What are those mysterious white blocks in Tacoma?
Drivers who frequently use I-5 in Tacoma have probably seen stacks of giant foam blocks near the old Puyallup River Bridge.
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I received this text during Seattle’s Morning News on 97.3 KIRO: “What are the huge blocks of foam used for down here by the Puyallup River bridge project in Tacoma? They are on the old span, and they are 8′ x 8′ big white blocks of Styrofoam. Dozens of them.”
Cara Mitchell with the Washington State Department of Transportation said they are just what they appear to be, giant foam blocks called Geofoam. The blocks are used in lieu of fill-dirt for support in construction projects. Drivers are traveling over them right now.
“They have been in place, and people are driving on them now when they take the 28th Street on-ramp to northbound I-5,” Mitchell said. “They are also being used on the new mainline northbound I-5 alignment.”
Geofoam weighs less than typical fill dirt and works well in wet soil, like the environment around the Puyallup River in Tacoma.
“The soils are very soft and sandy, and instead of using traditional fill dirt, which weighs a lot more, this product weighs much less,” Mitchell said.
Geofoam makes sense in this project because there are some old utility pipes in the area, and contractors didn’t want to crush them under the weight of the new freeway.
Why are we just seeing them now?
Mitchell said they used to be stored out of sight, but now the old northbound bridge makes a great storage area.
“Earlier in this project, we had them stored down near the 28th ramp so they weren’t as visible,” she said. “Definitely with them sitting up on the old bridge deck, absolutely everyone can see them right now.”
The foam blocks are going to be used in the new southbound Puyallup River Bridge project that will begin next year. They have also been used in other mega-projects such as the Alaskan Way Viaduct replacement and the 167/I-405 interchange in Renton.
Mitchell said the blocks are also cheaper than traditional fill dirt.