Tunnel’s lower deck arriving in Seattle piece by piece

This week precast panels of the SR 99 tunnel’s lower deck are arriving in Seattle. This marks the beginning of an important stage of tunnel construction.

The upper deck of the tunnel’s double-deck highway has been under construction for some time; it was more than half complete by the time the tunneling machine Bertha finished mining in April. The contractor, Seattle Tunnel Partners, is building the upper deck and most of the tunnel’s other interior structures using the “cast in place” method. Our timelapse video shows this method in action as crews assemble formwork, place rebar, and pour concrete to create the tunnel’s walls and upper deck.

The tunnel’s lower deck will be built in a much different manner. While our tunnel progress tracker shows work has not yet begun on the lower deck, that’s a little misleading. All of the roadway panel construction is actually finished. Concrete Technology Corporation in Tacoma, Wash. has been building the panels since 2017 began. What remains is to bring the panels to Seattle and install them in the tunnel.

 

The lower deck will be made up of 1,152 concrete panels. Each panel is 32 feet wide (the full width of the two-lanes-plus-shoulders roadway) and roughly eight feet long. There are four types of panels, each serving a different role in the tunnel’s future northbound roadway. A specialized crane (called a precast gantry crane) built to slide along temporary rails inside the tunnel will lift and place the slabs on top of the corbels (ledges) on both sides of the tunnel. 

The red mobile crane inside the tunnel, sitting atop rails placed above the corbels on each side of the tunnel.

Crews will then tighten the slabs together (called post-tensioning), bolt them to the corbels, and later pour a layer of concrete atop the panels to create a smooth driving surface.

Saving time by starting from the middle
Most of the tunnel’s interior structures and its mechanical, electrical, and plumbing systems are being installed in a linear fashion, starting from the south. The lower deck will take a different route. Crews will begin installing the lower roadway at a point about two-thirds of the way through the tunnel from the south (a spot roughly beneath Virginia Street and First Avenue). From there, they will move south as they place the deck panels.

This starting point and direction of work is due to the spatial constraints involved in tunnel construction. During mining, a truck delivered tunnel ring segments to Bertha from the south. Building the lower deck would have blocked this delivery access, shown here:

A special truck delivering curved tunnel segments into the tunnel portal

Even after Bertha finished building the tunnel, crews still needed that vertical space for the red travelers and material deliveries involved in constructing the tunnel’s corbels, walls and upper deck. So while that work finishes up at the far north end of the tunnel, STP’s plan will save time by working their way back south through the tunnel, then returning to finish up the northern third of the roadway. Assembling the lower deck is expected to be complete in 2018.