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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 45 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />utilities, and other structures along the trench. Trench backfills should be uniformly moisture- <br />conditioned to within about 3 percent of optimum moisture content prior to placement in the <br />trench. <br />As specified by the City of Everett Standard Specification (COE, 2023a), trench backfill shall be <br />spread in lifts and compacted by mechanical tampers of the impact type approved by the City <br />Engineer. The backfill material shall be placed in lifts not exceeding 12 inches in loose <br />thickness. Thinner loose lifts may be necessary to achieve compaction where handheld <br />compaction equipment such as jumping jacks or plate compactors are used. The contractor <br />should develop compaction methods that consistently produce adequate compaction levels. <br />During placement of the initial lifts, the trench backfill material should not be bulldozed into the <br />trench or dropped directly on the pipe. Furthermore, heavy vibratory equipment should not be <br />permitted to operate directly over the pipe until a minimum of 2 feet of backfill has been placed <br />over the pipe bedding and compacted to an in-place density of at least 92 percent of the <br />maximum dry density as determined using ASTM D 1557. <br />Each lift shall be compacted to the density specified below: <br />1) Improved areas such as streets and sidewalks shall be compacted to 92% of maximum <br />dry density to within 3 feet of subgrade. The last 3 feet shall be compacted to 95% of <br />maximum dry density. <br />2) Unimproved areas or landscape areas shall be compacted to 90% of maximum dry <br />density. <br />A significant cause of trench settlement is inadequate shoring practices and inadequate <br />compaction during shoring removal and backfilling. Special care must be taken to obtain good <br />compaction up to the edges of the excavation as the shoring is removed. Moreover, attention <br />must be paid to ensure good compaction around manholes. <br />Compaction tests shall be as required by the City Engineer, but in no case less than two tests <br />every 200 feet of trench (one at subgrade and one at 50% of trench depth) (COE, 2023a). <br />4.10EXCAVATIONS AND CAVING <br />HWA anticipates that obstructions such as large woody debris, logs, or construction debris may <br />be encountered during pipeline trenching and structure excavations. The project specifications <br />and cost estimates should account for potential removal of obstructions during utility installation <br />and deep excavations associated with the proposed improvements. <br />HWA does not expect the fill and wood materials at the site to be self-supporting; caving of <br />sidewalls during excavation activities should be anticipated. Prospective contractors should