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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 17 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />expects that pile driving will result in localized soil densification within, and slightly around, the <br />existing pile groups. <br />4.2.3 Post-Liquefaction Residual Shear Strength <br />Residual shear strengths for the liquefiable soils were developed using a weighted average of the <br />results of the Idriss (2007), Olson and Stark (2002), Idriss and Boulanger (2007), and Kramer <br />(2008) relationships. The residual shear strengths assigned are a function of the equivalent clean <br />sand SPT value, (N1)60cs, the potential for void redistribution, and the initial effective overburden <br />stress. At locations where (N1)60cs is less than 10, HWA assumed void redistribution effects could <br />be significant, which gives an appropriate conservative estimate of residual shear strength. <br />Residual shear strength parameters of the liquefiable soils were calculated to evaluate global <br />stability under static loading conditions for the post-liquefaction event. <br />4.2.4 Liquefaction-Induced Settlement and Down Drag Loading <br />The potential for liquefaction-induced settlement was evaluated at all boring locations. The <br />methodologies used were developed by Yi (2010) and Andrus (2010) and are generally based on <br />the relationship between shear wave velocity, cyclic stress ratio, corrected SPT blow counts, and <br />volumetric strain. Using these methods, HWA estimates liquefaction-induced settlements of <br />approximately 4 to 24 inches could occur across the site following the design seismic event <br />(2,475-year return). <br />Down-drag loading on buried structures, including deep foundation elements, occurs when the <br />surrounding soil settles or otherwise moves downward relative to the buried element. Downward <br />movements on the order of ¼ inch are sufficient to fully mobilize negative skin resistance or <br />down drag. As indicated above, liquefiable soils within the project site are expected to undergo <br />liquefaction-induced settlement as a result of the design earthquake. The down drag loads should <br />be accounted for in the design of the proposed improvements. It is important to note that the <br />proposed ground improvement located west of the Effluent Pump Station is intended to mitigate <br />liquefaction-induced slope instability and lateral displacement only. The proposed ground <br />improvement does not prevent liquefaction-induced settlement and applicable down-drag loads <br />should be accounted for in design of all the proposed improvements, including the Effluent <br />Pump Station. <br />4.2.5 Liquefaction-Induced Slope Instability <br />Initiation of liquefaction is triggered by the generation of increased pore water pressures within <br />the liquefiable soils. As the pore water pressures increase, the soil loses shear strength. When the <br />soil is fully liquefied, the soil shear strength is at its lowest level, referred to as “residual shear