AASHTO Journal, 15 May 2015
The Washington State Department of Transportation is relying on deep sea divers to replace aging cables on the Interstate 90 floating bridges across Lake Washington, connecting Seattle and Bellevue.
The $3.4 million safety and preservation project that began May 11, WSDOT said, will see specially trained contractor crews from Orion Marine replace 21 “frayed and corroded anchor cables.” Those cables connect the concrete bridge pontoons to anchors on the lake bed, stabilizing the bridges and holding them in place during windstorms.
The anchor cables that will be replaced over the next three months are 24 to 29 years old, and WSDOT said “they are regularly subjected to strong winds and pounding waves.”
Generally, the agency said, the cables should be replaced every 20 to 25 years. This project will upgrade a fraction of the 108 anchor cables holding the bridge in place. A similar project replaced 30 anchor cables on the I-90 and State Route 520 floating bridges in 2010.
“Worn cables pose a higher risk of breaking during a windstorm,” said Archie Allen, WSDOT bridge superintendent. “Broken cables increase the risk that the bridge could be damaged, resulting in a long-term closure. Regular inspections and replacement help keep the bridge open and safe for travelers.”
The agency says while there may be occasional overnight lane closures on the floating bridges, most of the work takes place underwater and inside the pontoons so most drivers won’t be directly affected.
In a project that replaces one cable at a time, divers will descend in daylight hours as deep as 180 feet to disconnect the cables from lake bed anchors. Each cable will then be winched to the surface. After the first diver has fully decompressed, a second diver will descend to the lake bed with a new cable to securely fasten it to the underwater anchor.
WSDOT said the bridges carry nearly 177,000 vehicles a day, and are an essential link to the Interstate 5 and Interstate 405 corridors, the Port of Seattle and business areas on both sides of the lake.
