February 29, 2008

Modern Improvements

Peat bog highway beds under intensive study

Complex tangle of interchanges need support

KORKY KOROLUK

correspondent

OLYMPIA, WASH.

A peat bog might seem to be an unlikely place to build a highway interchange, but that’s exactly where the first bridge of an interchange near Seattle was built in 1940, and engineers with the state transportation department have been living with it ever since.

Since then, traffic growth has dictated more bridges, turning the interchange into a complex series of structures carrying higher traffic loads than they were designed for. And as the peat bog moves, so, too, do the bridges.

The interchange is on the shore of Lake Washington, which separates Seattle and Bellevue. Part of it lies on the Mercer Slough, a peat bog, and part over an open stretch of water to Mercer Island. Interstate 90 connects there with both state highways and local arterials in an interchange that looks, from the air, like a handful of cooked spaghetti dropped on the kitchen floor.

Why build on a peat bog? Tim Ditch, bridge supervisor with the state transportation department just laughed at the question. “That was done before my time,” he said. “I guess that would be some kind of engineering question from a long time ago.”

George Comstock, regional bridge inspection engineer with the state, also laughed. “I don’t really know,” he said, “and I don’t think I want to go there.”

Comstock is the driving force behind a newly-installed Global Positioning System (GPS) that is gathering data from which he hopes to discover exactly why and how the bridge moves.

A total of 22 GPS devices have been installed, he said, and they are gathering information on both horizontal and vertical movement. The system was provided by Leica Geosystems. With the cost of wireless communications and a server added in, the total came to about $385,000.

He explained Lake Washington is used for floodwater storage, and its level varies each year by about two feet.

“We believe the lowering lake level activates lateral movement of the peat,” he said. “In other words, you drop the lake level, and starting right at the edge of the water, the peat starts sliding into the lake.

“It kind of works its way up into the structure. Then, when the lake level rises, that lateral peat flow stops occurring and the bridge actually will ‘snap’ back into position. “At least, that’s what we think is happening, and that’s what we’re trying to verify.”

The interest is more than academic, he explained, “because this is a transportation corridor that’s going to be around for a long time.”

“When we put light rail in here, or put in the next bridge, we want to be in a position to know how to create the structure so that it will handle the load.”

Since the first bridge was built, continuing lateral movement of the 60-foot-thick peat bed has resulted in damaging deflections to the pile-supported structure and to a major waterline that runs parallel to the highway.

The Leica system detects motion in the bridge as small as one millimeter. It not only measures deflections in real time, it provides automated alarms when movement exceeds pre-determined thresholds.

Until now, Comstock explained, “we’ve been collecting information while knowing that we had problems doing deck-joint measurements.”

The bridge consists of multiple spans in one rigid frame, he said, and there is a deck joint that separates the spans within the frame.

“But we couldn’t tell which one was doing the movement, or if they were both working together. We could only say that this span was moving relative to that span, but we couldn’t get any absolute information.”

The Leica system, he said, “is giving us that absolute information.”

Comstock said there will likely be enough data gathered within 18 to 24 months to provide a precise picture of the bridge movement, enabling engineers to design better solutions to the problem and avoid future, similar problems.

At the end of that time, he said, the sensors will be taken out and reinstalled at another bridge that is causing his department some concerns.

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