Contractors who worked on Maine’s Sagadahoc Bridge have the distinction of constructing North America’s longest precast concrete span. Erected by the balanced cantilever method, the precast span measures 420 ft. (128 m).
The bridge, which was complete Aug. 1, 2000, connects the city of Bath and the town Woolwich, and replaces the aging Carlton Bridge.
The 70-year old Carlton Bridge, a steel truss lift bridge that carried U.S. Route 1 and railroad traffic across the Kennebec River, had “reached the end of its useful life,” according to Maine Department of Transportation’s (MDOT). “Studies conclude that given its age, present condition and volume of vehicular traffic carried, repairs to the bridge would cost nearly as much as building a wider and more modern replacement,” reported MDOT.
While planning for replacement of the Carlton Bridge began in 1981, it was not until 1996 that the first move toward construction was made.
Sine 1981, a series of engineering studies, MDOT, working through the Carlton Bridge Advisory Committee composed of city officials, merchants, and citizens of Bath, Woolwich and Arrowsic, determined that a new, four-lane bridge had the greatest potential for reducing project traffic congestion on the Carlton Bridge.
When a consensus was reached in 1996, MDOT entered its first design/build effort.
MDOT allocated $74 million for this project, $60 million in federal funds.
The team of Flatiron Structures LLC, headquarted in Longmont, CO, and Figg Bridge Engineers Inc., Denver, CO, entered a proposal of $46.6 million for the precast segmental construction.
Contract and Construction Phases
Made up of 12 spans and totalling 2,972 ft (906 m) in length, the bridge opened 30 days ahead of schedule. MDOT awarded the following three contracts for the project:
Contact 1 included the construction of the new river span and the Bath approach and was awarded under the design/build process by the end of August 1997. The new river span was constructed during the first year and a half of the project.
The construction of the the realigned and widened Bath approach began in the fall of 1998.
Contract 2 encompassed widening the Woolwich approach to provide four travel lanes, a center running lane, bicycle lanes on both sides and a sidewalk on the south side. This contract was awarded in the more traditional design/bid/construct process.
Contact 3 involved the rehabilitation and improvement of the Carlton Bridge for train traffic. This $10-million contract included the maintenance, electrical work to upgrade the lift span, mechanical work and installation of locks on the lift span.
Floating Cofferdam
Ben C. Gerwick Inc., San Francisco, CA, was contacted to design a safe and economical cofferdam system for the six main river piers. (see www.gerwick.com)
The design for the piers was based on using 8-ft. (2.5 m) diameter drilled shafts. A typical pier foundation is located in 45 ft. (13.6 m) of water and contains four drilled shafts supporting a 33-ft. by 36-ft. by 12-ft. deep (10 by 11 by 3.6 m) footing block with the top of the footing about 3 ft. (1 m) below low tide.
This design, according to the contractor, is very efficient because it significantly reduces the depth at which the footing is constructed. However, it does create the problem of how to construct an underwater footing suspended 28-ft. (8.5 m) off the river bottom.
In order to address this problem and reduce the amount of work in the river, Ben C. Gerwick Inc. proposed the following construction sequence:
• Pre-install the drilled shafts using a two-stage template.
• Construct a precast footing shell on shore and attach a temporary steel follower cofferdam.
• Launch the cofferdam and tow it to the bridge site.
• Position the cofferdam over the drilled shafts and fix it in position with four spud piles.
• Lower the cofferdam down over the pre-installed drilled shafts with jacks located on top of the spud piles.
• Lock the footing to the drilled shafts by placing a 4-ft. (1.2 m) deep tremie seal.
• Dewater the cofferdam and construct the footing and pier shaft in the dry.
• Flood and remove the follower cofferdam for reuse on the next pier.
This construction sequence minimized work in the river and allowed the drilled shaft installation to proceed concurrently with the onshore fabrication and launch of the cofferdams.
Installation of the drilled shafts started in October 1997 and the first precast footing shell with follower cofferdam was launched April 28, 1998.
The Sagadahoc Bridge spans the Kennebec River approximately 150 ft. (46 m) north of the Carlton Bridge and provides 75 ft. (23 m) of vertical clearance for river traffic. For more information visit www.state.me.us/mdot/constpg/bath.htm.
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