3800 RIVERFRONT BLVD Geotech Report 2023-05-04

I March 26, 2019 HWA Project No. 2015-061-21 Task 600-7 I Manual (WSDOT, 2017), and using the BridgeLink tool (BridgeLink, 2017) developed by WSDOT. I Table 2. Seismic Coefficients for Evaluation Using AASHTO Guide Specifications calculated by USGS Seismic Hazard Map IPeak Spectral Spectral Horizontal Bedrock Bedrock Site Coefficients Peak Horizontal SiteII Class Bedrock Acceleration Acceleration Acceleration at 0.2 sec at 1.0 sec AccelerationPGA, (g) PBA, (g) Ss,(g) Si,(g) Fpgu Fa Fv D 0.370 0.837 0.247 1.23 1.165 2.106 0.455 Based on the above parameters the Peak Ground Acceleration, PGA (As) for site class D at the project site is 0.455g. 1 4.2.2 Liquefaction Hazard I Liquefaction is a temporary loss of soil shear strength due to earthquake shaking. Loose, saturated, cohesionless soils are highly susceptible to earthquake-induced liquefaction. Recent experience and research have shown that certain silts and low-plasticity clays are also susceptible. Primary factors controlling the development of liquefaction include the intensity and duration of strong ground motions, the characteristics of subsurface soils, in-situ stress conditions and the depth to groundwater. To evaluate the liquefaction susceptibility of the soils along the I project alignment, the simplified procedure originally developed by Seed and Idriss (1971) and updated by Youd et. Al. (2001) and Andrus and Stokoe(2000), was used. I For this site the PGA used to evaluate liquefaction is 0.455g. The analyses indicate that several zones of loose sand and cohesionless silt are expected to liquefy during the design level event. The depths and thicknesses of the potentially liquefiable materials vary along the road alignment Iand are identified in the geologic cross-sections Figures 4A—4D. The cross sections in Figures 4A &4D show predicted liquefaction within portions of the soil I units below the refuse materials. HWA does not believe that liquefaction will occur within the refuse for several reason. First, while groundwater was noted within the refuse it was highly variable in initial depths reported and groundwater observed at shallower depths is likely due to Iperched groundwater systems. As a result, the thickness of the saturated portions of the refuse are likely less significant than noted in the boring logs. Second the material makeup of the refuse is considered to not be conducive to allowing for the buildup of hydrostatic pressures high Ienough to mobilize the larger waste materials scattered throughout the refuse layer. If a large enough area of appropriately sized soil participles with extremely low amounts of larger refuse material existed within refuse layer, some limited amount of liquefaction could occur. However, Geotechnical Report Revision 3 26 2019.doc 9 HWA GEOSCIENCES INC.