3500 TERMINAL AVE Geotech Report 2022-10-25

Port of Everett—South Terminal Wharf& Electrical Upgrades—Phase 2 111 Liquefaction Potential Liquefaction is a phenomenon caused by an increase in porewater pressure that reduces the effective stress between soil particles, resulting in the sudden loss of shear strength in the soil.Granular soils that rely on inter-particle friction for strength are susceptible to liquefaction until the excess pore pressures can dissipate.Sand boils and flows observed at the ground surface after an earthquake are the result of excess pore pressures dissipating upward,carrying soil particles with the draining water. In general,loose, saturated sandy soils with low silt and clay contents are the most susceptible to liquefaction.Silty soils with low plasticity are moderately susceptible to liquefaction under relatively higher levels of ground shaking. For any soil type,the soil must be saturated for liquefaction to occur. Liquefaction can cause ground surface settlement and lateral spreading. We used empirical methods to estimate liquefaction potential based on the standard penetration test (SPT)and the cone penetration test(CPT)data obtained at the site.The procedures outlined in Idriss and Boulanger(2008)and Idriss and Boulanger(2014)were used to evaluate the factor of safety for liquefaction based on the SPT and CPT data, respectively. For our analysis of the 72-year,94-year, 144-year,224-year,and 475-year hazard levels,evaluation of liquefaction potential was performed using earthquake magnitudes between 6.5 and 7.0, and peak ground accelerations(PGAs)of 0.156,0.181,0.224, 0.270 and 0.346 g,respectively. Based on the available subsurface data and analysis results,the native silty sand below the construction dike is likely to liquefy in the northern portion of the site(near Section D-D'),as shown on Figures 3 through 6. Further south,the native material becomes generally non-liquefiable,with the exception of a 5-to 10-foot-thick lense of very loose silty sand encountered in HC-2 at a depth of 15 feet below ground surface,and a 5-to 10-foot-thick lense of loose silty sand encountered near ground surface at HC-1 at the southwest corner of the site. Based on our interpretation of the data in CPT-3 and HC-3,the dredged backfill(ESU 1)is also likely to liquefy at all considered hazard levels below the groundwater table. Further upland,some explorations indicate that limited or localized zones of soft or loose liquefiable deposits are present in the native sand below elevations of approximately-10 feet MLLW. Post-Liquefaction Vertical Settlement Post-liquefaction settlement occurs because liquefiable soils are redistributed and become denser after an earthquake.The ground surface settlement is not typically uniform across the area,and can result in significant differential settlement. We estimated liquefaction-induced ground surface settlement using SPT corrections by Idriss and Boulanger(2008)and volumetric strain formulations by Yoshimine et al. (2006),as well as CPT correlations (Cliq 2006)based on procedures by Idriss and Boulanger(2008).We calculated ground surface settlement only from the volumetric strains in the upper 80 feet.This is a reasonable assumption for ground surface settlement, because research has shown that volumetric contractions at depths greater than 60 feet may not manifest as surface settlement(Cetin et al.2009),therefore our settlement estimates are likely analytically somewhat conservative. Y 19232-01 HARTCROVISER December 6,2017