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Construction on the new Interstate 74 bridge reached another milestone this past week.

For about 15 hours last Thursday, crews poured a massive amount of concrete into a large cofferdam to build the foundation for a pier that will support two of the new span's basket-handle arches.

The arches, which will tower above the bridge, are not just its most prominent architectural feature, but they will also serve to support the structure.

The work began Thursday, officials said, and involved sending 95 truckloads of concrete by barge from a staging area in downtown Moline to the cofferdam in the middle of the river.

Inside the dam, which had been pumped free of water, they carefully guided the concrete into place.

Danielle Mulholland, project manager for the Iowa Department of Transportation, said that Thursday was an important benchmark in the construction timeline for the bridge.

"This is the first very, very large concrete pour," she said.

For months, crews have been drilling shafts to support these foundations, filling them with smaller amounts of concrete.

This concrete pour was of a different magnitude.

In all, 933 cubic yards of concrete was put into place last Thursday.

The foundation, one of several to be built, is a concrete slab measuring about seven-feet in height and approximately 59-feet each in length and width.

The foundations, which are below the water line, will provide support for even larger concrete pedestals that will, in turn, provide a base for the vertical concrete ribs that support the arches.

Mulholland said it will be this spring or summer before the support structures will become visible above the water line.

Planners say they are building the I-74 bridge to last 100 years, and they have been eager to show off the work going into supporting the span.

The I-74 bridge's Twitter account last week posted two videos of the process.

Construction of the new bridge is expected to take three-and-a-half years. Work began last summer. Officials say the Iowa-bound span is expected to be completed and put into use in 2019. That will accommodate traffic while the Illinois-bound span is built.

The entire project is slated for completion in late 2020. The cost of the I-74 corridor, which extends from 53rd Street in Davenport to a mile south of Avenue of the Cities in Moline is estimated at $1.2 billion.

The north stretch of the corridor, from downtown Bettendorf to 53rd Street has not yet been approved for funding. This part of the corridor is expected to cost about $150 million.