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What are those huge white blocks being used on the Sammamish River bridge?

If you’ve driven past the Sammamish River bridge project on 68th Ave NE in recent weeks you may have notice the giant stacks of snow white blocks piled on the river bank, and wondered what that stuff was.

The answer is geofoam, a lightweight expanded polystyrene used to provide a lightweight fill that will not add significant loads to the supporting soil. The material is non-biodegradable and will not affect soil or groundwater, according to an article in Roads & Bridges, an industry website.

The geoform blocks are placed under the bridge to protect the soil embankment and is topped with a concrete slab to distribute the load evenly over the geofoam mass. The concrete also protects the foam from potentially damaging solvents. The article says the geofoam was first used in King County on the York Bridge project in Redmond and has been successfully used in 20 states according to the Federal Highway Administration.

The foam reduces the cost of the bridge replacement by cutting the time and cost of hauling in tons of construction fill needed for support of the roadbed.

To facilitate traffic flow uniformed police have been directing traffic through the NE 170th St. and 68th Ave NE intersection during the day. Starting Oct. 26, nighttime utility work will begin. Workers will be installing underground conduit and wiring from 7pm to 6am.

If you have more questions about the project, visit the construction website for current information or send your questions to Email:

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