Crews Create Land Over Water for New Riverwalk
By John Gregerson, ENR Midwest
Now that a small flotilla of barges has largely retreated, the main branch of the Chicago River no longer resembles a rush-hour jam on Lake Shore Drive. However, crews still have plenty to do on land to propel the second phase of Chicago's Riverwalk—a $43-million, 900-ft-long walkway along its south bank—to a Memorial Day opening. In mid-December, crews were pouring substructural and structural concrete for River Theater, one of three 300-ft-long themed "rooms" bracketed by bascule draw bridges spanning the river. As with the adjacent Cove and Marina Plaza, crews also were completing cap work and formwork for walls, stair footings and planters, in addition to the installation of granite pavers.
Come spring, they will regroup to begin work on the third and final phase of the $98.6-million, 0.7-mile project, a commercial and recreational venue that upon completion in 2016 will extend nine city blocks, from Lake Michigan through the heart of downtown. As with phase two, the Chicago Dept. of Transportation (CDOT) and a project team including Chicago-based contractor Walsh Construction will orchestrate the activities of up to 12 barges in efforts to extend existing frontage 25 ft into the 200-ft-wide channel and construct underbridge connections to join each of the block-long rooms. They will deploy divers to investigate existing conditions and round-the-clock crews to negotiate the narrow time frames provided to drive 75-ft-long caissons through water and soil to bedrock. And, as before, builders will have only half the width of the river to "build land on water," says CDOT project manager Oswaldo Chaves.
With phase three, "we should have a better idea of what to anticipate, based on our experiences with phase two," says Dan Gross, senior resident engineer with Chicago-based Alfred Benesch & Co., Riverwalk's structural engineer. "On the other hand, you never know what lies beneath riverbed."
Nor could team members have anticipated extreme weather that froze the river last winter and delayed phase two's start date by a month, nor heavy spring and summer rains that flooded portions of new structure and substructure, delaying work while crews pumped water back into the river. Last July, a 50-ft-long, 20-ft-wide barge carrying a 21,800-lb telescopic boom lift took on water and sunk to the river's bottom, requiring a diver to secure a pair of cables to the lift and a crane to hoist it out of the water. Crews then refloated the barge by pumping air into it and water out of it.
Team members agree the mishaps have been outweighed by the prospect of creating a venue to rival Chicago's Millenium Park by introducing fishing piers, walking trails and performing arts to the city's central business district. "It's a project that's been planned and thought about for 30 years," says Gina Ford, principal with Watertown, Mass.-based architect Sasaki Associates Inc., a member of a design team that includes Ross Barney Architects and landscape architect Jacobs/Ryan Associates, both of Chicago.
The $9.5-million phase one, extending 17 ft into the river, was completed in 2009, but funding issues and administrative changes in city government, including election of a new mayor, delayed construction of phases two and three. In the interim, the project's architects collaborated with Benesch for phase two, "evaluating numerous structural iterations that supported emerging architectural concepts while addressing issues of constructibility," says Sasaki project manager Zach Chrisco.
A chief issue was land—or a lack of it, says Chaves. Because closing Wacker Drive, a major east-west artery extending alongside the river, wasn't practicable, all signs pointed to a solution that best supported staging the majority of construction on water. Last February, crews began executing a plan that called for demolition of existing landscaping, sidewalks and sheet pile caps at the site of each room, followed by installation of continuous steel sheet pile wall opposite existing dockwall; placement of crushed stone backfill in the 25-ft gap separating new and existing walls; and construction of sheet pile caps, structural slabs, concrete sidewalk and pavers.
"It proved the most logical and cost-efficient approach of leveraging the waterway and mobilizing the equipment required to construct structure and substructure," Chaves says.
Team members took a different tact with each of the three underbridges, specifying precast concrete "tubs" positioned atop four newly installed caissons. Then, workers fitted them with rebar before crews poured concrete to create a solid cap.
