A new place to stroll above the SR 99 tunnel

Frequent observers of the project’s construction cameras may have noticed some white blocks being placed next to the north operations building at the corner of Sixth Avenue North and Harrison Street.

Crews are building a public plaza that will be a centerpiece of the landscaping at the project's future north portal. The reconnection of Harrison, Thomas and John streets will make walking between each side of SR 99 a more intuitive and pleasant experience. Pedestrians as well as drivers will have an easier time traveling east-west between Uptown and South Lake Union.

Should you take a stroll to this plaza, you'll be treated to a close-up view of the north portal operation building's fan room. Through the building’s glass wall you'll see the four centrifugal fans (topped on the roof by the distinctive yellow stacks) that are central features of the tunnel's ventilation system.

Air will play another role in this plaza, in the ground beneath your feet. Those white blocks forming the base of the plaza are blocks of geofoam, a high-strength, low-density synthetic material (think of a very strong version of Styrofoam). Used in many types of construction projects, geofoam has been used in several viaduct-replacement projects as a substitute for fill soil.

Why is geofoam preferable to fill soil? Because it can handle and disperse loads, but is 98 percent air by volume, making it much lighter than soil. Using it to fill the space between the tunnel and the plaza built atop it dramatically reduces the weight the tunnel must support. This reduces overall project costs by lowering structural engineering costs.

The plaza is scheduled to be complete when the tunnel opens to traffic, currently scheduled for early 2019.