Robust design behind the Tanana River Bridge

Written by Jenifer Nunez, assistant editor

Alaska's longest bridge is designed to withstand floods, large debris, ice jams, road and rail traffic and just about anything else the wilderness can throw at it.

 

by Mat Fletcher, P.E., S.E., vice president, Hanson Professional Services Inc., and Mischa Wanek-Libman, editor

Undiscovered territory is no longer a label that can be applied to Alaska, undeveloped, however, is still an apt description for portions of the state, which makes grand infrastructure feats all the more impressive, as well as all the more important to the communities in which they serve.

Alaska Railroad Corporation’s (ARRC) Tanana River Bridge is one example of this. The 3,300-foot bridge was built near Salcha and currently serves as a permanent replacement for a seasonal ice bridge, allowing military personnel year-round access to the Joint Pacific Alaska Range Complex, a million-acre training site.

While the bridge sits about 13 miles from the nearest tracks, it is still a key component in ARRC’s Northern Rail Extension. The bridge is the first phase in extending the existing railway 80 miles from the Eielson Branch at North Pole to a point near Delta Junction. According to ARRC, the line will ultimately provide improved transportation options for the U.S. military, local transportation options and freight movement. The bridge will serve as the primary access point for equipment, materials and construction personnel for nearly 75 percent of the extension project, which will traverse 60 miles of undeveloped land that has no supporting infrastructure for construction activities.

ARRC utilized the construction manager/general contractor (CM/GC) method to deliver Alaska’s longest bridge. The CM/GC team consisted of Hanson Professional Services, Inc., which served as the prime engineering consultant for design of the bridge, and Kiewit Infrastructure West Co., which served as the general contractor. The project also utilized HDR Alaska Inc. for permitting, hydrology, land acquisition and construction management and Shannon & Wilson Inc. for geotechnical engineering, foundation construction recommendations and pile-driving observation.

“The [CM/GC] method is beneficial for projects like the Tanana River Bridge because it improves constructability and economy with contractor input early in the design process,” said Mat Fletcher, vice president at Hanson. “The contractor and engineer can work together on some design options that allow for concept level pricing by the contractor to identify the best solution. Similarly, it is also helpful to the owner to identify true project costs for non-standard items, such as project access. This was especially true for the Tanana River Bridge.”

The bridge team faced a series of challenges out of the starting gate including a construction locale, which many outside of Alaska would consider remote, that had the potential for harsh environmental conditions; a large, glacier-fed, braided river that could change course from season to season; a potentially unstable river bed because of/caused by debris, ice jams and glacial melt and a bridge that needed to be designed for multiple modes of traffic.

“For me, I think the interesting aspects included the scale of the bridge combined with the logistics of building a project in this part of the world. The primary structural components were relatively straightforward, but the size of the elements and the total bridge length was of a larger scale that most typical bridges. The logistics effort included: building access into the river two times, shipping girders from China and bringing in all the materials necessary to complete the project,” said Fletcher.

Before the design process could progress, finding the ideal spot to construct the bridge was one of the first priorities. ARRC and HDR referenced aerial photographs going back to the 1930s and found a location with a single braid-plain that had remained relatively stable during an 80-year period in which to build the bridge. However, that area experienced a 75-year flood event during the design process that eroded the right bank 400 feet back from where the bridge abutment was to be placed. The erosion required the design team to shift the skew of the bridge in order to place the abutment on remaining shoreline without significantly lengthening the bridge.

Once the location had been confirmed, the focus shifted to designing the bridge to accommodate both rail and road traffic. Initially, the design called for the bridge deck to consist of steel beams supporting direct fixation rails and steel grating for the roadway surface. However, late in the design process, the deck design was amended for road vehicles only as a cost containment strategy because rail use of the bridge is still years away. The current steel deck is designed to be bolted on the top flanges of the girders and can be removed and replaced with floor beams and the bi-modal-style deck at a future date.

