Photo courtesy of GeoEngineers GeoEngineers developed an unusual two-wall shoring approach for 1918 Eighth Avenue’s tight sight in downtown Seattle.

It was a classic “between-a-rock-and-a-hard-place” challenge: A 34-story office tower with below-grade parking was to be constructed on a site hemmed in on two sides by major downtown streets, and on the other two sides by a high-rise condominium and an office building.

In order to buttress the construction of ground-level walls after the site had been excavated almost 75 feet down, there was a need for supporting tieback anchors. Traditionally, buildings under construction are supported by tieback anchors that extend below adjacent buildings. But on this site the anchors would exceed the 65-foot right-of-way or be impossible due to the proximity of the next-door condominium building. It was a challenge that threatened cost overruns and delays.

GeoEngineers, in response, developed an unusual two-wall shoring approach for the building’s east side that enabled the excavation and construction to proceed. The firm also used nontraditional shoring methods for the other three sides of the site, creating a “truncated no-load zone” that involved threading tiebacks through deep foundations supporting the commercial building next door, and shortening the tiebacks to prevent them from crossing over one another underground.

It was a solution that will prove helpful for projects in other tight urban environments, advancing the design of deep excavations in the Northwest.