BNSF spokesman Steve Forsberg said the guess is the riprap and metal are a result of old rail lines dumping the debris in the river to stabilize areas subject to erosion.
"The only thing that we can safely say at this point in time is that we encountered unexpected obstructions and that we're studying, as a team, with the Coast Guard and the contractor, potential solutions," Forsberg said.
BNSF is working with the U.S. Coast Guard as part of the 1940 Truman Hobbs Act that requires bridges to be altered if they impair the ability to navigate the river channel.
The railway hired Ames Construction out of Minnesota as the main contractor for the project, though the business has employed several local subcontractors.
Both BNSF and the Coast Guard believe the issue can be resolved, but the cost of the shift and the length of delay are not yet determined.
Roger Wiebusch, bridge administrator for the 8th Coast Guard District out of St. Louis, said the new plan may require the span part of the bridge to be shifted 75 feet to the east to be able to find bedrock for a solid foundation for the pier.
"I think the important thing is to get piers in the right place where it's going to be safe for navigation to get through the area," Wiebusch said. "I think there will be some minor changes, but the navigation span will stay the same."
The construction will increase the navigation span from about 150 feet to more than 300 feet, which is the length Wiebusch said it would remain. He said he expects the new plan to be ready in a matter of weeks.
Forsberg said the large metal and rocks had only caused a problem where the eastern pier would have been placed. Because it's still the early stages of the project - construction began in earnest in November - Forsberg said the foundations for either of the piers have not been completed.
Both BNSF and the Coast Guard agree moving the piers will set the construction back for now, but the time can be made up during the long construction period. Construction is expected to continue into 2011.
"The overall construction timeline had a lot of overlapping timelines, and any construction schedule was always subject to weather and other conditions," Forsberg said. "I don't know that this has had a significant impact on the overall project's timeline."
Forsberg said work continues off site, with suppliers fabricating spans. Work continues on the bridge - a crane has been parked near the eastern part of the span for the past couple of days - but Forsberg said it has just been there to secure the pier in place.
Prior to the delay, Forsberg said previously he expects the shaft foundations to be completed by the end of this month.
The Burlington bridge project has long been on the U.S. Coast Guard's list of bridges in need of repair, because it is one of the top three in the United States most often hit by barges or other towing boats. Between 1992 and 2001, the bridge was struck 92 times.
Congress slowly has been building up appropriations for the 118-year-old bridge. The lawmakers secured enough funds last spring to get the project started, thanks to stimulus dollars.
According to a release by BNSF, the bridge is used by about 30 trains a day. The bridge opens about 300 times a month during the navigation season to let river traffic pass.