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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 47 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />The contractor should assume that most of the excavations required for the construction of the <br />proposed improvements will require temporary shoring due to poor soil conditions and a high <br />groundwater table. Sheet piles typically extend below the bottom of the excavation at least 1.0 <br />times the depth of the excavation (e.g., a 20-foot-deep excavation typically requires a 40-foot- <br />long sheet pile) and should be deeper as necessary to lengthen the hydraulic flow path and <br />achieve a stable bottom. <br />As mentioned in Section 4.10, HWA expects that obstructions may be encountered during <br />installation of temporary shoring. Pre-drilling or digging to facilitate driving of sheet piles may <br />be necessary. The contract should contain provisions for addressing obstructions encountered <br />during sheet pile driving. Excavations can likely be accomplished with conventional excavating <br />equipment such as backhoes and excavators. <br />The contractor should be responsible for the temporary shoring system design and should be <br />required to present a submittal of the design details for review by the Engineer prior to <br />construction. Design, successful installation, and removal/partial removal of the temporary <br />shoring system should be the responsibility of the contractor. Additional temporary excavation <br />and shoring details are provided in Section 4.13.1 of this report. <br />4.12 DEWATERING <br />Locally, dewatering with wells or well points may allow unsupported excavations below the <br />water table to be to be safely sloped or shored; however, large scale or prolonged dewatering is <br />likely to cause settlement of nearby existing structures or utilities. Settlement monitoring for <br />structures not already supported on piles should be included in the project specifications. <br />Where feasible, the use of water-tight shoring may reduce the extent of dewatering required. <br />Where dewatering will be completed adjacent to settlement-sensitive structures, pre-construction <br />condition surveys and settlement monitoring of those structures should be performed. Where soil <br />conditions permit, sheet pile groundwater cutoff walls may be used to limit the dewatering <br />impacts to nearby structures. <br />For dewatering of excavations, once the groundwater level is lowered below the base of the <br />excavation, the system should be maintained until the structure is backfilled. Failure to maintain <br />the dewatering system until backfilling is substantially complete may result in ground loss and <br />settlement of the proposed structure. The contractor should provide appropriate backup systems <br />to prevent a failure of the dewatering system. The contractor should assess the potential need to <br />mitigate hydrostatic head pressure from artesian groundwater conditions to achieve a stable <br />excavation bottom.