After more than a decade of wrestling with technical challenges, Sound Transit is finally building the world’s first train tracks on a bridge that floats.

The water level below the Interstate 90 bridge typically fluctuates 2 feet annually, as spring runoff enters Lake Washington and the Army Corps of Engineers maintains boat passage at the Ballard Locks. Hinged transition spans near each end of the bridge change their slope as the floating pontoons rise or fall.

Wind gusts, waves and traffic push the bridge in other directions, and the 300-ton light-rail trains coming in a few years will add new forces.

Yet through the next century of bridge motion, the rails must stay precisely parallel to avoid a derailment.

Contractors this summer are fastening these customized tracks and ties in the former I-90 center express lanes, the most innovative segment in a $3.7 billion light-rail corridor that will run from Seattle’s Chinatown International District to Bellevue and Overlake, scheduled to open June 2023.

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“It’s sort of Sound Transit’s moon shot, which is a term overused a lot this week, but it is. Nobody’s ever done this before,” said John Sleavin, deputy executive director for design, engineering and construction management, during a media tour Tuesday.

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Americans just commemorated the 50th anniversary of the first astronauts who walked on the moon July 20, 1969.

Keeping the bridge buoyant despite the added trackway weight is the first necessity. Engineers once considered scraping off a few inches of concrete road deck, but dropped that strategy. Contractors are instead replacing concrete barriers, which wouldn’t stop a train anyway, with cable fence to prevent falls into the lake.

Concrete blocks called plinths that support the rails are 9 inches thick instead of the typical 15, and built from a lightweight mixture, Sleavin said. Steel bars are used to connect the plinths instead of heavy concrete ties. Rock ballast is being reduced inside the pontoons.

Track supports rest on layers of rubber to disperse stray current from the electric-powered trains and prevent corrosion of steel rebar inside the bridge. The plinth blocks are fastened with epoxy to the deck, to avoid thousands of bolt holes.

The project’s biggest innovations are the eight platforms Sound Transit calls “track bridges” where rails pass about three feet above the hinged bridge joints.

Basically, the rails will stay in line while the 43-foot-long platforms beneath pivot or shift under the influence of lake motion, even as trains go 55 mph.

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That’s feasible because the 17 track ties on each platform rest on 17 pairs of flexing bearings that resemble black wafers. The bearings are similar to seismic dampers that isolate the roof of CenturyLink Field and many Seattle towers from earthquake motion. As the floating bridge moves, the flexible parts within the platform should pivot only one-half of a degree, Sleavin said.

The technique was called “a brilliant solution” by John Stanton, a University of Washington professor of civil engineering who researches earthquake preparedness. Otherwise, Sound Transit might have settled for joints that slide within the rail connections that are less resilient and possibly would require lower 35 mph train speeds.

Some 200,000 travelers who cross the bridge daily can see new concrete blocks, where steel frames will be mounted to carry the trains’ overhead electric power wires.

The innovations aren’t cheap. Sound Transit spent an unusually long 50 months to complete final engineering, and $53 million just on floating-bridge redesigns.

After that, the agency approved an extra $225 million in the construction contract with Kiewit Pacific, which currently totals $685 million for tracks and stations between Seattle and South Bellevue. Much of the cost increase went to adding steel tendons that compress and strengthen the concrete pontoons, a method known as post-tensioning.

The goal is to assure the bridge, built in 1989, exceeds its 100-year design life despite punishment from 600,000-pound trains. Besides rail costs, taxpayers spent $283 million to squeeze in a pair of I-90 mainline carpool lanes, leaving some motorists outraged that the state relinquished scarce road space in the express lanes.

Sleavin said he’s hopeful the floating bridge work will continue trouble-free. The latest progress report cites other spots as “closely monitored issues” that could slow construction, such as Judkins Park and Mercer Island stations wedged into the freeway median; a legal spat with Seattle City Light in Bellevue’s Spring District; and water seepage into the excavated Bellevue tunnel.

The 14-mile extension remains on schedule to open in mid-2023, followed by two stations in Redmond by 2024, serving an estimated 50,000 passengers per day. A trip from Bellevue to Seattle would take 20 minutes.

Sound Transit crews fasten rails to the I-90 floating bridge. This is the first time  rails have been installed on a floating bridge. (Ellen M. Banner / The Seattle Times)
Sound Transit crews fasten rails to the I-90 floating bridge. This is the first time rails have been installed on a floating bridge. (Ellen M. Banner / The Seattle Times)