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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 43 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />in the southwestern portion of the property. HWA understands that the City has decided to not <br />mitigate liquefaction and lateral spreading of select utilities as part of this project (BC, 2023b), <br />and that reconstruction of some utilities after a design seismic event may be required. However, <br />the proposed ground improvement buttress will provide some mitigation against seismically <br />induced lateral spreading for utilities and structures located east of the improvement, and HWA <br />understands that some utilities and associated structures will be supported on piles to mitigate <br />static and liquefaction-induced settlements. <br />For static design, flexible utility connections, capable of tolerating 2 to 4 inches of long-term <br />differential settlements, should be used where proposed utilities connect to existing lines. <br />HWA also recommends using flexible connections where proposed utilities transition from soil- <br />supported to structure-supported (i.e., seismic response of the soil will be different from the <br />structure and differential seismic displacements may concentrate stresses at these transitions and <br />damage structures). The magnitude of differential movement that the flexible connections will <br />need to tolerate will depend on the seismic response of the structures. <br />4.9.1 Utility Subgrade Preparation <br />The utilities will be underlain by historic fill soils (including woody debris) and bay deposits. <br />These soils are highly variable and are not expected to provide adequate support without <br />stabilization. To stabilize subgrade soils along the alignment, HWA recommends that these soils <br />be overexcavated a minimum of 2 feet below utility pipe invert elevations and replaced with <br />permeable ballast. If suitable soils are encountered at the invert elevation of the proposed <br />structure, overexcavation is not required. The permeable ballast should be a quarry product and <br />not a gravel pit product. Once the permeable ballast is compacted, a geotextile separator fabric <br />meeting the requirements of Table 3 in Section 9-33.2 of the 2024 WSDOT Standard <br />Specifications should be placed above the permeable ballast to prevent pipe bedding migration <br />into the voids of the permeable ballast, as shown on Figure 7. <br />4.9.2 Utility Pipe Bedding <br />Pipe bedding material should consist of CSBC as specified in Section 9-03.9(3) of the 2024 <br />WSDOT Standard Specifications. Pipe bedding should provide a firm uniform cradle for support <br />of the pipes. A minimum 6-inch thickness of bedding material beneath the pipe should be <br />provided. Prior to installation of the pipe, the pipe bedding should be shaped to fit the lower part <br />of the pipe exterior with reasonable closeness to provide uniform support along the pipe. Pipe <br />bedding material should be placed in layers and tamped around the pipes using hand equipment <br />to obtain complete contact. To protect the pipe, bedding material should extend at least 12 inches <br />above the top of the pipe.