SR 19 / Alder Creek Interchange
A new diamond interchange with two precast girder bridges, 1.8 miles of widening, two roundabouts and a separated shared-use path.
What made it hard.
The interchange had to be built over a live four-lane highway carrying 48,000 vehicles a day, with a salmon-bearing creek crossing the alignment and a gas main running beneath the new bridge footprint.
The in-stream work window was limited to six weeks, and the corridor could not be closed during the summer tourism season.
How we built it.
We used accelerated bridge construction: precast girders and deck panels were fabricated off site and erected during two weekend closures. The gas main was relocated during an early-works package before the bridge foundations began.
In-stream work was completed with a temporary clear-span crossing, keeping equipment out of the creek bed. Roundabout construction was phased to maintain one lane in each direction during peak hours.
Where it landed.
The new interchange opened to traffic four days before the Thanksgiving holiday. The separated path eliminated a quarter-mile gap in the regional trail network, and the corridor now moves an estimated two million more vehicle-hours efficiently each year.
On site.
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