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Saturday, October 5, 2002 - Page updated at 12:00 AM

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Sinking street gets foam foundation

Seattle Times Eastside bureau

For 30 years, Southeast Eighth Street has been slowly sinking into the soft muck of the Mercer Slough peat bog.

Buildings have shifted, the road has rippled and cracked and two whole city blocks are 4 feet closer to the core of the Earth than they were when built.

Yesterday, road crews took a step the city hopes will stem the sinking for at least 20 years: They inserted 3,000 slabs of plastic foam into the ground.

The white construction material, called Geofoam, is extremely light — about 60 pounds per cubic yard, compared with about 3,000 pounds for typical road fill. That, say engineers, is the key to saving the street from its continuous decay. Because it's so light, Geofoam drastically reduces the weight on whatever is below it.

"It's not going to eliminate settlement altogether, but it's definitely going to slow it down," said Dave Cieri, capital-investment program manager for Bellevue's Transportation Department.

The street could still sink 12 to 18 inches in the next 20 years, but the foam will stabilize it enough so the asphalt on top should keep its shape, Cieri said.

The project, which will put a 700-foot-long, 44-foot-wide swath of foam below Southeast Eighth Street between 112th Avenue Southeast and 114th Avenue Southeast, will cost Bellevue about $3.8 million.

The foam was selected over various other options, including putting in a bridge with pilings for $10 million.

This is not the first time there have been problems with land built near the Mercer Slough.

First settled by white homesteaders in 1869, the 2-mile waterway has been manipulated by people ever since. In 1916, the Army Corps of Engineers opened the Chittenden Locks, lowering Lake Washington and the slough by about 9 feet. This drained the marshes around the slough, exposing a blanket of mushy peat soil.

The mostly man-made land is far too close to the water table beneath it, and by the time office buildings were being constructed in the 1970s, the ground was slowly sinking.

The sinking has caused problems such as poor drainage and flooding for Bellefield Office Park on Southeast Eighth, confirmed Katy Strausborger, spokeswoman for Equity Office Properties, which owns the park. She said the property owner and engineer hope the foam will help.

Geofoam, which looks like the stuff used for drinking cups or packing but is much stronger, has been used in construction for about 30 years but has become really popular in the U.S. only in the past five or six years, said Rich Kay, who manufactures the foam being used in Bellevue at his Kent facility.

The material also can be used for walls, to shore up embankments, as landscape fill and for frost protection.

Last year it was used to stabilize bad soil on the tideflats at the Port of Tacoma and was recently installed in offramps and a bridge in Longview, said Kay.

Work began on Southeast Eighth Street in June, when workers tore up the street, dug out dirt and gravel, repaired leaky water lines and built a storm-drainage system.

Yesterday they began installing two layers of foam, attached to each other by gripper plates to form one giant, foamy foundation that engineers say will also help stabilize the street during an earthquake.

On top of that goes a plastic mesh, 12 to 20 inches of crushed gravel, and two layers of asphalt.

The foam should be installed within a few weeks, and drivers probably can start using the road again in November, Cieri said. The entire project is scheduled to be completed this spring.

Natalie Singer: 206-464-2704 or nsinger@seattletimes.com.

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