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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 41 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />settlements (i.e., biodegradation of wood debris) and static settlements. Where woody debris is <br />encountered at the invert elevations of the proposed improvements, HWA recommends that these <br />soils be overexcavated a minimum of 2 feet below structure bottom-of-footing elevations and <br />that foundations should bear on a 24-inch-thick leveling pad of crushed surfacing base course <br />(CSBC). If suitable subgrade soils are encountered within excavated bottom-of-footing <br />elevations, then the leveling pad thickness may be reduced to 6 inches. <br />A 2-foot-thick layer of CSBC is recommended below Lift Station 46. CSBC should meet the <br />requirements in Section 9-03.9(3) of the 2024 WSDOT Standard Specifications, except that it <br />should be a quarry product and consist of crushed rock, not crushed gravel. CSBC should be <br />compacted per the recommendation of Section 4.13.3 prior to placement of structures. The <br />leveling pad should extend a minimum of 2 feet beyond the perimeter of the mat/footing <br />foundation. <br />4.8 BELOW-GRADE STRUCTURES <br />HWA understands the proposed improvements include below-grade structures such as the <br />Effluent Pump Station, Lift Station 46, vaults, conveyance piping, and manholes. The Effluent <br />Pump Station and other proposed structures (designated manholes, vaults, etc.) will be supported <br />on deep foundations; Lift Station 46 will be founded on shallow foundations at a depth of about <br />32 feet bgs (elevation -15 feet). Other structures throughout the site (e.g., vaults, conveyance <br />piping, manholes, etc.) will be founded on shallow foundations at variable depths between <br />approximately 3 to 20 feet bgs. Due to the potential for biodegradation settlements and <br />anticipated liquefaction-induced settlement, HWA recommends that the exterior walls of all <br />buried structures supported on shallow or deep foundations be covered with a minimum of two <br />layers of 10 mil polyethylene plastic sheeting prior to backfilling to minimize additional down <br />drag loading on the foundation elements. <br />Where buoyancy is a concern and/or extended base slabs are used to resist buoyancy, the <br />polyethylene plastic sheeting should be placed between the backfill and the native soils. The <br />contractor should cover the walls of temporary excavations with polyethylene plastic sheeting <br />prior to backfilling the excavation. <br />The lifespan of 10-mil polyethylene plastic sheeting when buried is often in the range of 10 to 20 <br />years when used as a vapor barrier. However, the polyethylene plastic sheeting is intended to act <br />as a bond breaker, and although some degradation of the polyethylene plastic sheeting will occur, <br />its capability to act as a bond breaker between soils undergoing settlement and the buried <br />structures is not diminished. <br />Recommendations for design and construction of below-grade structures are provided in the <br />following sections.