21 November 2011
A dramatic float-in operation marked the final stages of a fast-track scheme to rebuild the bridge over Lake Champlain in New York state.
As local residents gathered at the end of August to witness the arrival of the main arch span of the new Lake Champlain bridge, it was little more than a year since the contract to build a replacement bridge had been let.
The former bridge, which had provided a connection across the lake between Vermont and New York states for the previous 80 years, was closed to traffic in October 2009 after consultant HNTB deemed it at risk of collapse (see box).
Since that time, the only link across the lake has been via a free ferry that was put into place when the bridge was closed. But a US$69.6 million fast-track scheme to rebuild the link is now nearing completion with the bridge due to be finished by the end of the year. The operation carried out by contractor Flatiron in August involved the float-in of a 900t tied network arch span which was then raised into position on to the steel superstructure of the bridge to complete the new fixed link between the two states.
The process to move and install the huge arch structure attracted a lot of interest from sightseers, with crowds gathering before dawn on the morning of 26 August, waiting in anticipation for the Lake Champlain Bridge arch to be set afloat. Spectators brought with them cameras, folding chairs, mugs of coffee and sweatshirts to ward off the early-morning chill.
Many of the onlookers had spent the week camping near Port Henry NY, where the 123m-long modified network tied arch had been taking shape over the past few months. Flatiron crews prepared for the operation for months, says project manager Mark Mallett, pushed by the bridge’s accelerated construction schedule at every step along the way. “The schedule has been breathing down our neck from the get-go, right from the day the original 1929 bridge was found to be in danger of imminent collapse and closed,” says Mark.
The harsh northeastern weather also took its toll on the bridge’s construction, and crews watched helplessly as the water level at Lake Champlain rose to 31m – 150mm higher than the previous record, set a century ago. “The flooding came when we were getting ready to assemble the arch, and our whole assembly yard was under water for two or three weeks,” Mark said. “We essentially just had to let Mother Nature take her course.”
The Flatiron team monitored the weather closely in the days leading up to the arch lift. They knew they needed a smooth, glassy surface so the four tugboats could manoeuvre the arch, floating on deballasted barges, across the lake and into place below the already-constructed piers. They would have only a few inches for negotiation on each side, and Hurricane Irene had already begun her climb up the East Coast. Flatiron crews tracked the hurricane every three hours in the days leading up to the 26 August lift.
“We had an hourly schedule prepared so that even if the lift went slower than anticipated, we would be structurally secure well before the storm hit this area. Therefore the decision to proceed as early as safely possible on that day was confirmed the day before,” said Mark. The sun rose on a perfect morning and the watching crowd was treated to the once-in-a-lifetime view of the eight-storey arch skimming across the lake.
Rendering of the footway lighting
The arch carefully navigated around two sunken shipwrecks, completing its 3.2km-long journey in just two and a half hours. By 1:30pm, Flatiron crews had hooked the arch onto the four massive jacks which raised it 24m into place. The contract for the new Lake Champlain Bridge was awarded to Flatiron Construction Corporation back in May 2010 by the NYSDOT and includes construction of the new bridge and approaches, as well as removal of the temporary ferry facilities in both states once the bridge is open to traffic.
Work began in June 2010, and continued throughout the winter season. The construction covered three major work phases: the concrete substructure, the steel and concrete superstructure, and the approach roadways. The substructure consists of two concrete abutments and seven concrete piers; six of them are founded on a total of 32, 1.8m-diameter drilled shafts in the lake.
Near the NY shore, the casings extend about 12m deep, while in the deeper main channel areas near the midspan of the bridge, the casings are more than 30m long. The seventh pier, which is the one closest to the Vermont shore, is founded on a spread footing on rock on the shore. The 1.8m-diameter steel casings were driven through the lake bottom soils until they reached bedrock.
Once they reached rock, carbide cutting-teeth on the bottom of the casings were used to drill the casing a minimum of 150mm into the rock. Once seated in rock, the casings were cleaned and a 3m to 4.6m-long rock socket was drilled into bedrock at the base of the casing. The casing was then fitted with heavy spiral steel reinforcement and filled with concrete. The completed drilled shafts were topped with a reinforced concrete pile cap, located at water level.
The seven piers are essentially similar except the height of the pier stems varies depending on location. Once the piers were complete, the steel girders were erected on top of the piers. Flatiron assembled the network tied arch span at the Valez Marine in Port Henry, NY. As described above, the arch was floated on barges to the bridge site and lifted into place on the ends of the NY and VT approach structures during a major operation in August. The centre span was erected without its concrete deck to limit the weight of the lift to just the structural steel components of the structure.
To expedite the deck construction, precast concrete panels were used for both the roadway surface and the walkways, which will be located on the outside of the arch. These panels were staged on barges, lifted by crane to one end of the arch and moved into position using rollers. After the arch was loaded with the weight of the precast panels, the final VT and NY approach deck pours were progressed. The transverse space between each precast deck and sidewalk panel will be filled with concrete in a series of small transverse closure pours.
Once all the transverse closure pours are completed and cured, the precast concrete deck panel system will be post-tensioned. The final concrete pour on the VT approach span was completed at the end of September, and a similar pour on the NY approach span was completed at the beginning of October. This completes the placement of large concrete pours, and only the smaller transverse and longitudinal closure pours and some sidewalk pours remain.
The bridge was closed in October 2009 when inspectors and engineers determined that it was unsafe for use. The New York State DOT and Vermont Agency of Transportation had been working to try and address the deteriorating condition of the bridge since 2006, initiating a capital improvement project in 2007. But the issue became critical in 2009 when a series of in-depth inspections and tests highlighted significant deterioration of the unreinforced concrete substructures of the bridge.
Closure of the bridge was recommended, and follow-up inspections, including a comprehensive underwater inspection of the piers, confirmed the fragility of the substructure elements well below the water. The severity of the deterioration at water level and the wide cracks reported below the water level on all the piers, reinforced the consultant’s recommendation to close the bridge.
Moreover, HNTB reported that it was impossible to assess the condition of significant portions of the caissons and pier stems without carrying out underwater excavation. This was problematic due to access constraints such as the presence of shallow water that prevented access with barge-mounted excavation equipment, and very soft lake mud. The report explained that there were several major factors that accounted for the conditions encountered.
“The frozen bearings have resulted in increased bending and shear forces at the piers. Freeze/thaw and ice abrasion damage at the water level has reduced, and will continue to degrade, the piers’ axial and flexural capacity. It appears that static ice pressure exerted by thawing ice on the piers closest to the shoreline has caused the formation of large cracks roughly 8ft to 10ft (2.4m to 3m) below the water surface. The lack of reinforcement results in large cracks which exceed 3/8” (10mm) in many cases.
“We have evaluated various methods to rehabilitate the pier foundations in order to reopen the bridge to traffic in the shortest possible time and to permit the potential for future comprehensive rehabilitation of the important bridge. All rehabilitation alternatives evaluated require the contractors and engineers to work in close proximity to the existing bridge and their safety during the rehabilitation operation cannot be guaranteed, given the overall fragility of the structure, particularly in winter months where further freeze thaw damage and ice pressure are anticipated. If any major cracks were to develop diagonally in the pier or deterioration further reduces the flexural or shear capacity of the piers, abrupt failure could ensue. Wind loads, temperature induced loads, and ice loads, or a combination of the three could trigger collapse.
“The risk and safety for personnel working in close proximity to the existing, fragile bridge is too great to permit rehabilitation in any form. Moving forward, the existing bridge should be razed in a controlled manner eliminating the risk of sudden, potentially catastrophic, bridge failure,” the report said.