You'll be driving on styrofoam with new SR-99 off ramp

SEATTLE -- Rebar is going down over a freshly-poured load distribution slab on what will be the new northbound State Route 99 off ramp to Dearborn Street, but it's what's underneath it that's unique.

It's the same stuff they use to make styrofoam coffee cups. It's geofoam, huge styrofoam blocks that are holding up the bridge that'll be the last exit before the new SR-99 tunnel. Drivers can take it to get to downtown.

Had Washington State Department of Transportation used soil -- instead of geofoam -- the short-term "Seattle squeeze" would have been a long-term nightmare.

"This closure would have probably lasted six months instead of three weeks, so it's a huge difference " said David Sowers, WSDOT deputy program administrator. "So the little extra cost of the geofoam certainly outweighed the six months of no 99."

It has to do with SODO's own soil.

"There's marine deposits that were here thousands of years ago, but they're soft, they're loose and they compress when you put a significant load on them," Sowers said.

Building with geofoam, there's no need to wait for the ground to settle. It's seismically stable and 100 times lighter than dirt. With the foam weighing about one point per cubic foot, WSDOT said the off ramp will float over the ground in the event of an earthquake.

The stable structure gets help from the first-of-its-kind flexible bridge, built with memory-retaining metal rods and bendable concrete composite. It's designed to sway with a strong quake and then snap back to its original shape.

It's just one of many features that WSDOT said will make this portion of SR-99 one of the safest roads to travel during seismic activity.