Mississippi Riverdance

Replacing two bridges spanning the Mississippi River puts pumping ingenuity to the test

One of the key components to ensuring a successful concrete pumping project is positioning the pump for maximum production and optimum reach. Impediments to doing so—lack of available space, structures blocking access and so on—can dramatically affect those efforts. Take that pumping effort out onto the water, however, and an entirely new set of variables is added. That’s what Lunda Construction and its subcontractor Sun Concrete Pumping are dealing with as they replace a pair of existing bridges linking Iowa and Illinois in the Quad Cities area. A combination of difficult on-water access, unique mix designs, inclement weather and massive volumes (to name just a few) have all made their presence known at one point or another. But meeting those challenges with innovation and solid pump performance—including a fleet of Schwing pumps in triple-pump, barge-to-barge configurations—both firms are past the mid-point of the project and on track to bring Quad City cross-river traffic into the 21st century.

Shafts and Slabs
To anyone who lives in the upper Midwest, the name Lunda Construction is as familiar as a cold winter morning. The contractor is attached to almost every major bridge project undertaken in the region, testament to the expertise they bring to each project. So it’s no surprise that, when the I-74 spans were slated for replacement due to a combination of traffic far beyond its designed capacity and ongoing wear, Iowa DOT committed to the Black River Falls, Wisconsin-based company. A good deal of that experience is being brought to bear on the I-74 bridge job, according to Garrett Clifton, one of the contractor’s project managers.

“We started this project in July, 2017 and are moving along nicely, despite the challenges of working over water and dealing with weather issues such as bitter cold and flooding,” said Clifton. “Working with Sun Concrete Pumping and a fleet of pumps ranging from 36-meter to 61-meter units, we’ve established a really nice routine to keep things moving. There are sections of the bridge—the abutments, approaches and associated ramps, for example—that are still on dry land, but the bulk of the structure is over water and we’ve done a very good job of making those pours happen quickly, safely and efficiently.”

Work on each bridge’s 28 (on water) piers started with a series of drilled shafts and steel casings which measure between seven and 10 feet in diameter, followed by construction of a coffer dam and pumping of a seal slab for the arch piers, plus the use of a footing tub form system that is lowered in the river, and pumping of the seal slab for the approach piers. The drilled shafts were barge-pumped using an S 39 SX pump, with concrete volumes ranging from 30 to 150 cubic yards per shaft. Following installation of sheet piles for the coffer dam, each seal slab was pumped, according to Shawn Jackson, the pumper’s Eastern Iowa manager.

“To create each slab, we lowered the boom, with a discharge hose attached, into the water and, once we hit river bottom, lifted up about eight inches and started pumping,” he said. “The concrete comes up around our pipe for two feet, which eventually seals the bottom of the coffer dam. We really don’t even see what we’ve poured until the hole is dewatered. The volumes for the seal slab pours varied according to the size of the pier it would eventually support, but some were in the 400- to 1,000-yard range. In fact, at this point, I believe we’ve pumped more concrete under the water than above.”

Offshore Account
Despite their previous experience with such matters, both Clifton and Jackson are in agreement that positioning the pumps for each offshore pier pour was an exercise in logistics. While the piers closest to shore could be accessed from land using either a 58-meter or 61-meter unit, things began to get interesting from that point forward, according to Clifton.

“For Pier 3, which was located about 200 feet from shore, Sun double-pumped from a 46-meter to a 39-meter pump to give us the reach we needed,” he said. “Then, to pour the 840 yards of concrete needed for Pier 14, we used a 46-meter pump onshore and two barge-mounted 58-meter pumps, one at roughly mid-point, the second moored alongside the pier. That was probably a distance of nearly 500 feet, but it got the job done nicely.”

One of the biggest challenges Lunda faced was trying to determine the ideal barge placement that would both allow them to “spud” (drive pipes into the river bottom, thereby providing barge stability for working) and also provide the reach they needed for each pour. While pump selection was based on the amount of reach needed, that was not always as cut-and-dried as one might think, said Clifton. “Obviously, the height of the structure being pumped or the distance off the barge is key. However, some of the areas in which we had to work could get very congested, forcing us to set the barge with the pump truck off a bit, which dictates the need for a longer boom. But the pumper had all the equipment we needed.”

