Figg/PCL Civil Protects Vermont River with Cantilever Bridge Construction
The replacement for two of the oldest I-91 river bridges in Vermont is essentially complete after more than three years of complex construction procedures to protect the scenic, environmentally sensitive West River in Brattleboro.
The design-build team of Figg Bridge Engineers and PCL Civil Constructors built a single replacement for the northbound and southbound I-91 bridges using the balanced cantilever construction method as part of a $60 million bridge improvement project for the Vermont Agency of Transportation (VTrans). The existing twin steel truss bridges were built around 1958 during the early stages of construction of the 177 miles of I-91 in Vermont. According to VTrans records, these bridges experience Annual Daily Traffic volumes of approximately 22,000 and are functionally obsolete and structurally deficient.
Their replacement is a 1,036-foot, arching concrete box girder structure that soars up to 100 feet over Route 30, the West River and the West River Trail. Two piers, each consisting of four curved columns, divide the bridge into three spans of 263 feet, 515 feet and 258 feet, respectively. Officials chose balanced cantilever, segmental construction for the bridge for reasons both environmental and pragmatic.
Protecting the Environment
A tributary of the Connecticut River, West River rises in the Green Mountains in the town of Mount Holly and generally flows southwardly for about 54 miles to Brattleboro, where it joins the Connecticut River. Its watershed covers 423 square miles, most of which is forested, with the upper river supporting wild native brook and wild brown trout, while Atlantic salmon occur in most of the river. This nearly pristine watercourse offers opportunities for fishing and recreational kayaking and canoeing. Complementing this portion of the river is about 6 miles of the parallel West River Trail, which follows the route of a long-abandoned narrow-gauge railroad. In recent years the trail has been restored by volunteers and is now a popular attraction providing access to hiking and other recreational activities.
In order to help protect the environmentally sensitive river and trail, engineers decided that balanced cantilever construction would offer minimum impact, primarily because with this method there is no need to install falsework to support the long main span.
Furthermore, the structure can be built segmentally - constructed in short sections at a time unlike that of a traditional method that constructs a bridge in very large sections. Segmental bridges are economical for long spans, in particular if the construction site is restricted. If the bridge is made of concrete, the segments may be precast or cast in place.
Precast vs. Cast-in-Place
For precast construction, the concrete segments are constructed on the ground, then moved to the bridge site and erected using cranes or other hoisting systems. However, the sheer size of the new I-91 bridge ruled out this technique.
The bridge is designed to accommodate both north-bound and south-bound barrels of I-91. Each barrel consists of a 10-foot median shoulder, two 12-foot travel lanes, and a 16-foot outside shoulder. Add to this the 1-foot railings and a median barrier, and the total out-to-out width of the bridge is almost 105 feet. The box girder is trapezoidal in cross-section, with a top width of approximately 70 feet plus a 17-foot cantilevered wing on each side to carry the outside shoulders and railings. Bridge segments are about 15 to 16 feet long, with depths ranging from approximately 13 feet near mid-span and abutments, to roughly 31 feet at the piers. Precast box segments for this bridge geometry would be too large and heavy to transport from casting yards and hoist at the bridge site.
Additionally, casting the structure in relatively small segments in place can improve concrete quality control, while the repetitive nature of the process yields manpower efficiency over time. All aspects considered, officials decided that casting in place would be the most practical, economical way to build the bridge.
A Balancing Act
With the balanced cantilever method, after a pier is constructed a superstructure, or pier table, is built on top of the pier to serve as a platform from which moveable falsework called a form traveler is launched for building the box girder. The form is cantilevered out from the pier table, and workers install rebar and cast the concrete for the box girder segment. When the concrete reaches the required compressive strength, transverse and longitudinal tendons are post-tensioned, the form is released and the traveler advances on the cured concrete to position the form for the next casting position. On the opposite side of the pier, workers cast an equally sized box girder segment on a cantilevered form traveler, and follow the same procedure.
Segments are cast alternately in opposite directions to maintain the expanding structure's balance. Each added segment is connected to the preceding segment and supported by post-tensioned steel tendons that have been placed within ducts not affixed to the concrete.
After the concrete has been cast and set, tendons are stressed, or post-tensioned, by pulling the tendon ends through fixed anchorages while pressing against the concrete. Tremendous forces are applied to the tendons, resulting in permanent compression being applied to the concrete.
Getting the Job Done
PCL Civil followed this procedure, constructing 14 segments on each side of each pier, for a total of 56 segments. In addition, crews formed and placed a closure segment at each abutment and a closure segment for the center of the 515-foot middle span. Before constructing the middle closure segment, the contractor placed counterweights of 130,000 and 200,000 pounds, respectively, at the tips of the cantilevers near the abutments, to balance the structures. Next, at the middle closure location, crews longitudinally jacked the cantilever tips apart with a force of 1.2 million pounds, cast the mid-span closure segment, removed the end counterweights, and post-tensioned the tendons.
Work on the new I-91 bridge began in fall 2013, with all traffic being shifted to the existing southbound I-91 bridge and construction of the foundations for piers and abutments getting underway. Drilled shafts were installed for the two piers, while piles were driven for the southern abutment, and spread concrete footings were constructed for the northern abutment. By January 2014, demolition of existing I-91 northbound was underway, and in June 2014, construction of the piers was advancing using EFCO forming systems for the gracefully curved columns that architects called for to create an aesthetic cathedral-like appearance. Form liners were installed over the columns and concrete placed within the gap to yield a 4-inch relief simulating Vermont stone.
Construction of Pier 1 proceeded during the winter of 2014-2015, with enclosures built around the pier and pier table so that forms and rebar could be pre-heated and concrete kept warm enough to cure, despite the winter cold. Pier 2 work was in full swing by summer 2015. And by winter 2015-2016, construction of Pier 1 cantilever segments was fast approaching the middle of the main span. Several months later, in May 2016, work on Pier 2 segments had begun.
Completing the Arch
The final mid-bridge closure segment was cast in January, and the entire bridge is expected to be completed this spring.