LYNNWOOD — The intersection at Ash Way and Maple Road was built on a peat bog. That’s been a documented problem as far back as 1998.
The peat can’t support that kind of weight, and the crossing has been sinking about an inch a year. Adding more pavement just makes the road heavier, and worsens the problem, said Robert Victor, a project manager for the city of Lynnwood. Now a permanent fix is scheduled.
Maple Road is considered a critical east-west link around Alderwood mall. Snohomish County, which owns part of the intersection, is partnering with Lynnwood. They are sharing construction costs of about $7 million, said Steve Thomsen, the county public works director.
The plan is to drive 8-inch-wide steel piles into the solid ground beneath the peat. It will require hundreds of piles, each averaging 50 feet long, Victor said. Similar systems are used to support piers and bridges.
The work is designed to raise the road about five feet. That would bring it above the 25-year floodplain for Swamp Creek. Where it sits now, the intersection has to be closed several times a year for flooding, said Jared Bond, Lynnwood’s environmental and surface water supervisor.
“There are bathtubs that happen whenever we get a large rain,” Victor said.
That affects traffic for the mall as well as Costco and surrounding businesses.
The construction includes about 700 feet of Maple Road and 600 feet of Ash Way. It is scheduled to start in spring 2018, after an open house, and could last five months. The road will be closed during the work, as well as the nearby Interurban Trail, with detours planned.
“We’re trying to get it done in the summer before shopping season,” Victor said.
About 7,300 vehicles travel Ash Way each weekday and about 5,900 use Maple Road. After construction, drivers are unlikely to notice the piles, which will be covered with infill.
The project includes culvert improvements for fish passage as part of a statewide court ruling.
Lynnwood has similar problems — roadway sinking into peat — on 196th Street SW near Wilcox Park and Scriber Creek and on 44th Avenue W near I-5.