Naheola Bridge

Details
Location: Choctaw and Marengo counties, Ala.
Service: Bridge modification
Project Cost: $2.9 million
Client: U.S. Army Corps of Engineers, Mobile District
Owner: Meridian and Bigbee Railroad Co.
Completed: 1996

Description
Naheola Bridge crosses at mile 173.5 of the Tombigbee River, connecting Choctaw and Marengo counties in Alabama. The bridge, constructed in 1934 under a U.S. Army Corps of Engineers bridge permit, is owned by the Meridian and Bigbee Railroad (M&BRR).

The bridge consists of a 161-foot vertical lift truss span and two 207-foot approach truss spans with wooden trestles at each end. The eastern trestle is unusually long, measuring approximately 2,200 feet, while the western trestle is only about 60 feet in length.

The lift span provides an effective horizontal clearance of 150 feet at the navigation channel. Decking was laid across the bridge about 40 years ago to provide a road surface for highway traffic. Alabama Highway 114 now crosses the bridge.

The U.S. Army Corps of Engineers proposed to excavate a navigational channel adjacent to the east bank along a straight line for 300 feet either side of the bridge. This provided a 185-foot navigation channel through the east span.

The Corps of Engineers originally proposed a seven-foot-thick concrete collar surrounded by, and post-tensioned to, the existing pier stem. This collar would be supported by four-foot-diameter caissons placed around the perimeter of the existing pier. Placement of the caissons would require removing several timber bents of the bridge and temporarily closing the bridge to all traffic. Steel sheet piling would be placed along the channel edge at the face of the collar to provide earth retainage in the immediate vicinity of the existing pier.

Challenges
The bridge is located on a bend in the river. Cross currents in the river cause tows to drift sideways into the lift span pier.

Several accidents have occurred because of the bend in the river, the river current and the narrow navigation opening. Tow operators now use the east span, when possible, to take advantage of the slack water. This usage has led to problems.

During periods of high water, the east span does not provide adequate vertical clearance. Also, as barges pass near the east bank they are occasionally damaged by rubble placed by the M&BRR. The railroad adds the rubble in an attempt to stabilize the pier.

Services
Hanson was retained by the Corps of Engineers to provide contract plans and technical specifications for the pier stabilizing system.

During the final design process, we rearranged the caissons by placing a row of five-foot diameter caissons in front of the existing and an eight-foot diameter caisson in each rear corner. This revision to the U.S. Army Corps of Engineers original concept will allow the bridge to remain open to both train and vehicular traffic at all times.

Raising the elevation of the bottom of the collar to the approximate ground level was another Hanson innovation. This allows the contractor to install the caissons and the collar without the need for a cofferdam. It also minimizes disturbance to the ground around the existing pier and eliminate the need to drive sheet piling until after the pier is stabilized by the collar and the caissons.

This project was an opportunity for Hanson to assist the Corps of Engineers in providing a better navigation channel for the users of the Black Warrior Tombigbee Waterway.