1918 Eighth Avenue Shoring Design

The 1918 Eighth Avenue building in downtown Seattle is a 34-story mixed-use office/retail tower with below-grade parking. The site is bounded on two sides by major downtown streets and on two sides by condominium and office structures. As a result, excavation—which ranged from 63 to 73 feet deep—had complex support needs. GeoEngineers worked with Ground Support to design a unique shoring system that was cost-effective and kept the project on schedule.

Approach

The greatest design challenge the project team faced was along the east wall of the structure. Easements could not be obtained from the adjoining condominium’s owner because the building’s foundation and shoring systems could not reasonably accommodate tieback anchors from the 1918 Eighth Avenue project. GeoEngineers and Ground Support developed an innovative two-phase approach that enabled the project to proceed.

• Designed a support system that included inner and outer shoring walls with sufficient soil between them to anchor tiebacks and allow for excavation and construction of the building core
• Removed inner wall and excavated the soil between the two walls during the second phase
• Used Peri Formwork—typically used in vertical applications—to provide lateral support between the newly constructed building core and the outer shoring wall
• The other walls were affected by adjacent buildings and narrow rights of way. The project team modeled and implemented a “truncated no-load zone” for the shoring and tieback anchors. This kept the anchors within the right of way and ensured the anchors would not cross each other.

Results

The east wall shoring system demonstrated that this two-wall approach is a viable alternative when adjacent structures cannot support tieback anchors, or if there are other obstacles to traditional shoring methods.

• Measured deflections on all the walls were less than the City of Seattle one-inch tolerance
• The shoring design was more cost effective than other alternatives and kept the project on schedule
• This innovative shoring approach is transferable to other sites, advancing the state of the art for deep excavations in the Northwest