The crown jewels of the new Interstate 74 bridge — its high-rising arches — are so complicated to build, transportation officials say, progress is slowed again.
Little headway was made in July on the arches that will rise above the westbound span of the new bridge, which originally was to be finished in just a couple of months. The anticipated completion of the westbound (Iowa-bound) span already has been pushed into the middle of next year, following a particularly harsh winter and multiple spring record floods in the work zone.
But the lead agency for the project, the Iowa Department of Transportation, was expecting progress on the arches to move more rapidly when Mississippi River floodwaters receded. While the river has been below major flood stage since mid-June, the arches remain at just 10 erected segments out of a total of 30 that will go into each span.
"The contractor has experienced some challenges with installing the stays and pulley system," said Danielle Alvarez, I-74 project manager for the Iowa DOT. "It’s a difficult process and requires extreme precision, because the stays will be used to adjust the angle of the arch segments to ensure the correct fit.
"The true arch design is unique and challenging, and we are working closely with the contractor to identify and resolve issues as quickly as possible."
More specifically, the delays are being caused by difficulty in getting cables from their concrete housings that are built into a pier near the shoreline to thread through the blue/green towers that stand between the arches and the shore. The cables must then be placed onto the arch segments themselves, because they will be used to help direct the positioning of each added arch piece.
"It’s taking time, because the stays must be meticulously placed and threaded through the mechanism at the top of the tower," Alvarez elaborated last week in an email. "This process is critical, and everything must be positioned accurately before any new segments can be added to the arch.
"As we mentioned, if the alignment of the arch is off this early in the process, it would create fit-up issues down the road. Accuracy is key at this stage."
The cables running from the housings (also called anchorages) to the tower are back stays, and the ones running from the towers to the arches are called fore stays. The towers are more than 200 feet high, so the process of precisely looping cables through them with cranes is tricky work.
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Once the cables are placed, though, the progress on adding more arch segments will happen more quickly, Alvarez said.
"The contractor is also planning ahead for when the arch is near completion," she said. "They are coordinating with the U.S. Coast Guard to obtain approval for briefly closing the navigation channel when the last few segments of the arch are ready to be set.
"The arch meets in the middle of the navigation channel, and the contractor will need to station a barge and their equipment in the channel in order to set the last few segments. The channel can only be closed for brief periods of time and requires prior approval from the USCG."
At this point, it does not appear it will be necessary to request a channel closure in August, because the arches will not be ready, she said.
By using the pulley system for guiding the arches toward their meeting point in the center, the contractor avoids the need for long-term obstructions in the navigation channel. Barge traffic already took a major hit this spring when long-term flooding closed the river to navigation.
"We had hoped that the arch erection would proceed smoothly after flooding receded, but the challenges with the stays and controlling proper arch alignment have slowed our progress," Alvarez wrote. "Progress on other parts of the bridge continue, including constructing the deck over the piers south of the arch as well as pre-assembling the arch flooring system (the section that will run the length of the arch and support the concrete deck)."
Much of the arch work is being done by members of Ironworkers Local 111, which is based in Rock Island. Earlier this summer, 53 ironworkers from 111 were working on the bridge, union leaders said, and another 30 were expected to be called into action by July.
While many ironworkers continue to work on the bridge decking for both spans, very little activity has been underway near the arches.