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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 26 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />• Definition of refusal conditions. Soil-cement columns should extend below the deepest <br />liquefiable layer and into the dense outwash bearing layer. If obstructions or refusal are <br />encountered, additional soil-cement columns adjacent to the location of refusal will need <br />to be installed. <br />• Sampling (e.g., wet grabs) and testing requirements to confirm the soil-cement column <br />meets the designed unconfined compressive strength at 28 days. <br />• A pre-drilling program to address potential obstructions. <br />• A pre-production test program to confirm design requirements, installation procedures <br />including installation within the woody material. <br />• A general jet grout/deep soil mixing spoil return management plan outlining waste <br />containment methods, treatment, and removal for soil cement column spoils developed <br />during the ground improvement process. <br />4.5 FOUNDATION OPTIONS <br />The project site is underlain by liquefiable soils that extend to depths of approximately 10 to 80 <br />feet. As described in Section 4.2 of this report, HWA estimates that the subgrade soils may <br />experience liquefaction-induced settlements of 4 to 24 inches across the site following the design <br />seismic event (2,475-year return). The depth to the dense to very dense advance outwash bearing <br />layer rapidly increases from about 30 feet bgs at the location of boring HWA-13P on the east <br />side of the Port Gardner property to over 100 feet below ground at the location of boring HWA- <br />10P near the west side of the property. <br />As part of the design process, HWA considered different applicable foundation types, including <br />shallow foundations (i.e., spread or mat footings) and deep foundations (i.e., drilled shafts and <br />driven piles). These foundation types are discussed below. <br />Spread Footing Shallow Foundations: Shallow foundations are the most common foundation <br />type used in construction. Shallow foundations are often defined as foundations that have a depth <br />less than or equal to its length and width. Shallow foundations are often the most cost-effective <br />foundation type where shallow soils provide adequate bearing capacity to support structures with <br />a reasonable footing size. Shallow foundations can be constructed using standard excavation <br />equipment including back-hoes and hydraulic bucket excavators. Disadvantages of shallow <br />foundations include additional excavation, earth work, and surface disturbance required to <br />prepare the bearing layer to cast the spread footings. <br />In general, a shallow foundation is very susceptible to differential settlement. Additionally, <br />construction of spread footings could require excavation that may expose contaminants. Based