By the numbers
8,000: Approximate number of geofoam blocks to be used in Salmon Creek Interchange Project.
As the forklift lurched toward a nearby truck, its prongs began to slide between two foam blocks loaded on the trailer. An unmistakable squeak followed, like Styrofoam being jammed into a cardboard box.
No, that's not packing material stacked up along Northeast 139th Street in Salmon Creek. It's the foundation for the road itself.
"They're not hugely different than what you would find wrapped around your TV when you open the box," said Andrew Fiske, a geotechnical engineer with the Washington State Department of Transportation.
Here's the key difference: "However, we can dictate the strength requirements," he said.
Those blocks are actually made of expanded polystyrene, commonly referred to as geofoam. The material is produced like packing foam, but made to be stronger, more dense and more uniform in its composition, he said.
Construction crews are using the relatively uncommon material at Clark County's Salmon Creek Interchange Project. The geofoam blocks will help hold up the approaches to a new bridge at Northeast 139th Street, plus new freeway ramps connecting to it. Crews are building the span as part of a $133 million effort to remake the convergence of Interstate 5 and Interstate 205.
WSDOT has used geofoam on a handful of projects in other parts of the state, said area engineer Leon Winger, but not in Southwest Washington.
Several factors made the Salmon Creek project a good candidate for geofoam, Winger said. Soft, unstable soil on the site was among the biggest, he said.
"The ground is Jell-O underneath that," Winger said.
Using crushed rock or earthen fill to support the bridge approach would make it more prone to shifting or settling in an earthquake, Fiske said. So instead of strengthening the soil to hold a heavier load, crews simply made the load itself lighter, he said.
Gravel fill weighs about 130 pounds per cubic foot, Fiske said. A cubic foot of geofoam, by comparison, weighs about a pound and a half, he said.
Workers are using more than 31,000 cubic yards of geofoam on the Salmon Creek Interchange Project, according to WSDOT. It will take almost 8,000 geofoam blocks, most measuring 3 feet by 4 feet by 9 feet, to fill that space.
Once the foundation is set, workers will cut the foam to match the slope and shape of the road, Winger said. Sand and a rubber liner will help finish the structure. The road will sit on top, and concrete walls will enclose the entire foundation, he said.
"You'll never know that foam was ever part of the fill," Winger said. So, no squeaking.
The Salmon Creek Interchange Project is scheduled for completion next year. The new 139th Street bridge will give the area a second east-west thoroughfare, over the freeways, which WSDOT hopes will alleviate heavy traffic on nearby Northeast 134th Street.
Geofoam won't replace traditional fill in all cases, but it may become more common as construction projects become more conscious of seismic risks, Winger said.
"It's not a new technology," he said. "But it's a good solution."