Lightweight foam, instead of retained dirt, has been installed beneath a new Alaskan Way Viaduct segment.

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When the state opens a new Alaskan Way Viaduct segment to traffic on Halloween morning, thousands of motorists and bus commuters will be riding over foam.

The lightweight material has been installed, instead of retained dirt, where the road bed rises to meet new Highway 99 bridge spans in Sodo, now being completed.

The spans fly over railroad tracks, and will carry three permanent southbound lanes in 2016. Until then, two lanes of cars in each direction will squeeze onto that overpass for four years, until a deep-bore tunnel to South Lake Union is done and new northbound spans open. The detour, which includes a winding surface portion near Safeco Field, will add several minutes to an already congested commute.

The state Department of Transportation (DOT) ordered Geofoam brand high-density polystyrene, because the Sodo surface is made of unstable fill soil, dumped over tide flats in the early 20th century. The weight of dirt, and the process of packing it tight (as Seattle has done on its Mercer Street rebuild), would cause the weak Sodo surface to sink as much as 8 inches. That would have destabilized the old viaduct, still being used this summer by traffic a few feet away, said DOT engineer Paul Johnson.

Installation of the polystyrene, pre-cut into blocks like puzzle pieces, goes faster than soil or concrete, Johnson said.

Workers already have installed the white blocks and covered them with sand and gravel. But travelers can still glimpse them this week, if they look right while driving on the old southbound lanes past Safeco Field.

Similar foam has been used on a new curlicue flyover road and walkway where train lines divide Royal Brougham Way South, on an Interstate 580 junction near San Francisco Bay, and for light-rail systems in Utah and Colorado, to name a few examples.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com