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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 46 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />design their temporary shoring systems to account for potential obstructions in, and caving of the <br />native soils. <br />4.11TEMPORARY SHORED EXCAVATIONS <br />Trench excavation should conform to the requirements of Section 7-08.3(1)A of the 2024 <br />WSDOT Standard Specifications (WSDOT, 2024). Actual trench configurations and <br />maintenance of safe working conditions, including temporary excavation stability, should be the <br />responsibility of the contractor. All applicable local, state, and federal safety codes should be <br />followed. Temporary excavations in excess of 4 feet should either be shored or sloped in <br />accordance with Safety Standards for Construction Work, Part N, located in Chapter 296-155 of <br />the Washington Administrative Code (WAC). In the absence of groundwater seepage, the soils <br />anticipated within the trench zone along the project alignment generally classify as Type C soil <br />per Chapter 296-155 of WAC. The prescriptive maximum allowable excavation slope for Type C <br />soils is 1½H:1V (horizontal to vertical). If groundwater seepage is present, flatter slopes, <br />temporary shoring, and/or dewatering may be required. <br />Trench boxes are a worker safety device but do provide lateral support of the adjacent soil. <br />Where a trench box is used to support excavations, one or both sides of the trench may cave <br />against the box, especially if the soil is not properly dewatered. Caving may extend out on either <br />side of the trench for a distance approximately equal to the depth of the excavation. The potential <br />for caving can be reduced by routing stormwater away from the excavation and limiting <br />vehicular traffic or vibrations next to the trench. When the trench box is moved, precautions <br />should be taken to minimize disturbance of the pipe, underlying bedding materials, and <br />surrounding soil and/or structures. Trench boxes should meet the requirements in Safety <br />Standards for Construction Work, Part N, located in Chapter 296-155 of the WAC. <br />Additional bracing or sheeting may be required where the edge of the trench will be closer than <br />1.5 times the trench depth to settlement-sensitive utilities or structures. If bracing is needed to <br />support the trench walls, the temporary bracing system should be designed by a structural <br />engineer licensed in the state of Washington. <br />In general, excavations which extend more than about 4 feet below the groundwater level and/or <br />which need to stay open for more than a few days should be dewatered and supported. When <br />trench boxes or sloped excavations are not feasible, dewatering and slide rail shoring or the use <br />of steel sheet pile shoring, combined with dewatering wells installed inside the sheet pile <br />enclosure could be used. Recommended design earth pressures for cantilever and braced <br />temporary shoring are shown on Figures 8A and 8B, respectively. The shoring designs should <br />include hydrostatic pressure, as well as surcharge loads due to construction equipment, <br />stockpiled material, and vehicle traffic.