During one weekend in late August, a construction crew demolished a decaying I-91 bridge in Vermont, and then positioned a 700-ton replacement span onto new abutments on the same alignment using lateral slide - the first time this innovative technique has been used in the Green Mountain State. What's more, workers were poised to replace a sister bridge using the identical method during an upcoming weekend. 

PCL Civil Constructors, Inc., construction manager and general contractor for the I-91 project, is a wholly owned subsidiary of PCL Construction Enterprises, Inc., a group of independent construction companies located across Canada, the United States, the Caribbean, and in Australia. The companies have more than 4,400 full-time professional and administrative salaried staff and over 10,000 hourly tradespeople, and construct more than $7.5 billion in buildings, heavy industrial, and civil infrastructure each year. 

PCL Civil Constructors Inc. has the contract with Vermont's Agency of Transportation (VTrans) to replace bridges 43N and 43S, which carry I-91 northbound and southbound over U.S. Route 5 at exit 11 in White River Junction, a village in the Town of Harford. The two 50-year-old bridges, located about one half mile north of the junction of I-91 and I-89, are being replaced because of marked deterioration of the concrete deck, beams and abutments due to the ravages of heavy traffic, age and weather. 

VTrans called for the bridges to be replaced using Accelerated Bridge Construction methods (ABC) as part of its Accelerated Bridge Program. That program was implemented in 2012 to improve the condition of the state's bridges while reducing project costs through expedited project development, delivery and construction. Under the program ABC techniques coupled with brief road closures enable VTrans to replace disintegrating bridges and culverts across Vermont in a matter of weeks instead of entire construction seasons for conventional construction. 

The ABCs of Bridge Replacement

The Federal Highway Administration (FHWA) encourages states to use ABC techniques whenever feasible. In its manual, "Accelerated Bridge Construction" (Publication No. HIF-12-013, November 1, 2011), FHWA says ABC reduces traffic impacts, onsite construction time and weather-related time delay, and improves the safety of the traveling public and the flow of the transportation network. In addition, these techniques can minimize impacts to the environment and existing roadway alignment, limit the need for utility relocations and lessen the occurrence of right-of-way disputes.

Generally, ABC consists of building a replacement bridge on a site near the bridge to be replaced, cutting away and removing the old bridge, and, using one of several accepted techniques, transferring the replacement to permanent abutments. Oftentimes the highway carried by the bridge is closed for just one weekend. ABC techniques involve prefabrication of parts or even major portions of structures. These can include superstructures (bearings, beams, girders, trusses, and deck), superstructures with integral piers, or even complete bridges. FHWA points out that a key component to large-scale prefabrication is the methods used to move and install the elements. Cranes alone often cannot move super-sized prefabrications.

Lateral Slide Eases Impact

VTrans is using an ABC method called lateral slide, or skidding, for moving and installing the I-91 bridge superstructures. 

According to the agency, lateral slide was chosen primarily because it causes the least possible impact to the road users and the surrounding community. In general, the lateral slide method requires the new bridge to be built parallel with its finished location on a temporary support frame equipped with rails. 

The existing bridges have three spans supported by substructures consisting of two abutments and two piers, while the replacements are single span structures with substructures consisting of two abutments. The agency required the contractor to proceed with construction both under the existing bridges as well as next to them. New 3-foot-thick abutments were constructed on concrete footings, with three of the footings supported by plumb and battered micropiles drilled and grouted into bedrock, and a fourth abutment built on a cast-in-place concrete subfooting doweled into bedrock. At the same time, the contractor was constructing the replacement superstructures (bridge decks and girders) on falsework supported by eight shoring towers, four at each end of the spans.

Both existing I-91 bridges were to remain in service while construction went on underneath and next to the bridges. Travel lanes on U.S. Route 5 were reduced from three lanes to two, but traffic was allowed to flow in both directions throughout construction.

Building 700-Ton Spans on Falsework

The new bridges are approximately 128 feet long, with widths of 47 feet northbound and 49 feet southbound, respectively, measured from fascia to fascia of their f-shaped concrete bridge rails. Composite decks are 9 inches thick, consisting of 3-1/2-thick deck panels topped with 5-1/2 inches of cast-in-place high-performance concrete. Reinforcing steel for the cast-in-place concrete is level III corrosion-resistant stainless steel.

Six steel plate girders support the decks. Girders for the wider structure, Bridge 43S, have 50-inch deep webs compared to 38-inch webs for Bridge 43N. Heavy rolled steel beams serve as diaphragms to stiffen the enormous frames, including intermediate diaphragms W40x149s spaced 21 feet, 4 inches apart on Bridge 43S, and W30x90s spaced 16 feet apart on Bridge 43N.

Due to the sheer weight of the bridge components, very large cranes have been employed at the site. CCS Constructors Inc. of Morrisville, Vermont, is providing 250-ton and 200-ton Link-Belt Lattice Boom Crawler Cranes and 275-ton Grove and Liebherr Hydraulic All-Terrain Cranes for the heavy picks, according to CCS Spokesman Sam Davis.

Bazin Brothers Trucking of Westminster, Vermont is performing earthmoving for the two bridges.

Old Razed, New Placed - In One Weekend

Once the new foundations and superstructures were built, the bridges were ready for the slide. VTrans specified that one bridge be moved at a time. Bridge 43N was chosen for the first slide, with the weekend of August 28 through 31 targeted for the technique's Vermont premiere.

During this unique event, a portion of I-91 was closed and traffic rerouted onto the established detour route. Work began at 6 p.m. Friday, August 28th. A team of large excavators equipped with demolition hammers, in concert with crews wielding concrete saws and torches, began methodically reducing the existing bridge to truncated steel beams, cutout deck slabs and concrete rubble - all to be hoisted or pushed out of the way for disposal off-site. This phase of the work took 21 hours. 

Next, the new 700-ton bridge slid from its cradle of falsework, pushed by small, but powerful, hydraulic jacks, and proceeded slowly along lubricated travel beams to its final destination on new abutments.  That took another 10 hours. 

This was followed by the placement of the 86,000-pound precast concrete approach slabs on their engineered bed of stone and gravel and subbase material. Then crews installed the overall bituminous concrete pavement.  Elapsed time for the final effort was 26 hours.

VTrans opened the new bridge to traffic at 3 a.m. on Monday, August 31st. The massive, well-orchestrated weekend effort had taken just 57 hours. 

The lateral slide for the second bridge will take place on a future weekend to be announced by the agency.