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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 44 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />4.9.3 Lightweight Backfill for Utilities <br />The existing historic fill soils that are to be excavated along the proposed utility alignments <br />possess an in-place unit weight of approximately 100 to 115 pounds pcf. If the utility trenches <br />were backfilled with conventional trench backfill (weighing approximately 135 to 140 pcf) <br />consolidation settlement of the underlying soils would occur. HWA recommends using a utility <br />trench backfill weighing between 100 and 110 pcf. Granular materials that possess wet unit <br />weights within this range generally consist of clean sands with poor gradations. The lightweight <br />utility trench backfill should conform to the gradation, or similar gradation, provided below. <br /> <br />Sieve Size Percent Passing <br />1/2 – inch 100 <br />U.S. No. 4 100 <br />U.S. No. 10 85-100 <br />U.S. No. 40 30-60 <br />U.S. No. 60 2-20 <br />U.S. No. 100 0-10 <br />U.S. No. 200 0-3 <br />All percentages are by weight. <br />A similar gradation to the recommended above consists of Fine Aggregate Natural Sand as <br />specified in Section 9-03.2(1) of the 2024 WSDOT Standard Specifications. HWA recommends <br />that the contractor submit their proposed trench backfill source prior to import to allow the City <br />and design team to obtain a representative sample for testing to confirm that the material has a <br />unit weight within the recommended range. <br />Where lightweight trench backfill is used in roadways or parking lots, a separator geotextile <br />should be placed between the lightweight sand and the underlying base course, as shown on <br />Figure 7, to prevent sand migration into the voids of the base course. <br />The use of lightweight trench backfill will reduce potential consolidation settlement but will not <br />eliminate future settlement. The remaining historic fill soils may continue to undergo long-term <br />settlement (biodegradation) over the design life of the utilities. The utilities should be pile- <br />supported if the utilities cannot tolerate 2 to 4 inches of long-term settlement under static loading <br />conditions. <br />4.9.4 Trench Backfill Placement and Compaction <br />Proper preparation, placement, and compaction of the trench backfill is important to limit future <br />settlement of the ground surface along the trench. Failure to achieve proper compaction could <br />result in surface settlements on the order of several inches, resulting in distress to pavements,