What do a bridge in Scanlon, MN and Bob Dylan have in common?

• Dylan’s sixth studio album is called “Highway 61 Revisited” and the bridge is on Highway 61 in Carlton County.

• A songwriter and an engineer each make plans and assemble resources, in just the right amounts, to support the performance.

• Both a bridge and a song have the ability to transport you.

Carlton County engineers were singing the blues about the bridge, which was built in 1987. It’s a prestressed concrete girder bridge. According to the Minnesota Department of Transportation, 5,000 vehicles per day drive on it. Over time, the approach to the bridge coming from the west deteriorated, which required considerable maintenance. The surrounding soils are peat and red clay, a challenge to build on.

In 2015, a long-term remedy to the problem was implemented. Tire Derived Aggregate (TDA) was installed as lightweight fill. TDA stabilized the approach by creating a permeable, stable subgrade to float the road over the poor soils. The shreded tires interlocked and created a snowshoe effect to support the structure.

According to the Federal Environmental Protection Agency, red clay can weigh up to 3,000 pounds per cubic yard when wet. By comparison, TDA has a nominal weight of approximately 600 pounds per cubic yard.

For the bridge project, 3,120 cubic yards of TDA was installed underneath the bridge approach. This lightened the load by 7,488,000 pounds (3,744 tons) across the 400 foot bridge approach.

Carlton County engineers designed the bridge project, and Ulland Brothers, Inc. did the contract work. Carlton County has a successful history of combating red clay with the innovative use of TDA.