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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 27 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />on the soil conditions, shallow groundwater elevation, substantial and varying depth to the <br />bearing layer, presence of contaminants within subgrade soils, and the anticipated structural <br />loads associated with the proposed improvements, HWA believes that shallow foundations are <br />likely not a suitable foundation option for the Effluent Pump Station. However, it is a feasible <br />alternative for Lift Station 46. <br />Drilled Shaft Foundations: Drilled shaft foundations are commonly used to support heavily <br />loaded structures, structures that are to be founded over soft compressible soils, soils susceptible <br />to liquefaction, and in areas where it is desired to limit the areal extent of excavation for <br />construction of shallow foundations. The advantage of drilled shaft foundations relative to <br />shallow foundations is that construction of drilled shafts reduces the surface disturbance and <br />excavation requirements. Construction of drilled shafts is often more expensive than shallow <br />foundations; however, construction costs for smaller-diameter shafts that do not require <br />oscillator/rotator systems can be comparable to shallow foundations that require substantial <br />excavations. HWA understands that to limit excavation spoils due to presence of contaminated <br />soils, drilled shaft foundations are not a preferred foundation alternative for this project. <br />Driven Pile Foundations: Driven pile foundations are a deep foundation technique that consists <br />of driving steel or precast concrete piles to depths far below the ground surface. Piles are <br />typically installed in groups using an impact pile driving hammer. Driven pile foundations are <br />often used where poor soil layers extend deep below the ground surface and excavations are <br />impractical. <br />Based on HWA’s experience with similar soil conditions, the potential for presence of <br />petroleum-hydrocarbon-contamination within the near-surface soils, wide variation in depth to <br />bearing layer, potential for differential settlement, and the anticipated variation in structural loads <br />of the proposed improvements, HWA recommends that deep foundations for the project include <br />driven open-ended steel pipe piles, H-piles, or prestressed concrete piles. HWA anticipates <br />difficult driving conditions that cause damage to driven piles in localized areas based on <br />subsurface wood deposits and old timber structures that may have been buried during filling of <br />the site. <br />Drilled Micropile Foundations: HWA understands that some proposed improvement areas are <br />located where overhead power lines require a low-clearance installation method. Micropiles are a <br />small-diameter (less than 12-inch) drilled and grouted replacement pile system that is typically <br />reinforced. The micropiles penetrate suitable bearing strata and derive bearing capacity from skin <br />friction between the pile grout and surrounding soil. Installation of micropiles generally involves <br />drilling a borehole through the soil to a specified depth, placing reinforcement, and grouting the <br />hole.