Once the “sweet spot” for barge/pump placement was determined, the pumper would back the 46- or 58-meter pump up to the edge of the barge so that the hopper extended off the end. Then, the barge with the ready mixed trucks—concrete was generally delivered about 40 to 60 yards at a time on each barge, depending on logistics of pour—was positioned next to it and drivers would back up to the pump’s hopper, much as they would do on land.

To maximize efficiency, the concrete supplier, Hahn Ready Mix, used a fleet of 12 to 16 trucks working off two barges—four trucks out on the water unloading, four loading up on the next barge, four batching and four washing out and prepping for the next round. On pours that they knew would be slower, the number of trucks was cut to four sets of three.

Reaching Critical Mass
Because many of the elements on the I-74 spans were so massive (one pedestal structure at the base of the arch contains nearly 2,400 cubic yards itself) and the dimensions of each of the structures were so large, they fell under Iowa DOT’s “mass concrete” specs. In addition, many of the elements on the project—the approach columns, arch extended pedestals, arch crossbeam and ribs—are also specified for self-consolidating concrete (SCC), prompting Hahn Ready Mix, to make dramatic changes to the mix design.

“Each one of those things presents its own challenges, but a combined mass/SCC mix was a first for us,” said Griffin Hahn, the company’s production director. “To make that happen, we replaced 70 percent of the cement in the mix with granulated slag which, while bringing extraordinary strength, sets slowly and has a much lower heat output when it hydrates than cement. For the summertime work, we also injected the mix with liquid nitrogen to further minimize heating.”

To that last point, Hahn said that, if they had been running a dry batch plant, they could have simply replaced all the water in the mix with ice to keep temps lower. However, because they were operating a central mix or wet batch plant in which the concrete is completely mixed in the plant, they didn’t have that option.

“We needed concrete that, after an hour and a half out on the river, could still be around 55 degrees Fahrenheit in the middle of summer—ice was not going to cut it. So liquid nitrogen was the answer, and it was another first for us. We had a number of nozzles installed on the belt that feeds our central mix drum, and we sprayed all of our rock and sand with liquid nitrogen as it went into the drum, effectively cooling it before it was mixed. It worked like a charm.”


 Step Up to the Bar
While modifying and super-cooling the mix was effective in dealing with the parameters of the job, it also gave the pumper a real-time opportunity to see how its pumps would react to the heartier mix. According to Jackson, he could not have been more pleased—or more impressed—with how they responded.

“The pumps have definitely risen to the challenge out here,” he said. “At many points throughout the project, we were at 300 to 350 bar for every stroke—just maxing it out. When you subject a pump to that for 15 to 20 hours a day without stopping and have no problems, it speaks very well to how tough those pumps are built. In fact, there were times when we went 30 hours straight under those conditions—it was very impressive, even for us.”

 

 
Pumping West to East
Lunda completed their last pours of all the offshore piers in mid-February and will continue work on the structure’s iconic basket-handle arches before switching the focus to deck work.

“Once the westbound structure is open, it is expected that IDOT will allow periodic closure of a lane to allow us—in areas in which it is feasible—to pump the eastbound deck,” said Jackson. “In fact, in sections where westbound deck has already been completed, we’ve used a 58-meter unit to pump an adjacent pier and it was a nice luxury. But if we have to pour the decks from a barge below, we are ready for that as well. We have our sea legs now and know the pumps can more than handle it.”

The I-74 bridge is slated for spring 2022 completion.
 

 

SPECS:

General Contractor: Lunda Construction, Inc.—Black River Falls, Wisconsin
Pumping Contractor: Sun Concrete Pumping—Grimes, Iowa
Concrete Supplier: Hahn Ready Mix—Muscatine, Iowa
Design Services: Alfred Benesch & Company, Chicago, Illinois
Equipment: Schwing S 39 SX, S 46 SX, S 58 SX and S 61 SX truck-mounted concrete pump with placing boom