Project Overview
The Lake Pontchartrain Causeway features two bridges that span nearly 24 miles over water. The bridges, which were built in 1956 and 1969, are among the longest overwater bridges built in their time. And because they don’t have shoulders, when accidents occur, traffic on the bridges comes to a halt. As traffic in the region grew, the bridges were seeing 8 to 12 breakdowns daily, and 180 crashes per year, many of them rear-end collisions, says Christopher White, senior bridge engineer for Volkert. To improve safety and traffic flow, the owners wanted to add safety bay shoulders up and down both bridges to support stranded vehicles.
The use of precast concrete and ABC was key to the safe and successful delivery of this project.
“The original causeway bridge was built entirely out of precast components and was the first project ever to employ mass production and assembly-line techniques in fabricating and assembling a bridge,” White says. “Its use of prefabricated bridge elements and systems (PBES) and ABC was decades ahead of its time.”
In 2018, the owners embarked upon the upgrade to add 1008-ft pullover lanes at multiple locations in both directions. “The goal from the get-go was to add the most emergency stopping area for the least cost and with the least impacts to commuters,” says Carlton Dufrechou, general manager of the Greater New Orleans Expressway Commission.
The bridges support 40,000 vehicle crossings every day, which meant lane closures during construction were not feasible. So the project team chose an all–precast concrete solution, which included precast concrete cylinder piles, pile caps, and composite girder/slab deck units with barrier rails. “Using prefabricated components allowed for all major elements to be constructed off-site in a controlled setting and then transported and assembled on the bridge,” Dufrechou says. “This saved time, money, and almost entirely eliminated delays to our 40,000 daily commuters.”
Assembly-Line Construction
To speed construction, the contractor set up long-line fabrication beds along the waterfront. Each line could accommodate components for one complete 1008-ft safety bay, consisting of 12 spans for the northbound structure and 18 spans for the southbound structure.
Once one span was installed, the barge moved to the next span and that deck unit was transported into place. This process was then repeated for each span until the entire bay had been erected. Each bay was loaded in reverse order so that the final span loaded would be the first one erected at the bridge site.
Using the assembly-line method allowed the contractor to keep dedicated crews for each step continuously working somewhere along one of the lines. When the first line was ready for loadout, the pieces were transferred to a barge and the second line moved into assembly. Then the first bed was turned over to begin repeating the process for the next bay.
The long-line method also facilitated deck alignment at the bridge site. Assembly pedestals allowed vertical adjustments, using shims to ensure the constructed spans would best fit the geometry of the adjacent existing deck spans over water. This was critical due to the misalignment of spans that had occurred due to storms and minor vessel collisions with pier bents over time.
All of this was performed with no lane closures on the southbound structure, and only a limited number of night lane closures on the northbound structure. Throughout construction, the existing bridge rails remained in place to ensure safety for the traveling public and the workers.
“The bridge is ‘living proof’ that precast concrete is resilient,” White says. “So it only made sense that precast concrete would be used for this first major upgrade to the causeway in half a century.” |