FAIRBANKS —The state railroad says a planned bridge across the Tanana River is so big that contractors are scratching their heads about how they’ll build it and where they’ll find the material.

The bridge, more than a half-mile long, will be the largest in the state. It will connect the highway near Salcha to 1.2 million acres of military training ground south of the river, an area now reachable only by ice bridge or aircraft.

Construction is set to start next year and last into 2015. Brian Lindamood, a project manager for the Alaska Railroad Corp., acknowledged last week that there are many variables in the final plan.

The huge rocks needed to buttress the northern riverbank must come from somewhere. And there’s the bigger question of where to build the 80 12-foot-tall steel beams needed once work begins. Lindamood told a Fairbanks Chamber of Commerce committee that each beam will be 165 feet long and weigh 60 tons.

Contractors will need to find answers this winter.

Once construction arrives, it’ll be tough for Salcha to ignore. The project could attract 150 sets of hands on any given summer workday.

“We’re going to be spending six figures a month just to keep the lights on out there,” Lindamood said.

The project is all but financed, given a series of federal grants and one from the state Legislature. A final price tag should arrive by year’s end.

The tentative cost: $160 million.

The bridge will accommodate trucks and trains, with rail embedded in the roadway. It’s planned as the first act in a larger production, an 80-mile, $700-plus million expansion of the railroad to Delta Junction from its endpoint at Eielson Air Force Base.

The bridge’s tentative lead contractor is Nebraska-based Kiewit, although contract negotiations continue.

The bridge will sit on hammerhead, 10-foot-diameter column foundations. Each of those will in turn rest on a 576-square-foot pad supported by six-foot-diameter pilings sunk 110 feet into the riverbed.

“With piles that size, you don’t just go out and get a regular piledriving hammer,” Lindamood said. A big one is needed. “There’s a handful of them floating around the world. The last time we checked, the nearest one was in Singapore.”

Lindamood said by phone Monday that the contractor will need to import steel beams from the West Coast or Asia. Managers have talked about shipping them to the Kenai Peninsula. The railroad’s southern terminus is Seward. The barges also could land in Valdez, where trucks would drive the steel north, Lindamood said.

Enormous rocks would buttress a levee along the river’s northern bank. That would address the Tanana’s wandering nature and, in the process, protect scores of properties from floods. Lindamood said managers canvassed the Interior for the best and cheapest sources of rock, identifying a half-dozen sites between Fairbanks and Delta Junction. He said it’s proving tough to find spots that can offer that much stone while staying within budget.

Lindamood said the regulatory review surrounding the bridge project is also providing a challenge, with federal agencies pressing to know more about how the work could affect the environment. The Federal Emergency Management Agency also doesn’t like risk, he said. Lindamood said the number of questions illustrates the daunting nature of such a big project.

“It’s very difficult to go stand on the bank of the Tanana River and really get a sense of scope,” he said.

Contact staff writer Christopher Eshleman at 459-7582.