I-5 Stillaguamish River Bridge project making good progress

By Tom Pearce

Replacing an 81-year-old bridge deck full of ruts, cracks and holes is hard enough. Add to that the bridge being on a heavily-used highway, and you've got some serious challenges. But in the case of the I-5 Stillaguamish River Bridge, our contractor crews are rising to the occasion.

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I-5 Stillaguamish River Bridge
size 18 pothole.
While the steel superstructure of the bridge is holding up well, the old deck was worn out. We've repaired it a number of times, but with exposed rebar and potholes that dwarf a size 13 boot, it was time for a full replacement.

The $8.7 million project is scheduled for completion in mid-November. Since we reduced traffic to two lanes in each direction and shifted southbound I-5 to half of the northbound I-5 bridge, the contractor, Mowat Construction, has made good progress.

Working from north to south, the old southbound deck was removed. That took careful planning and a lot of skill. The 607-foot long, 48-foot wide, 8-inch thick concrete deck was cut into roughly 6-foot by 8-foot pieces that weigh up to 4½ tons, then each piece was removed by a track hoe fitted with a special lifting attachment.

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I-5 Stillaguamish River Bridge removed 6-foot by 8-foot deck section.
As the deck was being removed, another crew followed along to inspect the underlying steel framework. We were pleasantly surprised by what they found – most of the steel was in very good condition. We had to replace just four pieces, like this stringer, one of 216 that run lengthwise to support the deck. It’s a lot stronger than it looks, but we wouldn't want to leave it there for another 50-plus years, which is how long the new deck is built to last. The rest of the original steel was checked, then sandblasted and repainted to protect it for decades to come.

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I-5 Stillaguamish River Bridge rusted stringer.
Crews were still working on the steel framework when the concrete forms were placed. Once that was complete, the green coated rebar was put down. That will keep the concrete strong for another 50 years or more. The coating will help keep the rebar from rusting.

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I-5 Stillaguamish River Bridge green coated rebar.
There still is a lot of time-consuming work to be done, but we’re on schedule to open as planned in mid-November. Once the new deck is poured, probably starting in early October, it will take about three weeks to cure. When that’s done, the deck must be grooved to improve traction in the rain, the guard rails will be reinstalled and the road will be restriped for three lanes of traffic, along with other minor work. 

As the deck cures, the contractor crews won’t just be sitting around watching concrete harden. They’ll be busy repaving the bridge approaches. After all, the approaches have been handling traffic for 81 years too.

When everything is ready, the contractor will remove the barriers on southbound I-5 to restore traffic flow onto the new bridge deck. For northbound traffic, it’ll take a few days to remove the concrete barriers from the middle of the northbound I-5 bridge, then a couple more days to restripe the northbound bridge for three lanes, but we expect to have I-5 back to its original configuration, just in time for holiday travelers.

A note on traffic
Traffic flow was a big concern when this project was being developed. We had to take an interstate with three lanes in each direction, put in crossover lanes and reduce it to two lanes that are more narrow than normal, all while sharing a bridge that is wide enough for four lanes, but with small shoulders. 

At the Stillaguamish River, the two bridges handle a total of 80,000 to 100,000 vehicles per day. We anticipated backups at peak hours, but they've been less than we expected. The longest delays typically have been about 15 minutes, as some drivers have adjusted their travel times or selected alternate routes when their destination was Stanwood or Arlington. 

We appreciate the patience of motorists through this project. You've helped keep the backups manageable. Keep it up; in less than two months you’ll have a brand new bridge deck!