Deep Excavation Support: Innovative Shoring Design in an Urban Environment-Paper

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Tower 12, a high-rise development in downtown Seattle, required a deep excavation beside One Pacific Tower, an existing condominium with underground parking. An 18-foot-wide alley, the neighboring building’s foundations, and utility vaults constrained the available space for shoring. The west excavation reached a maximum depth of about 51 feet.<br /><br />To address these constraints, the design combined two temporary support systems. A soil nail wall supported the upper approximately 30 feet, using short, closely spaced 16-foot nails limited by the alley width. The lower portion was supported by soldier piles and steeply inclined, post-tensioned ground anchors, with capacities of up to about 200,000 pounds. Additional steel beams between piles improved face stability and stiffness. The hybrid system avoided internal bracing, reducing construction cost and schedule.<br /><br />Subsurface conditions included fill over stiff to hard glacial clay and silt, with perched groundwater anticipated above the regional water table. Construction took place from April to June 2015. Anchor testing verified the assumed bond strengths, and production anchors were proof-tested.<br /><br />Wall performance was monitored using an inclinometer and optical surveys of the excavation, nearby roads, and buildings. Maximum measured lateral movement was approximately 0.8 inch—below the City’s 1-inch limit and consistent with design predictions. The results demonstrate that combining soil nailing with anchored soldier piles can provide effective, low-movement excavation support where urban site constraints prevent use of a conventional system.

Keywords

Tower 12

Seattle excavation

deep excavation

soil nail wall

soldier piles

ground anchors

temporary shoring

hybrid support system

lateral movement monitoring

urban construction constraints