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Work has begun to replace a section of permeable paving on Davenport's Fairmount Street that has not performed as envisioned when it was installed during 2016 to reduce flooding from stormwater runoff.

Langman Construction, of Rock Island, received a bid of $300,000 to replace a 400-foot section of roadway with concrete, south of West Central Park Avenue to Lombard Street, Amy Kay, clean water manager for the city, said.

Work began Oct. 2 and should be finished by the end of November.

The permeable paving — pavers installed with small gaps between them to allow water to run down into the sub-surface, rather than sheet off or pool — has sunk, creating gaps on either side of the permeable section where it meets up with the concrete road, Kay said.

The problem was that the project was under-engineered, she said. "It wasn't robust enough to withstand the amount of traffic that the road gets, and there is a structural support issue with the soil below," she said.

That stretch of Fairmount carries an average of 8,500 vehicles daily, according to Iowa Department of Transportation data, she said. This includes trucks and buses.

The 2016 work was a grant-funded pilot project.

The construction now underway, designed by a professional engineering firm, will include the installation of more underground drainage tiles. The tiles will move stormwater into the rock layer built under the street when the pavers were laid, providing detention and infiltration of water, keeping with the original intent of reducing flooding, she said.

Kay defended the use of permeable paving as a way to reduce flooding.

"Because this project (on Fairmount Street) has issues, the perception is that permeable paving doesn't work," Kay said. "That's not accurate."

Two lessons learned from the project are that first, the cross-section of soil, rock and geo-fibers under the street must be designed to meet the demands of traffic and a great deal depends on the composition of the soil, she said.

Second, the composition of pavers also makes a difference. The pavers on Fairmount were made of concrete and some have "spalled," or deteriorated, through freeze-thaw cycles and the corrosive effect of road salt, Kay said.

Tougher materials are available, she said.

At the same time in 2016 that the Fairmount pavers were laid, a similar stretch was laid one block west on Gayman Avenue, a street that carries less traffic that also is slower-moving. Although there has been some sinking of the Gayman pavers, it isn't to the point of Fairmount, Kay said. "It's holding up much better."

Together the two areas were built to detain and infiltrate about 22,680 cubic feet of water.

The city has two other chunks of permeable payment, a two-block long stretch on Dover Court that was installed during 2014, and an area at 5th and Brady streets.

Dover Court "is holding up just fine, too," Kay said.

While Fairmount is torn up, the city advises drivers to use Division as the alternative north-south route because Lincoln remains closed through October and Clark has school traffic.

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