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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 25 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />shearing through the deep mixed zone; 1.3 for sliding, bearing, soil extrusion, and overturning of <br />the ground improvement block; and 1.1 against liquefaction triggered slope instability. <br />It is important to note that the soil cement column design is not required to fully mitigate <br />liquefaction across the site, but it needs to mitigate the lateral loads on the Effluent Pump Station <br />building and its connecting utilities. The foundation design for the pump station is required to <br />account for liquefaction-induced settlement and down drag loading despite utilizing ground <br />improvement. The contractor should provide all design calculations and working drawings to the <br />engineer of record for review. <br />Verification testing is required to confirm that the specified level of solidification (and therefore <br />liquefaction mitigation) has been achieved. Verification testing shall consist of cement grout <br />compressive tests, soil‐cement compressive tests, and cores advanced at locations of soil cement <br />test columns for a minimum of 3 percent of production columns to the full depth of improvement <br />to verify column diameter and 28-day compressive strength. The number of verification <br />compression tests can be selected based on the final soil improvement design. For areas not <br />achieving the specified level of solidification, additional grouting and subsequent verification <br />testing will need to be performed. Verification testing should be monitored, reviewed, and <br />approved by the representatives of HWA. <br />Deep soil mixing can generate spoils of approximately 10 to 40% of the treated volume, and jet <br />grouting can generate spoils of approximately 80 to 100% of the treated volume. These spoils <br />will have a high pH (typically ranging from 11 to 14) such that the spoils will need to be handled <br />and disposed of as Subtitle D or Subtitle C waste, depending on the pH. The contractor shall <br />dispose of the spoils as specified in Section 2-03.3(7)C of the Washington State Department of <br />Transportation (WSDOT) Standard Specifications (WSDOT, 2024a). Process water and surface <br />storm water from soil-cement column work should be managed in accordance with Section 8- <br />01.3(1)C WSDOT Standard Specifications. <br />The Special Provisions for Ground Improvement for the project should include these minimum <br />criteria to meet Project goals: <br />• Experience requirements for specialty contractors. Given the complexities of this type of <br />construction and the site, we recommend at least 5 years of experience and five projects <br />of similar size and depth. <br />• Real-time monitoring with automated data recording equipment to confirm that soil- <br />cement columns achieve the required dimensions and achieve a uniform mixture of soil <br />and cement.