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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 24 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />• Conducting jet grouting in close vicinity of nearby slopes could be difficult as the wet <br />soil mix column performs as a semi-liquid zero-shear-strength material immediately after <br />installation, which increases the potential for slope failure along the wet column. <br />• Typically, a more expensive method than deep soil mixing on a per-unit-volume basis. <br />• Spoils generated may range from 80 to 100% of the treated volume. <br />• Treatment of upper wood layers encountered in HWA’s explorations may require <br />installation of steel casing and pre-drilling before jet grout installation. <br />Jet grouting’s advantages include increased access capabilities (i.e., ability of equipment to <br />function in congested areas), precision in targeting the liquifiable layer, ability to accommodate <br />buried structures/debris, and lack of vibrations during installation (i.e., minimal disturbance to <br />adjacent structures). In HWA’s opinion, jet grouting is a feasible mitigation option for this <br />project. <br />4.4.2 Ground Improvement Design and Construction <br />In HWA’s opinion, the installation of soil cement columns by either deep soil mixing or jet <br />grouting techniques are feasible options to mitigate the impact of slope instability and lateral <br />loading during a large seismic event. Both techniques will accomplish controlled <br />densification/solidification to reduce the potential for lateral ground movement towards the slope <br />caused by liquefaction. <br />HWA has conducted stability analyses to evaluate the extent of the ground improvement <br />installation that would be required to satisfactorily mitigate seismic and post-liquefaction- <br />induced slope instability events. HWA’s preliminary analyses suggest that jet grout or deep soil <br />mixed columns would need to be installed in an area extending at least 40 feet west of the <br />Effluent Pump Station building. The soil cement columns should penetrate into dense non- <br />liquefiable soils (approximate elevation -70 to -80 feet, NAVD 88 Datum). For planning and <br />estimating purposes the soil cement column installation should achieve a minimum of 40 percent <br />area replacement ratio for the in-situ soil-cement grout mix with the minimum unconfined <br />compressive strength of 200 pounds per square inch (psi). <br />The proposed zone of improvement, area replacement ratio, and in-situ soil-cement unconfined <br />compressive strength shall be used as the minimum design guidelines for planning purposes. The <br />design zone of improvement, soil cement column installation diameter and layout area, <br />replacement ratio, and grout mix for the proposed ground improvements shall be determined by <br />the ground improvement contractor and be submitted to the geotechnical engineer for review. <br />The ground improvement shall achieve minimum factors of safety of 1.5 for global stability and