Cost Effective Excavation Support in Seattle-Paper

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The project involved constructing five underground parking levels for two 40-story residential towers in downtown Seattle, requiring an excavation up to 70 feet deep beside utilities, streets, and an existing seven-story building.<br /><br />A composite soil nail shoring system was selected as a cost-effective alternative to the originally permitted post-tensioned anchored system. Closely spaced steel vertical elements supported the excavation and allowed the upper soil-nail row to be installed below utility conflicts. Soil nails and reinforced shotcrete formed the main support system; the upper nails beside the neighboring building were post-tensioned to reduce movement. Removable pile tops and timber lagging were used in the upper four to five feet to meet City requirements for right-of-way elements.<br /><br />Construction ran from August to November 2018 and was completed in 12 weeks. Soil-nail testing identified several failures caused by bond loss between the bar and grout; additional nails were installed in affected areas. Nighttime excavation and hauling helped maintain the schedule.<br /><br />Four inclinometers and twice-weekly optical surveys monitored wall and nearby building movements. The maximum measured horizontal displacement at the top of the shoring was 0.9 inches, within Seattle’s one-inch limit; inclinometer readings likewise indicated limited movement.<br /><br />The project demonstrated that, in dense glacial soils, a composite soil nail system could provide adequate support with deflections meeting city requirements while costing less than the original anchored design. Soil nailing also avoided the need for blockouts in permanent walls to release anchor tendons.

Keywords

underground parking

Seattle excavation

soil nail shoring

composite shoring system

retaining wall

soil nails

shotcrete

inclinometers

excavation monitoring

utility conflict