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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 13 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />The soil conditions at the site are such that spread footing foundations are not considered suitable <br />for support of large or settlement-sensitive structures due to the predominance of compressible <br />fine-grained organic and liquefiable soils, which could lead to excessive differential settlement. <br />Small or lightly loaded structures which can tolerate several inches of total and differential <br />settlement could be supported on spread footings. Alternatively, a mat foundation could be used <br />to limit the effects of differential settlements. In general, HWA recommends that the proposed <br />structures, including utilities, to be located above liquefiable soils be supported on driven <br />displacement piles. <br />Slope stability analyses indicate that western portions of the Port Gardner property may <br />experience lateral spreading and flow sliding events under design seismic and post-liquefaction <br />conditions. In order to protect the proposed new pump station, HWA recommends implementing <br />ground improvement by the installation of soil cement columns by either deep soil mixing or jet <br />grouting techniques to mitigate the impact of slope instability and lateral loading during a large <br />seismic event. Ground improvements will mitigate the effects of lateral spreading for the pump <br />station foundation design; however, HWA understands that the foundations will be designed to <br />account for down drag loads associated with liquefaction-induced settlement. <br />Utilities and structures which are not pile supported can be expected to settle differentially with <br />respect to pile-supported structures. Utilities that cannot tolerate 4 to 12 inches of differential <br />settlement might require repairs after a seismic event. Flexible connections should be provided <br />where non-pile-supported utilities connect to pile-supported structures or utilities. <br />Portions of the site may encounter obstructions such as large woody debris, logs, or construction <br />debris during pipeline trenching and structure excavations. Additionally, the fill and wood <br />materials at the site are unlikely to be self-supporting; caving of sidewalls during excavation <br />activities should be anticipated. The contractor should design their temporary shoring systems to <br />account for potential obstructions in, and caving of the existing soils. <br />Shoring will likely be required for most of the proposed excavations at the site. When sloped <br />excavations or other conventional shoring techniques are not suitable to keep a safe and dry <br />excavation, HWA recommends the use of interlocking steel sheet piling to support the <br />anticipated deep excavations. The use of sheet piles will reduce the requirement for dewatering. <br />Dewatering inside the sheet pile enclosure would limit the area affected by dewatering. At <br />excavations adjacent to settlement-sensitive structures and facilities, such as the BNSF railroad <br />tracks and the Port of Everett’s Norton Terminal, shoring and dewatering efforts should be <br />completed in a manner that minimizes impacts to these features. Settlement monitoring of these <br />features should be performed to provide the City and contractor with information regarding the <br />impacts of construction activities.