(Above) Workers wrangle a pre-cast concrete bar that will be turned into a walkway underneath the Dearborn Street Bridge. (Click on images to view larger versions.)

September 6, 2014 – The first of the new Chicago Riverwalk’s under-bridge walkways is being installed. The 152-foot-long rectangular piece of concrete was cast off-site and delivered to the Dearborn Street Bridge on Tuesday morning.

It will fit under the south leaf of the bridge and connect the Riverwalk between stretches called Marina Plaza and The Cove.

(Above) Seen east of the Wells Street Bridge, the tugboat Buckley delivers the pre-cast concrete to the Dearborn Street Bridge on September 2 at about 10 a.m. (Photo by Chicago Department of Transportation.)

A tight workspace is the project’s biggest challenge. In a ten-minute video produced by Sasaki Associates, an international design firm working on the new Riverwalk, managers describe the project as surrounded by concerns.

“We’ve got two layers of roadway bordering the southern end of the project, we’ve got a waterway on the northern end, we’ve got seven bascule bridges, and then we’ve got all types of transportation surrounding that, from water taxis to pedestrians to motor vehicles,” says Zach Chrisco, a project manager for Sasaki. “It’s about as complex as you can get for a site work project but I think that we really enjoy that challenge.”

Zach Chrisco

While digging into the riverbed, workers must dodge tunnels, cables, and utilities. Divers are used to scout for obstacles, according to Dan Gross, Senior Resident Engineer for the engineering firm Alfred Benesch & Company.

“You never know what you’re going to find down there for sure until you go down in there,” says Gross. “We’ve had the contractor send divers down in there, make sure all the cables are located, [so] then when they’re driving the piles or drilling the caissons, we miss it. And we definitely don’t want to hit any tunnels or anything like that.”

Dan Gross

Construction extends no farther than 25 feet into the river to allow boats to pass.

During the week of September 8, workers will install water pipes and drainage structures, pump water out of spaces between old and new river walls and replace it with concrete, and build other wooden forms into which concrete will be poured.

• Video: Chicago Riverwalk under construction
• Previous story: First under-bridge river walkway arriving soon
• More photos: Construction of the Chicago Riverwalk