"The thinking was to flow forms down the river and incorporate them into existing structure rather than have divers construct formwork in the river," says Benesch project manager Matthew Hellenthal.
But divers did figure into plans for driving into bedrock the 75-ft-long, 6-ft-dia caissons—12 in all—upon which underbridges rest in order to ensure that crews didn't penetrate a subway line extending beneath the river once drilling began. After divers reported the precise locations of subway perimeters, team members further hedged their bets by locating vibration sensors in the tunnel to continuously monitor conditions during operations, Gross says.
Though drillers avoided the subway tunnel, they weren't as lucky with telephone equipment beneath the river that city officials say was undocumented. No major damage occurred, but work was delayed a week while crews regrouped to cut off the caisson, fill it and re-drill a second one several feet away. Operations also were interrupted, from mid-April to mid-May, when a mechanical hammer required to drive steel casing into soil malfunctioned and required replacement.
Once work resumed, it proceeded at a brisk pace. Because caisson drills required that adjacent drawbridges—three in all—be raised, crews were provided only a week to complete work at each location, during which time they worked 24 hours a day.
In each instance, workers first drove casings down to their required elevations, employing a caisson rig to drill out soil from casing interiors to required bearing elevations. Next, cranes lowered full-length circular rebar cages into the casings before operators of a 130-ft-long hose filled them with concrete—about 2,000 cu ft per casing—pumped from mixers on Wacker Drive.
As with other Riverwalk components, liner shells arrived at their locations via barges, the majority of which were loaded at a Walsh company yard southwest of Chicago's Loop, then guided upriver via tugboats. City officials say the completed installations are designed to withstand a hit from even the largest of barges traversing the river. "The objective was to develop a design that engaged all four caissons to resist forces in unison, which we were able to achieve through the detailing of the caps," says Hellenthal.
Work on the underbridges in some instances overlapped with remaining portions of the project, including installation of sheet piles forming the river wall and H-piles that anchor Riverwalk to riverbed, transferring loads to layers of earth capable of supporting it.
Rather than work sequentially, "We essentially built out all three of the rooms simultaneously," Gross says.
"Site constraints were about as challenging as we've ever encountered, given the need to maintain traffic both on Wacker Drive and along the river," Chaves says. As with concrete pours, CDOT closed portions of Wacker during evening hours to stage deliveries of construction equipment and materials.
It was largely up to Walsh and the project's engineers "to keep crews working and out of each other's way," says Chaves, who notes the task entailed choreographing the movements and locations of barges. "Their sizes varied," says Gross, "but at a minimum we had a larger and smaller one in each of the bays where we were building out into the river."
Prior to driving sheet piles and H piles, divers again were recruited "to account for the locations of tunnels, cabling and other subsurface facilities," says Benesch project manager Kurt Knaus.
As operations proceeded, crews employed hydraulic hammers to pound crane-held H-piles into riverbed at 10-ft intervals. After pile drivers drove steel sheet into riverbed, workers installed steel tie-rods and walers to stabilize the emerging wall.
Upon completing operations, workers pumped water out of the 25-ft gap between dockwalls and river walls, then deployed crane-held clamshell buckets to backfill spaces with tons of crushed aggregate.
"Operations required several large pumps, both on barge and landside, to dewater those spaces," says Gross. Divers simultaneously extended intake and drainage pipes from dockwall through sheet pile so river water could continue serving the mechanical systems of surrounding buildings. They also laid new electrical and plumbing conduit in support of new lighting and commercial uses for Riverwalk.
Next came extensive concrete pours to construct pile caps and other structural elements, including a deadman extending between old and new river walls.
Now that phase two is nearing completion, "there's a sense of excitement about all the opportunities Chicagoans will have to experience the river," says Chaves. In addition to themed spaces, visitors will encounter bicycle paths, benches, recreational spaces and sustainable native plant landscaping to support habitat protection. "It's not every day you're provided the opportunity to create land," says Chaves, "or put it to these types of uses."