3535 RIVERFRONT BLVD Geotech Report 2023-02-22

September 24, 2018 HWA Project No. 2015-061-21 Table 1.Design Seismic Coefficients for IBC 2015 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 0.2 sec. 1.0 sec. Ss.g Si,g SDs.g SDI,g Fa Fv D 1.306 0.496 0.871 0.497 1.0 1.504 The design peak ground acceleration (PGA) is taken as 0.4 times the design spectral acceleration at 0.2 seconds (SDs). Therefore, the resulting design PGA is 0.35 g. We utilized a horizontal coefficient, ki,, equal to PGA/2 or 0.175 g for our pseudostatic stability analyses of the river bank slopes. 4.2.2 Liquefaction Susceptibility Evaluation The potential for soil liquefaction effects must be considered during the design of any soil- 111 supported structure. Soil liquefaction is a phenomenon wherein loose, saturated, granular deposits temporarily lose strength and behave as a liquid in response to moderate to strong earthquake shaking. We evaluated liquefaction susceptibility of saturated fill (where it was observed to be granular) and native alluvial soils. Utilizing the simplified procedure originally developed by Seed and Idriss (1971), and updated by Idriss and Boulanger (2004), we evaluated the liquefaction susceptibility of granular fill and alluvial soils where they were encountered below the design ground water elevation for the 3-Acre Park site. The simplified procedure is a semi-empirical approach which compares the cyclic shear stress required to initiate liquefaction (or cyclic resistance ratio —CRR) to the cyclic shear stress induced by the design earthquake (or cyclic stress ratio —CSR). The factor of safety relative to liquefaction is the ratio of the CRR to the CSR; where this ratio is computed to be less than one, the analysis would indicate that liquefaction is likely to occur during the design earthquake. The CRR is primarily dependent on soil density, with the current practice being to base it on the Standard Penetration Test(SPT) N-value for the soil, corrected for hammer energy considerations, fines content and earthquake magnitude. CSR relates equivalent shear stress caused in the soil at any depth to the effective stress at that depth and the peak ground acceleration at the surface. In accordance with WSDOT practice, we considered zones with factors of safety against liquefaction of 1.2 or less to be liquefiable. Layers greater than 50 feet below the ground surface were not analyzed, as the CSR model has not been calibrated for deeper depths. Since SDs is greater than 0.5 g, and SD1 is greater than 0.2 g,the potential for liquefaction must be evaluated using the site peak ground acceleration defined by ASCE 7 (ASCE, 2010) as the maximum considered earthquake geometric mean (MCEG) peak ground acceleration (PGA). For this site the MCEG PGA is 0.54 g. We evaluated the liquefaction susceptibility for each of the borings with Standard Penetration Test data for this design event. the approximate extent of Final Geotechnical Report-3-Acre Park.docx 8 HWA GeoSciences Inc.