Treacherous crossing
Suspected military ammo delays Shreveport bridge project

By Sam Barnes

Just one span had been removed when the Louisiana Department of Transportation and Development halted the $5.1 million bridge replacement project over 12-Mile Bayou in Shreveport.

Rumors were circulating that unexploded World War II ammunition was lurking in the Shreveport bayou, so contractor Blount Bros. Construction of Shreveport had to move crews to other projects while divers scoured the bayou bottom.

But after nine months of searching they had found only 155 empty mortar shells.

"They presumably had been left there by a nearby military base," said Gene Ferguson, Blount Bros. jobsite superintendent.

When construction resumed in the fall of 2003, Ferguson had to re-evaluate his schedule and "speed things up a bit to make up some of the lost time."

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Ultimately, an existing 28-ft.-wide, 500-ft.-long bridge will be replaced with a 42-ft-wide, 720-ft.-long steel and concrete bridge. Much of the extra width comes from the addition of shoulders that will make the roadway safer and more accommodating to increased traffic flow.

The bridge crosses beneath Interstate 220, which is a bypass around the city.

Pharr Bros. Construction of Blanchard removed the existing steel and concrete bridge in about four weeks.

"They tore it down with backhoes and removed the steel girders from the shoreline with a 240-ton crane (operated by Joyce Crane of Longview, Texas)," Ferguson said.

Demolished girders ranged from 20 to 64 ft. long and weighed about 200 tons, and were hauled to a nearby scrap yard.

"The concrete was sent to our crusher to be broken down into road base material," Ferguson said.

In December, Case Atlantic Co. of Clearwater, Fla., began drilling eight 78-in.-diameter, two 60-in.-diameter and 33 36-in.-diameter auger shafts as foundation for the bridge's superstructure.

Greg Wall, the project's engineer with the Department of Transportation and Development, said only four of the shafts were drilled in the bayou.

"Those were necessary to support the bridge's longest spans, measuring 180 ft. and 130 ft.," Wall added. The remaining two spans measure 115 ft. long.

To drill in the water, the Case Atlantic crew drove steel casings through the bayou bottom, inserted augers through the casings and drilled to depths of 110 ft. with the help of a land-based crane.

"They drilled through a mud slurry," Wall said. "As they pumped the concrete they took the slurry out and put it in a holding tank. They re-used the slurry for each shaft." TXI of Shreveport supplied Class S underwater concrete during the foundation construction that contained an additive to slow setting time.

"That allowed more time for reinforcing steel to be inserted into the casing," he added.

Each shaft was poured 4 ft. above the water line and topped with a single bridge column. The drilled shafts were completed in about three months.

"We were fortunate to get the substructure finished without having to deal with high water," he said.

The largest columns, located in the middle of the bayou, measure 66 in. diameter and 20 ft. 6 in. tall and the smallest columns at the bridge abutments measure 30 in. diameter and 1 ft. tall.

Using a Symons re-usable form system, the crew then formed and poured column caps measuring 7 ft. wide by 4 ft. deep with about 950 cu. yds. of concrete, all pumped from the shoreline.

Ferguson said the bridge's most striking feature will be its large steel girders, which reach as long as 130 ft. and as deep as 6 ft. 4 in.

"It's a bit unusual to have girders of this size. Directly overhead is the I-220 bridge and its girders are not nearly as big," he said, adding that the steel was necessary to support the lengthy spans.

"Steel placement was performed by our crews early this year with a 100-ton crane and 65-ton, barge-mounted tailing rig," Ferguson said. The steel was fabricated by Afco Steel of Little Rock, Ark.

"Afco delivered all of the steel in 'just in time' fashion, since we had virtually no laydown area," Ferguson said.

After the steel was placed, PAC Unlimited Inc. of Choudrant installed metal pans as support for the bridge decking, which was topped with an 8.5-in.-thick layer of concrete for the driving surface.

"We used pump trucks with 140 ft. of reach from both sides of the bayou during the pouring of the deck concrete," Ferguson said.

The remainder of the bridge is made of 11 20-ft.-long flat deck bridge sections poured directly on top of the bridge caps without precast girders. All of the concrete contains reinforcing steel tied by Johnny Horton Contracting of Shreveport.

The bridge will be finished after PAC slip forms the concrete barrier rail on both sides of the bridge.

In other work, about 1,300 ft. of roadway is being reconstructed on the east and west sides of the bayou. The two-lane road sections will accommodate the new bridge's higher gradation and will have 10-ft. shoulders.

"On the south side we're shifting it to the west to take out some of the curves," Ferguson said. "The north side will be widened with new shoulders and ditches."

The reconstruction began in August when Blount Bros. removed the existing asphalt roadway down to the base, then came back with new soil embankment, 12 in. of soil cement and 4 in. of asphalt driving surface.

"We expect to finish the entire project by mid September," Ferguson said.

Useful Source:

For an update on the progress of the project, go to: http://www.dotd.state.la.us/construction/shreveport.html#CADDO