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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 40 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />Resistance to lateral forces on non-pile-supported structures will be provided by a combination <br />of base sliding friction and passive earth pressure against the buried portions of the structure and <br />its foundations. Base sliding friction should be evaluated using a coefficient of friction of 0.5. <br />Passive earth pressures in permanent backfill soil should be estimated using an equivalent fluid <br />pressure of 240 pcf. Where concrete slabs are designed as beams on an elastic foundation, a <br />modulus of subgrade reaction of 125 pci should be used. <br />4.7.3 Hydrostatic Uplift Pressure <br />To resist buoyant/uplift forces, HWA recommends that the mat extend outside the perimeter of <br />the structure’s walls, such that the weight of soil bearing on the extension will resist uplift. <br />Recommendations for estimating uplift resistance are presented in Figure 6A. As indicated on <br />Figure 6A, the maximum design groundwater elevation should be assumed at the ground surface <br />(i.e., backfill material could be saturated by infiltration during periods of heavy rainfall). <br />4.7.4 Static Settlements <br />Excavations for proposed structures at the site are expected to extend to depths of approximately <br />3 to 32 feet bgs. Soft to loose, moisture-sensitive soils are likely to be exposed at the base of <br />these excavations and relaxation (heave) of the soils should be anticipated due to a reduction of <br />vertical stresses (i.e., removal of overlying in-situ soils). Placement of fill soils (structure <br />excavation zone backfill) may result in a slight net load increase on underlying soft soils due to <br />the backfill soils being heavier than the soils being replaced. <br />Structures founded within, or above, the fill soils should be designed to accommodate 2 to 4 <br />inches of long-term settlement (due to recompression and secondary consolidation of organic <br />silts, peat soils, and wood material). <br />Additionally, increases in load could cause primary consolidation settlement along the utility <br />alignments if conventional granular trench backfill is used. These settlements are expected to <br />vary across the site. To limit the potential for additional primary consolidation along the utility <br />alignments, HWA recommends that light weight uniformly graded sand be used as trench <br />backfill. More details about lightweight backfill are presented in Section 4.9.3 of this report. <br />4.7.5 Shallow Foundation Overexcavation/Subgrade Preparation <br />HWA understands that Lift Station 46 and other structures throughout the site (e.g., vaults, <br />conveyance piping, manholes, etc.) will be founded on shallow foundations at variable depths <br />between approximately 3 to 32 feet bgs. These structures will likely be underlain by historic fill <br />soils (including wood debris) and bay deposits, which have the potential for biodegradation