By Mason Nichols
The history of Chicago is intertwined with the waterway that runs directly through it. Settlers in the area relied heavily on the Chicago River for the burgeoning lumber and meatpacking industries in the 19th century. Along with industrialization came increased use of the river for trade, resulting in Chicago becoming the fastest growing city in the world for several decades.
Today, Chicago is as busy as ever with nearly 3 million residents and continues to be a central hub for business and leisure. While use of the river for commercial purposes has declined, increased attention has been given to its aesthetics and recreational potential. The Chicago Riverwalk project aims to capitalize on the river’s beauty and central location with the construction of a 1.25-mile promenade connecting Lake Michigan with the heart of the city.
To make the new riverwalk possible, project owners turned to precast concrete.
Building land on water
According to Dan Gross, P.E., resident engineer for the project and senior vice president for Alfred Benesch & Co., constructing the riverwalk is similar to building a pier. Crews first drilled a series of caissons to support the underbridge structures on which the walkway would be built. Illini Precast of Westchester, Ill., then manufactured nine precast concrete “tubs” – varying from 45-to-84 feet in length and measuring 10 feet wide by 4 feet high – to function as the main component of the underbridges. Workers also had to widen the existing abutments 20 feet to accommodate the riverwalk.
“This is unique for us in that we’re basically creating land where there was none,” said Oswaldo Chaves, engineer with the Chicago Department of Transportation.
With limited storage space for project materials and the largest precast tub weighing 175,000 pounds, a unique solution was needed to deliver the structures to the job site. CDOT collaborated closely with each party involved, including the Army Corps of Engineers, Metropolitan Water Reclamation District and the Coast Guard, to ensure successful transport via barge.
Crews finished erecting the underbridge structures by fitting each with rebar and pouring additional concrete, forming a solid cap. Precast concrete pavers were then placed on top of each underbridge, completing the walkway.
The precast advantage
Before approaching the project, contractors narrowed their options for the underbridges to two materials – cast-in-place or precast concrete. According to Chaves, casting in place would have been more expensive.
“You’re building everything in water, so you would have to do a cast-in-place pour and formwork in water,” he said. “Given the complexity of that, doing a precast structure is more cost-efficient.”
Craig Wagenbach, project manager with Illini Precast, agreed.
“Precast is a great solution for this construction application, because casting concrete underwater is a rather expensive proposition,” he said.
Cost savings wasn’t the only advantage precast concrete brought to the project. The ability to manufacture the tubs in a controlled environment and ship them to the site helped crews stay on schedule.
“You manufacture the precast off site while you’re working on everything else or drilling the caissons, and then, when you’re ready, you just bring this right in and you’re not having to form,” Wagenbach said. “Forming and pouring – especially in the river – would take a longer time than just floating this in, setting it down and tying it in.”
When the Chicago Riverwalk project is completed in November 2016, it will provide residents and visitors alike with many benefits.
“The project offers a continuous path from the lakefront all the way to the west side, to the turning basin of the river,” Chaves said. “It also allows you to walk along the river without having to go up to street level and reduces street-level traffic.”
Gross added that nearby restaurants and bars should see increased business thanks to the riverwalk.
Although the river’s role in crafting Chicago’s history continues to evolve, one constant will remain for at least the next several decades – the durable precast concrete products that helped make its newest amenity possible.
Mason Nichols is the managing editor of Precast Solutions magazine and NPCA’s external communication and marketing manager.