Designing the bridge’s foundation to account for scour provided a unique challenge to the team. Scour using 150-foot spans was expected to be more than 50 feet deep at the piers based on hydraulics modeling, which would result in overlapping scour holes. In addition, geotechnical analysis found that the bridge’s foundation piles would not have the support of bedrock. This conundrum of not knowing how to calculate scour without knowing the size and spacing of the piers, but not knowing how to design the piers without knowing the scour sent the team into what it calls a temporary loop. The team broke the loop by performing two-dimensional and, eventually, three-dimensional hydraulic analyses to reach a conclusion on scour depth.

The team turned to the University of Illinois at Urbana-Champaign’s Ven Te Chow Hydrosystems Laboratory to construct two flumes using a 1:50 scale model of the piers and 11 tons of crushed walnut shells (determined to best represent the soils within the river) to get to the bottom of the scour question. The 3-D model analyzed various combinations of flow volumes and log jams and while the results showed an agreement between the calculated and measured scour, the analysis helped minimize previous concerns regarding the unsteady nature of the overlapping scour holes.

The bridge’s superstructure consists of 20 spans of 165 feet, using four girders per span to cover the length of the bridge. The team notes that the four-girder solution was not the lightest, but became the desired solution over a three-girder option to remove the need to design and fabricate the bridge with fracture critical criteria. The bridge piers, one of the primary cost drivers, were designed using a conventional pier type with six-foot diameter piles constructed using a cofferdam approach. The team went with a conventional pier approach, because the bridge required long spans in order to maximize the hydraulic opening and minimize scour. The pier stem was designed as a round column with an increased concrete cover on the rebar cage. This round column presents river debris and ice fields a similar face, as they could approach the bridge in a channel braid from any angle. Additional concrete thickness will help protect against potential abrasion.

Once design was complete, construction of the bridge was driven by a single factor: The impact of the seasons on the Tanana River. Temperatures can swing between an occasional 90 degrees in the summer to negative 60 degrees Fahrenheit in the winter. When the temperature dips that low, negative impacts on equipment, materials and manpower curtail work. In the summer, high floodwaters also limited construction activities.

Beginning in 2011, work on the bridge took place July through November with a plan of building the north bank levee upstream to downstream until the first abutment location was reached. Then, a temporary causeway and trestles were constructed across the river to construct the 19 in-river piers. The causeway and trestles had to be removed and reconstructed each season to avoid blocking the river during the annual spring ice breakup. Kiewit’s cofferdam construction plan allowed for up to five piers to be constructed at one time, which helped the project progress.

The bridge’s 80 girders were manufactured in China and shipped to the Port of Valdez before making the final 330-mile trek to the construction site by truck. Girder erection began in late fall 2013 and, by early November, all girders were in their final positions. Ballast pans were placed by Thanksgiving of 2013 and by the 2014 spring breakup, pier construction was complete, allowing for the final removal of the temporary causeway.

Therest of the summer of 2014 was spent on bolt-up and final demobilization efforts before the project was declared complete with a ceremony in early August 2014.

As with all large projects, Fletcher points to coordination and planning being the cornerstones of a successful project, as well as the success behind the Tanana River Bridge design and construction.

“Simply because of the logistical challenges of working in the Interior of Alaska, extra attention was paid to those items. In addition, the local stakeholders including the surrounding community and environmental agencies were brought in to the project early and updated frequently about upcoming activities on site,” said Fletcher. “I think that the Alaska Railroad was instrumental in keeping the public informed in an effort to be a good neighbor. I would say that the design and construction team communicated well, particularly in the planning stage, to push the project forward and work through the inevitable challenges that arose.”

Some information for this article came from the paper “Alaska Railroad Corporation – Planning, Design & Construction of the Tanana River Bridge” presented at the 2014 American Railway Engineering and Maintenance-of-Way Association (AREMA) Annual Conference in Chicago, Ill., Sept. 28-Oct. 1, 2014. The full paper is available in the Proceedings of the 2014 AREMA Annual Conference.

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