Seattle needs deep-sea divers to handle a bridge restoration project. Starting today, divers will descend as deep as 180 ft to replace aging and worn anchor cables on the Interstate 90 floating bridges between Seattle and Bellevue.
The three-month project involves removing and replacing 21 frayed and corroded anchor cables. Divers from Orion Marine will replace thick steel cables that connect the bridge’s concrete pontoons to anchors on Lake Washington’s bed.
At between 24 and 29 years old, the cables that stabilize the bridge, especially during storms, have decades of wear from wind and waves pounding them.
“Worn cables pose a higher risk of breaking during a windstorm,” Archie Allen, Washington State Dept of Transportation bridge superintendent, says in a statement. “Broken cables increase the risk that the bridge cold be damaged, resulting in long-term closure.”
The $3.4 million project has divers disconnecting the cables from the anchors. The cables then get winched to the surface. A diver will descend with a new cable and secure it to the existing anchor. The project runs one cable at a time. Each cable secures inside a bridge pontoon and extends at an angle in to the lake.
The project involves five divers on site at a time, with just one heading into the water at any point. Depending on the depth they are working, a diver will be allowed to stay underwater between 45 minutes and two hours. After the first diver is fully decompressed, a second diver will continue work. Each diver will get monitored for signs of distress using a video and audio feed.
The 21 cables range in length from 310 up to 750 ft. Laid end to end, the new cables would stretch from sea level to 12,005 ft, nearly the same height as Mt. Adams.
A similar project took place in 2010, replacing 30 anchor cables on both the I-90 and State Route floating bridges, with 15 of those for the I-90 bridges. This round of 21 cables is just a part of the 108 anchor cables holding the twin bridges in place.
Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He also writes for Popular Mechanics, Sports Illustrated and more. You can follow him on Twitter at @tdnewcomb or visit his website here.