Repairs to a Louisiana-Mississippi bridge that shifted 6 in. are under way almost three years after the problem was diagnosed, but the exact cause of the shift is still unknown.
|SHIFT Mississippi River bridge's bearings leaned as a result of a 6-in. shift. (Photo top courtesy of Angelle Bergeron, bottom courtesy of Lousiana DOT)|
The problem was discovered in May 2001 when the New Orleans-based office of Modjeski and Masters was routinely surveying the Interstate 20 bridge over the Mississippi River between Vicksburg, Miss., and Delta, La., says Gil Gautreau, bridge maintenance engineer for the Louisiana Dept. of Transportation and Development. The 31-year-old, 13,358-ft-long truss bridge has a series of simple and cantilever spans and a suspended span.
One of the engineers noticed the rocker bearings at the Vicksburg end leaning dramatically, Gautreau says. Both of the river piers on that side had moved simultaneously and pulled the fixed truss simple span toward the west. There also was a gap in the finger joint on the roadway above. DOTD checked photos of the finger joint and determined the bridge had migrated at least six inches in six months. State and federal highway officials agreed that it was a hyper-serious problem, said Gautreau.
DOTD immediately reset the bearings, added more support to the open finger joint and trimmed a steel chord. In 2001, it hired Modjeski and Masters to design short-term remediation, determine the cause, estimate future movement and pose a long-term solution. Installed inclinometers have indicated no more movement so far, says Marshall Hill, resident state engineer.
In 2002, the DOTD awarded an $808,000 emergency repair job to New Orleans-based Boh Bros. Construction, LLC. The bridge is hanging between two cantilever spans and, since one cantilever moved and the other didnt, its loose on one end and tight on the other, says project manager Drue Wands.
If we had to put it out for bid, it would have taken us until July and we wanted to get the contractor in there in the cooler weather when bridge stresses are less, says Don Tolar, DOTD resident administrator. Boh Bros. has had to wait until steel fabrication is ready in mid-March. The contractor will enlarge the top of the pier where the movement occurred because there is no more space to accommodate relocation of bearings. The firm also will replace the expansion joint and jack up the bridge to shorten the suspended span by about 10 in. The 90-day project is to be completed by June and has bonuses for early completion or fines for late completion of $1,350 a day for up to 20 days.
Other than the finger joint, the movement has not affected the 25,000 vehicles that use the bridge daily. It originated at the caissons base and seems due to very deep, old soil failure that was activated, but Im not quite sure why, Gautreau says. A 2001 minor earthquake in Arkansas may be a factor. The bridge rises from the Louisiana banks about 100 ft to a bluff on the Vicksburg side, so natural settling also may have contributed, he adds.