What are those big, foam blocks along I-5, and why is WSDOT using them?

Q: I’ve seen large (foam) blocks being used in a couple of different road projects around the region lately (Interstate 5 in Tacoma, and the state Route 167 HOV lanes to Interstate 405 in Renton). How are they being used, what benefits do they bring for construction and what negatives may come from their use? — Neil S., University Place

A: The big, white blocks you’ve seen along I-5 are called Geofoam blocks, and the Washington state Department of Transportation is using them instead of fill dirt around the Puyallup River bridges in Tacoma.

The reason: They’re light enough that they won’t ruin utility lines buried in the area, even taking into account the concrete in which the blocks are encased during construction, department spokeswoman Cara Mitchell said.

The sandy soil near the river is a particular problem, and the foam blocks’ comparative light weight means they won’t cause the soil to settle too much to damage any pre-existing utilities underneath.

The blocks do have some drawbacks, Mitchell said: Their size can make them cumbersome, and they have to be shaped to a perfect fit and leveled before being encased in a protective membrane and concrete. If the membrane fails, “it would be the result of a much larger roadway problem,” she added.

The construction process with the blocks at the I-405 project went like this: Crews first dug out a section of ground that weighed more than the foam. They then placed the blocks together with commercial roofing adhesive, put reinforcing steel on top of the blocks and covered them in concrete.

The life span of the blocks is about 75 years, Mitchell said, the same as any other structure material WSDOT uses. The department has used the blocks for about 15 years, and they’re used around the country for similar projects.

This particular patch of I-5 construction — the northbound HOV lane expansion from Portland Avenue to Port of Tacoma Road — is expected to wrap up next month “barring any strange weather delays,” Mitchell added.

Cox Media Group