Challenges include the world’s only trackway on a floating bridge, which moves with Lake Washington’s water level and must be kept buoyant. It’s a key piece of the $3.7 billion corridor between Seattle and the Eastside.

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The cost to build light rail on Interstate 90 between Seattle and Bellevue has zoomed $225 million higher than Sound Transit once estimated, now that final engineering work has revealed the challenge of retrofitting the roadway.

Transit-board members voted unanimously to approve a contract increase Thursday afternoon with construction group Kiewit-Hoffman.

The additional money would come out of contingency funds for the Eastside line, without causing delays or a tax increase.

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The vote raises the contract amount to $712 million for 7 miles of work, from the International District/Chinatown Station to South Bellevue. It’s a crucial phase in creating a $3.7 billion East Link corridor to downtown Bellevue and Overlake, where train service is scheduled to begin in 2023.

The job includes building the world’s only trackway on a floating bridge. The roadway moves with Lake Washington’s water level and must be kept buoyant despite the weight of tracks, ties and trains to be placed in the existing express lanes.

Studies, tests and design have been underway for a dozen years.

Sound Transit is on the verge of winning final approval by state and federal governments to begin I-90 trackwork this summer, after engineers solved 23 technical issues raised by outside experts.

The $225 million change represents a 46 percent increase, compared with the $486 million budget for the I-90 trackway construction contract.

One factor is a recent decision to install steel frames within each of the floating pontoons, Ron Lewis, East Link executive director, recently told the transit board’s capital committee.

Steel frames will enable Kiewit-Hoffman and subcontractors to perform “post-tensioning,” which entails threading steel cables through the pontoons and tightening them at extreme forces to unite them into one rigid structure.

Post-tensioning is a common technique to compress and strengthen concrete structures, especially bridges.

The goal is to assure that the floating bridge, which opened in 1989, can resist cracking and meet its 100-year design life span despite thousands of trips by 600,000-pound trains. The contract increase includes a 7 percent contingency for Kiewit if needed.

“I-90 is in large part a retrofit. If you can imagine retrofitting a house versus building a new house where everything is square, in a retrofit you might have conditions that might not be perfect when you’re out there,” Lewis said.

The cost of seismic retrofitting also was higher than expected, as the agency examined the bridge’s tall columns near the shore and the difficulty of working on structures that are part of a busy freeway carrying traffic.

This isn’t a traditional low-bid project.

Kiewit was awarded a preliminary contract for pre-construction and management services in 2015, with the understanding it would likely lead to the full construction project. Kiewit and its partners have built other floating bridges in Washington, including a new Highway 520 crossing that opened last year.

Even a $225 million hike didn’t prompt Sound Transit to reopen bidding for the main construction phase. The cost is considered “fair and reasonable,” the transit staff report said.

The I-90 bridge contains unorthodox features, including eight 42-foot steel platforms under the rails that flex slightly, and smooth out the sharp angles where the bridge’s fixed spans and floating spans meet. The bridge must be seismically reinforced, and any stray current from the electric-powered railway must be avoided in the wet environment.

After spending $225 million in contingency money, the staff said $147 million would be left in reserves, assuming that future bids on electronic systems and four Bellevue stations stay on budget. And both I-90 and a shallow downtown Bellevue tunnel would need to avoid large overruns.

John Niles, a Seattle transportation analyst who opposes and closely follows the Eastside line, said the higher construction cost comes at no surprise.

“Burning down some of the contingency to be approved today is a trivial fiscal event for an agency as cash-rich as Sound Transit, with or without the excess [car-tab tax] from its controversial car-valuation scheme,” he said via email.

He called the line “bad for the planet,” saying it would add only an estimated 10,000 passenger trips per day beyond what buses could achieve, even after the huge investment of materials and energy to install rail.

The light-rail line was approved by voters in the Sound Transit 2 measure in 2008 and funded predominantly by sales and car-tab taxes collected on the Eastside. Total ridership is estimated at 50,000 boardings, not including a two-station Redmond extension that’s due in 2024, funded by new Sound Transit 3 taxes approved last fall.

The city of Mercer Island has sued to postpone I-90 light-rail construction, on grounds that the state Department of Transportation is failing to provide adequate access for the island’s I-90 motorists, once the center express lanes are closed to vehicles and converted to rail.