The Washington State Coliseum was built in the 1960s for the Seattle World’s Fair. In 2019, construction began on a major redevelopment to expand the arena. The redeveloped building — the Seattle Climate Pledge Arena — retains the historic roof but continues 30 feet (9 m) below the original arena floor to expand capacity.
Mortenson Construction commissioned Delve Underground and J.W. Fowler to design and build the Seattle Climate Pledge Arena Tunnel and associated south decline and north temporary shaft. This truck tunnel runs under an existing unreinforced masonry structure, originally constructed in the early 1920s and designated a City of Seattle Historic Landmark. The tunnel is approximately 180 feet long (55 m), with a finished width of 25 feet (7.6 m) and a vaulted roof 20 feet (6 m) high.
The tunnel was constructed using sequential excavation methods and supported using a pipe arch canopy, steel sets, and steel fiber shotcrete lagging. The south secant pile headwall is within 6 feet (1.8 m) of an existing unreinforced masonry building, and the tunnel passes within 3 feet (1 m) of the building’s foundations. The ground between the pipe canopy and the existing footings was improved before excavation using permeation grouting.
Delve Underground designed temporary support for the new tunnel and excavation support for the associated depressed cut decline. Delve also provided on-site design support services during support of excavation construction. This included reviewing building settlement and tunnel convergence data, observing soil and groundwater conditions, and on-site collaboration.
Because of the fixed opening date of the arena, the design phase was performed under an accelerated schedule. The tunnel and portal construction on this Delve Underground / J.W. Fowler effort was completed in 14 months. The design-build team identified a risk in using a microtunnel boring machine (MTBM) for the pipe arch canopy. Cobbles and boulders, expected in the subsurface profile, could be challenging for the small MTBM to ingest, causing schedule disruption or a sinkhole.
Therefore, Delve modified the design to allow auger boring to install the pipe-arch canopy. This method allowed face access that later was used to remove cobbles and boulders. The team also implemented a robust instrumentation program to monitor the unreinforced masonry building and promptly identified and addressed structural issues. The planned mitigations helped to gain approval for the tunnel below the Seattle Historic Landmark building.