Foundations for one of the new bridge’s piers were successfully installed by temporarily removing portions of the existing bridge’s road deck.
Both the Tappan Zee Bridge and its replacement partially share the same footprint where they approach the shores of the Hudson River, posing a logistical challenge for project engineers.
Originally, work on the new bridge’s piers in these overlapping areas was planned for late 2017, after the existing bridge’s demolition. But thanks to innovative engineering, the project team has developed plans to advance foundation construction in these areas. This April, the first such installation was successfully executed near the Rockland shoreline in a bold overnight operation unlike any other on the project.
To date, all of the bridge’s massive piles have been installed in the riverbed with large cranes and pile hammers, which require a great amount of overhead space. Overcoming the low clearance under the existing bridge at the shoreline — just under 30 feet — was achieved by temporarily removing two panels of the existing Tappan Zee’s road deck to establish a ceiling-less work area for crews to install the replacement bridge’s 75-foot-tall piles.
The first step was designing a floating frame that would act as a template for the pier’s foundations and provide a water-free workspace in which the construction team would operate. The 12-foot-tall steel structure, called a cofferdam, was moved snugly between two of the existing bridge’s piers and secured using mooring lines in shallow water near the Rockland shoreline. The cofferdam’s hollow, tub-like form was then drained of all water. This dry work area was then prepared for installation of the foundation piles.
As always, minimizing impacts on Thruway travelers was a key priority. The project team scheduled a single overnight operation last weekend when traffic volume would be at a minimum. Work began at 9 p.m., Friday, April 29, when maintenance teams deployed protective barriers, including a specialized barrier truck, on the deck of the bridge to separate traffic from the work area. Crews then swiftly removed two 50-foot-long portions of the Tappan Zee Bridge’s deck to allow cranes to place the foundation piles in the correct locations. The piles were then driven deep into the riverbed. All of the pier’s foundation piles were successfully installed and trimmed to the appropriate length by just after 8 a.m. on Saturday, April 30. The deck segments were then reinstalled, the lanes were reopened and traffic flow returned to normal.
The success of the operation was the product of careful coordination with the tri-state region’s state departments of transportation, the New York State Thruway Authority and Tappan Zee Constructors, LLC. Since the start of construction, similar collaborative efforts have resulted in remarkable solutions for the historic twin-span crossing, including the transition to cashless tolling and the installation of structural steel high above the Metro-North Railroad tracks near the Westchester shore.
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