Creating land on water is no easy proposition. Such a task is made especially difficult when it’s in Chicago, one of the most populated cities in the U.S. To help create a continuous walkway along the Chicago River from the heart of downtown to the lakefront, the Chicago Department of Transportation approved the only material that could meet all the project’s needs – precast concrete.
Utility Concrete Products of Morris, Ill., manufactured nine massive precast concrete underbridge “tubs” to support the river walkways. According to Tom Heraty, UCP’s vice president of sales and engineering, each of the tubs is 10 feet wide and 4 feet tall, ranging in length from 38 feet to 86 feet. The largest unit weighs nearly 164,000 pounds, representing the heaviest piece the company has ever manufactured.
Getting the precast tubs to the site represented a significant logistical challenge.
“In some cases, remote trailers were used because the tubs were so long,” Heraty said. “And you couldn’t get them downtown because the area is too congested.”
To solve this issue, each of the tubs was transported to a staging yard before being placed on specially designed barges. Once on the barge, the units were welded together, creating even larger pieces. The barges then floated the tubs down the south branch of the Chicago River to the installation site.
After arriving on site, the tubs – which were manufactured with a high-strength, high-performance concrete mix – were installed on piers via steel embed plates. Workers then applied a neoprene seal to ensure a watertight connection to the piers. Each tub was heavily reinforced and filled with concrete, creating a solid cap and completing the walkway.
In addition to boasting incredible size, some of the tubs required a special, cambered design.
“What this means is that the whole table the tub is poured on is slightly arched so that the middle is raised several inches above the edges,” Heraty said. “The calculations had to be done precisely so that when the piece is set and you pour the concrete in, the weight of that concrete pushes down on that camber.”
The cambered design allowed for a precise connection to a stainless steel canopy system, helping protect the walkway against runoff from the bridge decks above.
By selecting precast concrete for the project, the contractor saved significant time on site. Additionally, the finalized walkway, which opened to the public in October, will be able to serve Chicago residents and visitors alike for decades to come.
“Besides the on-site challenges of pouring within water, when you pour the concrete in a controlled environment, you’re going to get better results,” Heraty said. “You also end up with a lower cost of labor in a plant compared to doing this type of work on site.”