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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 42 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />4.8.1 Buoyancy and Uplift Considerations <br />Buried structures will experience an upward buoyant force when groundwater levels outside the <br />structures are higher than the fluid level inside the structures. The weight of the structure will <br />provide resistance to uplift forces. In addition, extending the base of the foundation beyond its <br />outside perimeter will increase uplift resistance of the structure. HWA recommends using a <br />design groundwater elevation equal to the ground surface. <br />If an extended base slab is used, the weight of the soil overlying the footing, such as compacted <br />structural fill or controlled density fill (CDF), can be calculated as shown on Figure 6A. HWA <br />understands that buried structures will be situated within properly backfilled and compacted <br />excavations per the recommendations of this report, and that some buried structures will have an <br />extended base slab to mitigate uplift. <br />Because HWA recommends installation of bond breakers to minimize down drag loading, side <br />friction should be ignored for buoyancy resistance calculations. <br />4.8.2 Lateral Earth Pressures <br />Bottom elevations of proposed manhole and vault structures, the Effluent Pump Station, and Lift <br />Station 46 will be on the order of about 13 to -15 feet (approximately 3 to 32 feet bgs). These <br />structures should be designed to resist the lateral earth pressures shown on Figures 6B and 6C. <br />As indicated in the figures, the maximum design groundwater elevation should be assumed at the <br />ground surface to account for the extreme event when the backfill material is temporarily <br />saturated. The earth pressures provided assume that the walls of the structure will not be free to <br />deflect, and at-rest earth pressures will develop. <br />4.9 UTILITIES <br />HWA understands that the proposed project will include the construction of several underground <br />utilities. Given the potential for settlement throughout the project site (i.e., biodegradation of <br />wood, consolidation settlement, and liquefaction-induced settlement), HWA recommends that <br />underground utilities be supported on deep foundations with flexible piping material. However, <br />HWA understands that not all the proposed utilities will be pile supported. <br />Seismic-induced liquefaction settlements are anticipated along the utility alignments. <br />Liquefaction-induced settlements of up to 26 inches may occur as a result of the design <br />earthquake. HWA expects these settlements to be differential in nature along utility alignments <br />and may cause damage or failure of the utilities. In addition, some of the utility alignments may <br />undergo liquefaction-induced lateral spreading during the design earthquake. The magnitude of <br />lateral spreading is likely to vary along the alignment with the greatest displacements occurring