Miss. Bridge Work Set to Cut Barge Hits, Ease Strain
By: Jeff Amy - ASSOCIATED PRESS
VICKSBURG, Miss. (AP) The structure of the Interstate 20 bridge over the Mississippi River keeps shifting.
Barges keeping hitting it.
Travelers keep getting delayed.
It needs to be fixed and Mississippi officials say two federal grants will allow them to do just that.
The first $9.8 million project will install underwater radar on the Vicksburg bridge and Mississippi's three other highway bridges across the Mississippi. The radar will inform tugboat captains on shifting currents, allowing them to maneuver better.
A second $4.25 million project will jack up the span off the piers and reset bearings that are being stressed by tiny movements of the river bottom. The movements are shifting the support piers.
Together, officials say the work will make crossing the Mississippi at the spot more reliable and guard against catastrophic damage to the I-20 span, which was built in 1972.
The Mississippi's strong and shifting current can make it hard for tugboat captains to judge their angle of approach to pass under a bridge. Before a new U.S. 82 bridge was built at Greenville, it was the most likely to get hit, said Michael Stokes, who manages the intelligent transportation systems program for the Mississippi Department of Transportation. Now, the rail and highway bridges at Vicksburg, just downstream from a bend, are the prime targets.
So far, the bridges have stood up to the pounding they've taken from errant barges.
But every time there's a collision, officials have to shut down the bridge for two to six hours to inspect and make sure things are safe.
That can cause epic traffic jams, and burden people who need to cross the river for medical care, work or school. River traffic also gets shut down, forcing barges to tie up and wait.
On the Vicksburg bridge, as well as on bridges at Helena-West Helena, Greenville and Natchez, Mississippi is installing underwater Doppler radar that will measure currents and provide real-time information to captains, allowing them to make adjustments and hopefully avoid collisions. The radar and under-bridge cameras also will allow officials to detect minor collisions that may go unreported today.
“It's still relatively new technology to river traffic,'' Stokes said, saying 10 or fewer systems are installed elsewhere.
Officials also will try to ease traffic congestion by installing cameras on the roadways and traffic message boards for motorists. Mississippi, Louisiana and Arkansas will cooperate on managing detour routes — which are 125 to 250 mi. round-trip — when a bridge is closed. Message board will warn motorists in time to give them more detour options.
“The main thing is for us to give travelers real-time information,'' Stokes said. He said if motorists slow down when there's an accident on a bridge, it will make additional wrecks less likely.
While MDOT tries to keep cars and trucks moving, it would like to get the bridge to hold still.
A small piece of river bottom which holds two of the bridge's piers is moving toward the Louisiana side of the river, and slightly downriver.
“The site is a unique site, geologically speaking, said state bridge engineer Nick Altobelli.
So far, the expansion joints that allow the bridge to grow and shrink with temperature have tolerated roughly 9 in. (23 cm) of westward movement. Observant motorists may notice that there's a noticeable bump on the Mississippi side of the bridge, though, where the expansion joint is stretched out, and much less of a bump on the Louisiana side, where it's jammed tight.
The downriver movement is actually less — maybe only an inch — but much more of a worry for engineers. Altobelli said that's because the bridge span rests on bearings atop the piers. The bearing aren't designed to tolerate upriver and downriver movement, only expansion and contraction that moves the span from bank to bank.
The plan is to jack up the cantilever span and set it back down, easing the strain on the bearings.
Officials also would like to stop the movement permanently, but that would cost a lot more money. A plan exists to drill down through the moving river bottom and pour a shaft of reinforced concrete through the layer, essentially nailing it in place. That's estimated to cost nearly $70 million, though, Altobelli said.