Two of the four cracked Highway 520 pontoons in Lake Washington will soon be towed to drydocks for repair.
Pontoon T, weighing 11,000 tons, will be towed to Portland next week.
Then in July, the giant Pontoon W, forming the eastern end of the floating structure, will be taken to Harbor Island.
The other two pontoons will be fixed in the lake, using a steel coffer dam. The coffer dam will encircle one end of the pontoon at a time. The water is pumped out to create a dry environment around the work zone, for an estimated six to eight weeks of repair work per end.
State officials and contractor Kiewit-General-Manson are using both drydock and coffer techniques so that the four cracked pontoons can be fixed simultaneously, to save time, said spokeswoman Debera Carlton Harrell of the state Department of Transportation.
The cracks resulted primarily from design errors by the state DOT engineering corps, and might wind up costing $100 million, including delays, before the bridge is completed sometime in 2015. This cost would, essentially, cancel the DOT’s earlier announced savings from low bids to build 33 pontoons at Grays Harbor, for an initial price of $367 million.
Dive inspections last winter mapped significant cracks on the undersides of the pontoons, where cracks on the end walls extended around a corner.
The main crack-sealing task will be to add steel cabling from either side, then cinch the steel from either end to compress the pontoon, a technique known as post-tensioning. Epoxy will also be injected into cracks, and some areas will be wrapped in carbon fiber.
Minor cracks of less than 0.006 inches, caused by errors in concrete-curing temperatures, have already been repaired by surface sealants.
Meanwhile, the tug Alaska Mariner is expected to reach the Hiram M. Chittenden Locks in Ballard by Friday afternoon with Pontoon A, a large piece that will form the west end of the floating roadway. A diagram of pontoon locations can be found here, and below is a diagram of the pontoon retrofit strategy.