State DOT admits mistakes on floating-bridge pontoons

SEATTLE — The Washington Transportation Department said Tuesday that mistakes were made in the construction of pontoons for the new Highway 520 floating bridge across Lake Washington between Seattle and Bellevue.

The department said it’s making repairs and design modifications to ensure a 75-year life for the bridge.

The exact cost of the repairs hasn’t been determined, but it will be in the tens of millions of dollars, Transportation Secretary Paula Hammond told The Seattle Times.

The changes are the result of findings by an expert review panel that was convened last year to review cracking in the first pontoons built in Aberdeen.

Underwater inspections revealed the pontoons’ worst cracks grew over the winter. The most serious cracks begin in the end walls, and wrap around the upper and lower edges to continue along the top and bottoms.

An untreated underwater crack can let in water at rates of 1 cubic foot per hour, according to a technical report on the bridge.

The panel agrees the pontoons are structurally sufficient, but it found the department didn’t follow standards of good practice to validate the pontoon design elements and did not strictly follow protocols for oversight on the construction site, the department said.

Hammond said disciplinary action will be taken against state bridge division staff who signed off on the design without running models that might have foreseen the cracking.

A total of 77 pontoons will make the world’s longest floating bridge.

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