Pontoon arriving in Seattle today

Pontoons for the 520 floating bridge are on the move as the summer construction season warms up in June and July, the state Department of Transportation said Thursday.

The latest pontoon built in Aberdeen is due in Seattle Friday while four others from the project’s first cycle are prepped for repair. Repairs will be made at offsite drydocks or on Lake Washington using a “coffer cell,” which is a steel box attached to the underside of pontoons.

The repairs were endorsed by an expert review panel after cracks were found in the original batch built by Kiewit Construction in Aberdeen. The cracks and leaks must be repaired to ensure safety and that the bridge meets it’s 75-year-long design life, the department says.

Kiewit/General/Manson, a joint venture, will tow one pontoon to drydock in Portland and another to Harbor Island in Seattle. The other two pontoons will be repaired on Lake Washington, the DOT said.

Cracks in keel slabs and endwalls on four of the first six pontoons will be sealed by injecting epoxy, adding post-tensioning across the ends and applying a carbon-fiber wrap to the keel slabs. Post-tensioning is the placement of steel cables through concrete and then tightening to increase strength. It’s common in construction, the DOT said.

The public can watch the arrival of the pontoon in Seattle late Friday through the locks in Ballard. Known as “Pontoon A”, it is the second of two cross pontoons that will form the ends of the new floating bridge. It will be moored temporarily at the east end staging area where crews will build columns atop the pontoon. It will later be moved to its permanent home at the west end of the bridge.

Longitudinal Pontoon T is expected to leave for Portland next week. The Portland and Seattle drydocks were chosen because of their capacity to support the 11,000-ton pontoons.

Using the offsite drydocks will enable the DOT to build the coffer cell, which will create a dry dock environment so the bottom of the longitudinal pontoons can be repaired.

Modifications were made in Aberdeen to Cycle 2 pontoons to include post-tensioning at the ends. Cycles 3 through 6 pontoons will include the change in design. The state is negotiating the cost of repair and schedule changes with the contractor.

The pontoons’ progress can be tracked on the DOT’s website.