Cracks, leaks and spalling discovered in gigantic, newly constructed concrete pontoons are bedeviling the effort to replace the world’s longest floating bridge.
The problems are raising serious concerns on the project to replace Lake Washington’s SR 520 Bridge, which has been in service for almost 50 years.
Museum of History & Industry, Seattle
|A 1970 storm sends waves crashing into the SR 520 floating bridge.|
During the first round of pontoon construction for the bridge—a 7,500-foot-long floating structure connecting Seattle and Medina—crews found spalling and cracking in the end walls of the structures, which are designed to keep the bridge afloat.
A later inspection found ballast water leaking between two cells, according to the Washington State DOT (WSDOT).
WSDOT has since established an expert review panel to evaluate the problems, recommend solutions, and improve construction on future pontoons.
In January 2011, WSDOT awarded a $586 million design-build contract to Kiewit-General Joint Venture to build the concrete pontoons and a six-lane bridge, among other things.
Pontoons of Pharaonic Proportions
Construction of the bridge’s floating foundation is scheduled in cycles. The cracking, spalling, and leaking were found in pontoons from cycle one.
The new bridge requires a total of 77 concrete pontoons:
• Two cross pontoons, each weighing over 10,000 tons, mark the ends of the floating section of the bridge;
• 21 longitudinal pontoons, each about 360 feet long and 11,000 tons, form the backbone of the bridge and support the roadway superstructure; and
• 54 supplemental pontoons, each weighing over 2,500 tons, stabilize and support the weight of the floating bridge.
In May 2012, crews found cracking and spalling after completing post-tensioning on a longitudinal pontoon. The problems were repaired by adding reinforcing steel to fix the spalling, replacing concrete, and injecting epoxy to fix the cracks, according to WSDOT.
Once repairs were completed, the first six pontoons were floated out of the casting basin on July 30. However, an inspection Aug. 22 found ballast water leaking between two cells in one pontoon and moisture on the inside of an end wall in another.
|The SR 520 floating bridge is approaching the end of its lifespan, according to WSDOT.|
Updates on the pontoon construction project from WSDOT stated that the leak was repaired almost immediately, but “WSDOT and the contractor are continuing to look into the cause of this moisture” found in the end wall.
“We’re completely confident that the pontoons we floated to Lake Washington in July are safe and structurally sound,” said Julie Meredith, SR 520 program director. “They’ll last 75 years or longer, and so will all the rest of the pontoons we build.”
According to the expert review panel, repairs “adequate for structural capacity” were made to all of the areas affected by spalling and cracking.
Nevertheless, “It's pretty disturbing to see something like that where normally that’s not the type of thing we find,” John Reilly, who headed the panel, told KOMO News.
|During the first round of pontoon construction, cracks were found in pontoon walls.|
The panel recommended adjusting the design and position of post-tensioning strands and ducts.
WSDOT engineers briefed the media Sept. 26 to share updates on repairs on cycle one and the modifications planned for future cycles.
“A repair plan was developed by WashDOT, and, working with our design-builder, that repair procedure was implemented,” Mike Cotten, SR 520 floating bridge and landings project director, said at the media briefing.
“We want to confirm that WashDOT would not have loaded the pontoons onto Lake Washington if we weren’t completely confident that they were structurally sound and safe to drive on,” Cotten said.
The Big Picture
With only two lanes in each direction, the SR 520 Bridge sees 115,000 cars each day. Nearing the end of their original service life, the bridge’s pontoons have become vulnerable to windstorms and its support columns are vulnerable to earthquakes.
The new pontoons are designed for a 75-year service life to withstand windstorms up to 89 mph and a 1,000-year earthquake event. The new bridge will provide a six-lane roadway with dedicated transit/HOV lanes, wider shoulders, a 14-foot-wide bicycle/pedestrian path, and the possibility to accommodate light rail in the future.
As of Oct. 1, a total of nine bridge pontoons had arrived on Lake Washington. Construction is scheduled for completion by December 2014.
This project is the first construction phase of the SR 520 – I-5 to Medina: Bridge Replacement and HOV Project, for which the Washington State Legislature has set a program budget of $4.65 billion.