Blue Bridge No. 538 - Bridge Retrofit

July 2016 Update

Snohomish County Public Works will be returning a 1,300 foot strip of Monte Cristo Grade Road, that is no longer in use, to its natural habitat. This project is part of Snohomish County’s effort to mitigate the environmental impacts of the retrofitting of Blue Bridge 538 that occurred in 2013.

Mitigation includes removing gravel and road material and replacing it with mulch and wood chips to support plant growth. Hundreds of native plants will be planted in the area and logs will be placed to support animal habitat.

Construction will begin on July 25, 2016 and is targeted to be completed in September 2016. No road closures are expected, but large trucks and operating equipment will be traveling through the area to access the work site. Workers will return in October 2016 to plant native vegetation.

View information postcard, July 2016.

Description

During 2013 the bridge was retrofitted with deep-drilled piers at the west end to repair damage caused by earlier flooding. The embankment received extensive reinforcement and an engineered log jam was installed on the upstream side to prevent erosion.

Since 2006, both ends of the bridge have required extensive hydraulic, structural, and environmental planning and construction to repair damage. An east side bank stabilization project was completed during 2010.

The bridge crosses the South Fork Stillaguamish River on the Mountain Loop Highway 12.06 miles outside of Granite Falls. It is a two-lane, 160-foot long structure built in 1954 consisting of three spans: a 160-foot long steel through-truss main span and two 24-foot cast-in-place concrete approach spans. The spans are supported on two concrete piers and two concrete abutments, all founded on concrete spread footings. View photos on Flickr. 
A major flood event in 2006 on the South Fork of the Stillaguamish River eroded approximately 150 feet of the left bank upstream from the east pier, causing the bank to recede approximately 25 feet. The flood also washed away the soil fill between the west pier and west abutment at the bridge.

 
Schedule
Retrofit/repair of the bridge at the west end was completed in 2013. View the newsletter below for more details and photos: