Concrete track supports on Interstate 90 floating bridge create new worry for Sound Transit

SEATTLE – Sound Transit has discovered another construction problem with its future Seattle-to-Bellevue light-rail extension, this time on the Interstate 90 floating bridge deck.

Inspections in late March found three concrete track supports where material had flaked away, out of a sample of 500, Jon Lebo, executive project director, said Thursday. There are 7,800 similar track supports, also known as plinths, on the bridge, he said.

The leading theory is that twisting forces knocked away some material during installation and calibration of steel rail-fastening parts, on the tops of concrete supports, Lebo said. Engineering consultants were hired to investigate, and samples of materials sent to an out-of-state laboratory, to confirm the cause, he said.

“We want to take a deep look at the plinths, out of an abundance of caution, to make sure everything’s fine,” said Ron Lewis, director of design, engineering and construction, in a phone interview. He expects that a change in rail-installation sequences should prevent further concrete damage.

The $3.7 billion, 14-mile corridor will miss Sound Transit’s June 2023 grand opening target, and possibly be delayed until 2024, as previously reported. Lewis said he doesn’t have a new date or cost figures. He anticipates paying contractors for lost time due to COVID-19 and last winter’s concrete truckers strike. Construction has already been underway for six years and provided 7.8 million hours of worker wages.

Lewis revealed the damage Thursday afternoon to the transit board’s System Expansion Committee, which is chaired by Claudia Balducci, a Metropolitan King County Council member from Bellevue. The committee members raised no questions or comments following Lewis’ brief, illustrated presentation.

The cost to solve the problem isn’t clear yet.

“Currently, there is no basis to assume the necessity of extensive repairs,” Lewis said in a staff report Thursday.

This is the latest problem for light-rail construction, following a more serious finding that concrete supports near the International District/Chinatown Station had wrong dimensions or weak materials. Hundreds are being rebuilt this year.

In another discovery, part of the Bellevue downtown tunnel was too tight for trains to get through, Sound Transit revealed in a new monthly progress report, without further details. But Lebo said Thursday the problem’s been solved, by removing a few inches of concrete in a non-structural wall that houses utilities and an emergency exit, where trains will curve into Bellevue Downtown Station.

Other previously disclosed issues include track adjustments in northeast Bellevue and partial reconstruction of the Redmond Technology Station parking garage, where some weak beams posed safety hazards.

The I-90 track supports, in the former floating bridge express lanes, are made from light concrete about two-thirds the normal weight, to keep the bridge buoyant. They’re fastened using epoxy and plastic gaskets, instead of steel bolts that would conduct stray electric current and gradually weaken the bridge decks.

The March inspections were triggered by suspected damage to nylon inserts, where bolts were secured to the small concrete plinths. The nylon threads can be “fragile,” said project spokesperson Rachelle Cunningham. While checking the nylon inserts, officials discovered concrete that’s flaking, or spalling, she said. Nylon damage could be related to looseness and unwelcome forces pulling against the concrete, Lebo explained.

Crews have modified how they handle the rails and attachments, and no further damage has appeared, Lewis told the board.

The Seattle-Bellevue-Overlake project, which will be called the 2 Line on transit maps, was approved by voters in 2008 and is expected to serve about 48,000 daily passengers. Construction has started on an extension to Marymoor Park and downtown Redmond to open perhaps in late 2024, as trips and job growth proliferate within the Eastside.