Audubon Bridge Construction Uses Oscillator

By Lisa Doyle | September 28, 2010

Work continues on the John James Audubon Bridge in St. Francisville and New Roads, La. The east side work trestle is now complete and the west side work trestle nears completion. Pilings for smaller bridges along the east side of the project are also being driven. Casing for the first permanent shaft in the Mississippi has been placed on the west side of the project, where the contractor uses a Leffer oscillator instead of the traditional auger method to excavate for the shafts.

The John James Audubon bridge will be 12,883 feet long, with a 1,583-foot main span. It will sit 65 feet high over the Mississippi to facilitate river navigation. When completed in 2010, it will be the longest cable-stay span in North America.

The $350-million bridge is part of the Louisiana Transportation Infrastructure Model for Economic Development (TIMED), and is managed by Louisiana TIMED Managers (LTM). It is being constructed by Audubon Bridge Constructors, a joint venture of Flatiron Constructors, Granite Construction and Parsons Transportation Group.

The west side of the project yields to much gentler landscape. Roadwork here required fill. Design for the roadway on the west side also took into account a local endangered species, the Louisiana Black Bear.

"The approach structure for the main bridge on the east side of the river is fairly long. In addition five bridges along the roadway east of the river that give the bears plenty of opportunity to cross," says Chuck Duggar, project manager, Louisiana TIMED Managers (LTM.) "Along the approach roadway west of the river which is predominately at-grade we're adding 8-foot by 10-foot box culverts in several areas to provide them with plenty of opportunities for crossing."

The west side of the project also consists of 4,050-foot-long bridge structure which will provide a railroad grade separation. Boh Brothers Construction Co., LLC, is subcontracted for this portion of the project.

Permanent 8-foot-diameter casings are used with 7.5-foot diameter temporary casings to construct the foundation shafts which are approximately 200 feet long. Test shafts for both the western pier foundation and the eastern pier foundation have been installed. Additional Test Shafts are being installed on the west side to assure shaft capacities are adequate.

A Leffer oscillator is being used instead of more traditional auger methods to install the casings and excavate the shafts. The machine rotates the casing 15 degrees either way, in addition to pushing up and down to sink the casing and to remove the temporary casing as the concrete is being placed. A variety of excavation methods are being used including a hammer-grab type bucket, a TOYO Pump and traditional air-lift. "They're trying to clean the shaft bottom area out so that it is as level and flat as possible without excavating below the bottom of the casing," says Duggar. "Toward the end of the excavation it's almost like dental work to get the bottom cleaned adequately. A mini shaft inspection device (mini SID) is being used to confirm the conditions of the shaft bottom before placing the rebar cage and pouring concrete."