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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 20 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />4.3.3 Post-Liquefaction Condition Limit Equilibrium Analyses <br />Geologic Profiles A-Aʹ and B-Bʹ were evaluated using the post-liquefaction residual shear <br />strength of the liquefied soil layers to evaluate the static stability of the slope following the onset <br />of liquefaction. HWA’s liquefaction analysis predicts that the loose to medium dense fill, bay <br />deposits, and recessional outwash deposits, where fully saturated, liquefy during the design <br />earthquake. The results of the pseudo-static LEA with failure surfaces below 1.0 are presented on <br />Figures F-7 and F-8 in Appendix F. The analyses suggest that failure surfaces under post- <br />liquefaction conditions extend as far back as approximately 140 to 300 feet behind the seawall. <br />4.4 MITIGATION OPTIONS FOR SLOPE INSTABILITY <br />Limit equilibrium slope stability analyses indicate that western portions of the Port Gardner <br />property may experience lateral spreading and flow sliding events under design seismic and post- <br />liquefaction conditions. HWA’s analyses indicate that all structures located within approximately <br />100 to 390 feet from the west edge of the property may experience some level of lateral loading <br />and movement; however, the structures that are located closest to the western edge of the <br />property will experience the largest lateral loads and movement and they will provide some <br />protection to the structures located eastward from some lateral loading and movement. In <br />HWA’s opinion, the existing clarifier (to be repurposed as the secondary combined sewer <br />storage) and the proposed Effluent Pump Station (both located along the western edge of the <br />property) are at the highest risk for lateral loading and displacement due to slope instability and <br />liquefaction during a design seismic event. <br />The extent of the area experiencing lateral displacement, and the magnitude of displacements, is <br />typically a function of the magnitude of the event. Larger displacements on the order of several <br />feet will likely occur during a large seismic event. These displacements are anticipated to be a <br />widespread phenomenon across the Everett waterfront; the magnitude of the lateral displacement <br />and damage is anticipated to be greatest on structures located closest to the waterfront. Damage <br />due to liquefaction could render many of the structures within the PGSF non-functional. <br />However, HWA anticipates that seismic performance of the proposed structures would be <br />commensurate with that of the existing infrastructure on the site. <br />As discussed in Section 1.3, HWA understands that the existing structures and the <br />additions/modifications to the existing structures will not be designed per the governing IBC <br />2021 seismic design code; however, entirely new proposed structures such as the Effluent Pump <br />Station, Lift Station 46, and the Generator Pad (serving Lift Station 46) will be designed to meet <br />the requirements of IBC 2021 seismic design code (as described in BC, 2023b). HWA has <br />evaluated various approaches to address seismically induced slope instability and lateral <br />spreading mitigation options concerning the Effluent Pump Station. In HWA’s opinion,