Report 7 HWA GEOSCIENCES INC.