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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 2 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />This report includes recommendations concerning several proposed improvements, including <br />Area 60 Effluent Pump Station, Area 40 Stormwater Treatment Structure, Area 40 Stormwater <br />Diversion Structure, Lift Station 46, Engine Generator and Fuel Storage Tank, Flushing <br />Drainage Structure, and connecting piping throughout the site. The approximate locations and <br />extent of these improvements are presented on the Site and Exploration Plan, Figure 2. <br />1.3 SEISMIC DESIGN APPROACH <br />As part of the seismic resiliency assessment initially conducted by HDR, conceptual methods to <br />mitigate the impacts of the design seismic event were evaluated. This study found that ground <br />improvements and/or pile-support retrofits were impractical, had significant limitations, and high <br />costs. For these reasons, HDR concluded, and the City concurred, that seismic retrofits of the <br />aeration basins and secondary clarifiers would not be implemented. <br />Evaluations and engineering analyses conducted by HWA have provided additional validation to <br />support the assessment that seismic retrofit of the existing structures is impractical and cost <br />prohibitive. This assessment has also been outlined by Technical Memorandum 31 (TM 31), <br />dated June 20, 2023, by BC. HWA understands that the existing structures and proposed <br />modifications/additions to the existing structures will not be designed to meet current IBC 2021 <br />and ASCE 7-16 seismic codes. However, all entirely new structures (including the Effluent <br />Pump Station, Lift Station 46, and the OMES generator pad) are being designed to meet seismic <br />loads as defined in IBC 2021 and ASCE 7-16. <br />2.0 FIELD INVESTIGATION AND LABORATORY TESTING <br />2.1 GEOTECHNICAL SUBSURFACE EXPLORATIONS <br />HWA’s geotechnical exploration program included advancing eighteen borings, designated <br />HWA-01 through HWA-16P, HWA-17P-24/HWA-17P-24A, and HWA-18P-24, ranging in <br />depth from about 41.5 to 106.5 feet below ground surface (bgs). Additionally, two borings were <br />drilled in the eastern side of the site (HWA-19P-24 and HWA-20P-24) as part of the West <br />Marine View Drive Storm and Combined Sewer (WMVD) project. The borings were advanced <br />using a combination of hollow-stem auger and mud rotary methods. The field explorations were <br />conducted from April 14 through 26, 2022, by Gregory Drilling, using a CME-55 track rig, from <br />October 25 through 27, 2023, by Holocene Drilling, using a D-50 turbo rig, and from August 5 <br />through 7, 2024, by Holocene Drilling, using a D-70 turbo rig. Groundwater monitoring wells <br />were installed in borings HWA-10P, HWA-13P, HWA-14P, HWA-16P, HWA-17P-24/HWA- <br />17P-24A, HWA-18P-24, HWA-19P-24 and HWA-20P-24. Monitoring wells HWA-14P and <br />HWA-16P were installed a few feet offset of the borehole location to allow for well <br />development. HWA-17P-24/HWA-17P-24A, HWA-18P-24, HWA-19P-24 and HWA-20P-24