Working With Constraints
Portland International Airport decided to connect a new parking garage with their existing pedestrian tunnel. A 21-foot deep excavation, shoe-horned between two existing parking structures and the tunnel, was required. The existing structures’ pile foundations, shallow groundwater, limited workspace, and the plethora of buried utilities and obstructions severely limited the choice of excavation support systems. In addition, the use of internal braces was ruled out because of the constraints they would impose on the already crowded workspace.
Everything about this project was challenging, including getting an accurate map of where the existing infrastructure was located. The design management skills that we’ve developed over the past decade were invaluable as we coached the design/build team through developing a credible base drawing for our design.
A Longtime Collaboration
We worked closely with the structural engineer and ground improvement contractor—our longtime collaborator Pacific Foundation—to design a jet grout shoring system that would keep the excavation accessible, stable, and dry. The strong relationship was crucial when the general contractor found several differences between the record drawings we had been using and where things actually were buried.
Shiver Me Timbers
It really did seem like we were digging for buried treasure as we opened up the site for construction. It turns out that the tunnel had been built with sheetpiles that had been left in the ground right where we planned to build. We needed to redesign the shoring layout to work around the steel sheets, maintain a watertight excavation, and allow demolition once the hole had been excavated. It wasn’t a huge change in the shoring, but again we had to coach the team in how to survey and share the sheetpile location so that we had an accurate base drawing for planning our work.
Flexibility Leads to Success
The design continued to evolve through construction as new underground obstructions were discovered. A layer of drain rock caused problems with the jet grouting that required us to adapt the layout yet again. In the end, we were able to adapt our design to maintain a safe, dry excavation with minimal impact to the construction cost and schedule.