Summit Excavation Support

The Seattle Convention Center is a major economic stimulus for Seattle, Washington, drawing as many as 400,000 people to town each year for conferences and events. In 2008, the convention center’s owners embarked on an ambitious $2 billion expansion to double the facility’s available space and better meet the needs of the rapidly growing city.

GeoEngineers provided geotechnical and environmental services for this complex urban development project. The geotechnical team delivered shoring, earthwork and foundation recommendations for the 75-foot excavation pit and countless permanent and temporary structures for the 574,000-square-foot expansion.

The design and construction of the excavation support system required creative solutions due to extensive complex adjacencies created by the former bus transit station, adjacent cut-and-cover and bored transit tunnels, highway bridges, vacated city streets, project phasing, demolition of existing buildings/infrastructure, and heavy construction loading.

As geotechnical engineer of record, GeoEngineers performed sophisticated analysis to develop recommendations for excavation sequencing, shoring construction, and foundation design. GeoEngineers used finite element numerical modeling to predict the performance of sensitive structures under proposed shoring and excavation sequencing. This analysis was key to addressing concerns from multiple stakeholders who owned the sensitive structures.

Approach

Boren Avenue Bridge
The Boren Avenue Bridge’s proximity to the northeast edge of the project area caused several design challenges. Excavation would completely uncover the outermost battered pile supporting the bridge foundation, and the battered bridge foundation pile conflicted with the shoring and building walls. The project team (including KPFF and DBM Contractors) collaborated on a unique shoring design that allowed the building envelope to be installed according to the original plan, while maintaining temporary and permanent support of the Boren Avenue Bridge foundation.

Building on History
The Summit expansion is located on the former site of Convention Place, a large bus station built by the city in the 1980s. Although the transit facility was demolished in 2018 to make way for this project, a number of ground improvement structures remained, including permanent cylinder pile shoring walls surrounding the perimeter of the site. To save on construction and demolition costs, the project team developed unique excavation-support designs that allowed the existing cylinder piles to be reused instead of removed. By adding high-capacity tieback anchors and internal rakers, the team could include the cylinder piles in the excavation-support system, turning a potential difficulty into an efficiency.

Sequential Demolition of Existing Building and Shoring Installation
A turn-of-the-century building had to be demolished as part of the project, but its underground structure provided lateral support for the soil beneath Boren Avenue. After careful investigation, the team determined that the existing basement wall and floor slab could be used in the shoring design if additional lateral support could be provided. The team installed support tieback anchors through the basement wall on the upper two levels of the building and secured them with channel walers. The building could then be demolished while leaving the basement wall and slab on grade in place. This allowed the temporary shoring system to be installed behind the existing basement wall, which was then demolished in sequence with the mass excavation.

Modeling and Monitoring
The hybrid shoring design was complex and unusual, so the team took additional steps to validate the approach and monitor its performance. GeoEngineers performed numerical modeling to demonstrate that the sequence of excavation and installing an internally braced raker assembly would result in an acceptable amount of displacement of the bridge. The team also monitored real-time optical survey data to determine when to stop stressing each reaction anchor at the base of the raker assembly.

9th Avenue Bus Ramp
Excavation required the demolition of several ramps that carried King County bus traffic from a subterranean transit tunnel to the city streets above—and directly through the project site. The team found a way to maintain bus access throughout construction by building a shoring-supported earthen fill ramp with supports between building footing locations. At the upper end of this ramp, buses used a temporary 70-foot bridge to reach the street, allowing construction to continue beneath and maintaining bus traffic as required.

Pine Street Tunnels
The project site was immediately adjacent to several important bored and cut-and-cover tunnel transit facilities beneath Pine Street, and the team used a complex mixture of shoring strategies to protect them during excavation. The adjacent street was supported by an existing cylinder pile wall, and immediately behind it was a buried box tunnel, a bored tunnel, and driven pile supports for the adjacent Boren overpass.

DBM recruited Geoengineers to perform the complex finite-element modeling analysis for this wall. The design team used the results to analyze complex soil-structure interactions, calculate more accurate reaction loads for bracing and tieback design, address concerns from the owners of the adjacent structures, and provide accurate estimates of the shoring system’s expected performance.

Results

GeoEngineers’ geotechnical team developed creative excavation support designs that protected sensitive structures and allowed construction to proceed without significant impacts to adjacent roads and bridges. The completed Summit expansion will provide cultural, social, and economic value to Seattle for decades to come.