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WSDOT says cracks in new SR-520 pontoons are par for the course

You may have heard about some issues with the large concrete pontoons being built for the new 520 bridge across Lake Washington. Several of them sprung leaks and cracks after the cement set in Aberdeen.

But the state Department of Transportation says drivers don’t need to worry; the situation is under control.

Click the "play" icon below to see video of State Construction Engineer Jeff Carpenter describing how pontoon cracks are being patched to ensure they will last 75 years, as stipulated in the design plan:

KPLU went on a media tour of the floating construction site, just north of the old bridge.

Radio Script

Big cranes and construction barges line the shores of Medina, where the easternmost span of state-route 520 meets land. This is where the pontoons that have been floated in from Aberdeen and Tacoma are located now.

"What you’ve just crossed over from the east are the columns of what will be the fixed structure," said WSDOT's SR-520 Program Director Julie Meredith as she led the tour of the emerging replacement bridge. WSDOT wants to show more transparency as the repair process continues, Meredith says.

The big concrete floats are full of hollow chambers  and named after the letters in the alphabet.

“And we’re going to walk across pontoon W,” Meredith said. The pontoon is nick-named Wendy.

Each one is about the size of a soccer field. We pass some dive teams, who have been examining the underwater surfaces of the new bridge structures.

She takes us down a steep ladder, into the belly of pontoon V.

Inside the chamber, Meredith says this one is in good shape. Her voice echoes as it bounces off the concrete walls that have been marked with chalk in places, which she points out with a special flashlight.

"This is a different kind of crack, this is a shrinkage crack. And that's part of the repair treatment we did on that. Not a problem," she says.

In the next chamber there’s evidence of some major leaks, which have been patched with a special kind of epoxy-like substance. But WSDOT says they’ve brought in an independent expert and there’s nothing to worry about. 

“All of the pontoons that we made have cracking to some level. When we do concrete pours of a significant volume and size that we did, we expected cracking,” Meredith says.

Still, there are ongoing inspections to make sure the repaired pontoons won’t leak over the long haul. And the state is negotiating with the contractor to decide who will pay for the repairs and delay.

WSDOT is confident it can stay on schedule and move ahead with the 520 replacement work. It expects the new bridge to be in service for 75 years. It’s scheduled to open to traffic in the summer of 2015.

Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to