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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 12 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />• VOCs (including carbon disulfide and/or p-isopropyltoluene) were detected in both <br />samples at concentrations below MTCA CULs. No other VOCs were detected above <br />laboratory reporting limits. <br />• Of the RCRA 8 Metals, barium was detected in sample HWA-16P at a concentration <br />below MTCA CUL. No other metals were detected above laboratory reporting limits. <br />• Of the SVOC analyses, which includes cPAHs: <br />o cPAHs were not detected in any of the samples above laboratory reporting limits. <br />o The SVOCs naphthalene, acenaphthene, (3+4)-methylphenol (m,p-cresol), and <br />phenol were detected in sample HWA-14P at concentrations well below CULs. <br />No other SVOCs were detected above laboratory reporting limits in either sample. <br />• PCBs were not detected in either of the samples above laboratory reporting limits. <br />4.0 CONCLUSIONS AND RECOMMENDATIONS <br />4.1 GENERAL <br />The results of the explorations indicate that the existing structures of the PGSF are underlain by <br />loose to medium dense sand and organic soils, to a depth of about 30 to 100 feet below ground <br />surface, and glacially consolidated deposits at depth. <br />Following a large earthquake, HWA anticipates that some of the soils below the water table to an <br />elevation of about -70 feet (NAVD 88) will experience loss of shear strength due to soil <br />liquefaction. This could result in liquefaction-induced settlement and slope instability due to <br />lateral spreading and/or flow failure. HWA’s analysis predicts liquefaction-induced settlements <br />of approximately 4 to 24 inches in the soils across the site. If not properly designed, liquefaction <br />could also result in potential uplift of the buried structures, including the Effluent Pump Station, <br />Lift Station 46, and other buried structures extending below the groundwater level throughout the <br />site. <br />Due to the presence of liquefiable soils along the entire project site and anticipated liquefaction- <br />induced settlement, HWA recommends that the exterior walls of all buried structures supported <br />on deep foundations to be covered with 10 mil polyethylene plastic sheeting prior to backfilling <br />to avoid additional down drag loading of the foundation elements as a result of liquefaction- <br />induced settlement. Alternatively, the proposed improvements, including buried structures and <br />foundation elements, can be designed to account for the down drag loads.