MARYSVILLE — Drivers on State Route 529 will still be using the existing 85-year-old Ebey Slough Bridge between Marysville and Everett for a while yet, but its replacement recently reached another milestone in its construction progress.

“Feb. 15 was the end of the window we had to get the columns installed,” said Joe Rooney, chief inspector for the Northwest Region of the Washington State Department of Transportation. “From here until mid-May, they’re doing the structural work to get the piers ready for the girders. The earliest we expect to get those girders is May 15.”

Five piers will support the four girders running the length of the bridge. The two piers on each end of the bridge are supported by 10 columns each, with each piling measuring 48 inches in width, while the three piers in the middle are supported by four columns each, with each piling measuring 72 inches in width.

“The pilings in the middle needed to go down as far as possible, while the pilings on the ends didn’t need to be as deep,” Rooney said. “We wanted to minimize the number of pilings for those middle piers, but the pilings on the ends could be smaller and more numerous.”

The girders will span approximately 120-140 feet and will stand roughly seven feet tall. Each concrete pier is being constructed with gaps in its rebar for the girders, so that the steel girders can be embedded in the concrete. Rooney pointed out that this preplanning leaves zero margin for error.

“We’ll be grading everything so that the girders will sit exactly at the level they need to be,” Rooney said. “This can’t be off even by a little bit.”

According to Rooney, construction crews take the surrounding environment as seriously as they do the engineering of the bridge itself. To accommodate the fish which make Ebey Slough their habitat, Granite Construction worked through November of December of last year in order to have their columns and work platforms installed by mid-February.

“Check out these work platforms,” Rooney said, showing off the mostly clear wooden walkways around the piers. “You’re not going to see many that are this clean on other construction projects. We don’t want any debris getting out into the wetlands.”

The construction work on SR 529 extends north of the bridge to just south of its intersection with First Street in Marysville, as crews widen the road on its east side, carving out a new curb and gutter while marking off a new sidewalk area.

“We’ll have a six-foot sidewalk and a new bike lane,” Rooney said. “We’re replacing two 11-foot traffic lanes with four 12-foot traffic lanes, with sidewalks and bike lanes on both sides.”

The project is divided up into three stages, with the current first stage wrapping up in February of 2012. While all traffic will continue to use the existing Ebey Slough Bridge during stage one, northbound traffic will move to the new bridge during stage two, as construction crews realign SR 529 from the old bridge to the new bridge. Stage three kicks off in April of 2012 and is set to run to February of 2013, during which construction crews will remove the old bridge, complete the final paving and striping in the project area, and move all traffic to the new bridge.

For project details, graphics and video, visit www.wsdot.wa.gov/projects/sr529/ebeysloughbridge.