Plans to repair cracked 520 pontoons finally under way

Plans to repair cracked 520 pontoons finally under way »Play Video
SEATTLE -- The Department of Transportation is launching its plan to repair the first set of cracked pontoons for the 520 Bridge, which will cost in the tens of millions of dollars and won't be finished until at least March.

The four big pontoons floating on the lake right now look solid, and state experts insist they're structurally sound. But the Problem Solvers have obtained detailed crack maps from this past winter that show just how serious the problems are.

"These are big pontoons. They weigh 11,000 tons," said WSDOT project manager Dave Becher.

All four of the big pontoons are destined for big repairs. Diagrams of external cracks on pontoons W and V taken by divers last winter show troublesome cracks on the bottoms, or keels, of the pontoons. And the cracks are extensive on pontoon V.

"The whole idea is to actually seal the pontoons to prevent the water intrusion," Becher said.

Last fall the Problem Solvers uncovered extensive cracks and leaks in the first set of pontoons. Starting next week, pontoon T will be towed by dry dock to Portland by the same tug that is currently hauling pontoon A into Lake Washington.

In July, pontoon W, which is now floating in the middle of the lake, will make the trip to a dry dock on Harbor Island.

The other two pontoons will be repaired in the lake starting next winter, when a steel coffer dam is ready. It will create dry space around the areas needing repairs.

"It's really three very critical and different types of repairs," Becher said.

The first repair involves injecting epoxy into those keel cracks. Then steel post-tensioning strands will tighten the pontoons side by side. And finally, workers will add carbon fiber mats to some sections.

WSDOT won't put a specific dollar figure on what the repairs will cost.

"It's a difficult operation and it will cost tens of millions of dollars, and I'd hate to speculate more than that because we're in negotiations with the contractor," Becher said.

The repairs will also slow down completion of the bridge. Right now WSDOT's hoping to meet a new target deadline of July 2015, which is already a year later than the original plan.

Becher also said corroding rebar from salt water shouldn't be a problem because the pontoons won't be in the salt water for long.