Friday, March 05, 2010

BNSF Burlington, Iowa, bridge project adjusts

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Engineers who design facilities and structures live in a cycle of planning and planning again, the BNSF electronic employee newsletter reports. But sometimes, even with the best of planning and looking "around the corner," a surprise comes along that requires a different approach. That's what happened with the Burlington, Iowa, bridge project, which involves rebuilding a bridge originally built in 1867-1868.

BNSF is working closely with the U.S. Coast Guard to renovate the bridge. A lift-arm span will replace the swing-arm span built in 1868 to allow barge and other river traffic to pass through more easily.

"Our goal was to put the new piers around the old piers, built in 1868, so that the weight of the bridge rested solely on the new piers," said Steve Millsap, assistant vice president, structures. This plan was pursued to have the least impact on both marine and rail traffic during construction of the new bridge. As work began, it became apparent that building around two of the existing piers was going to be a problem.

"We knew there was debris on the river bed," Millsap said. "We just didn't realize how much and how deep. We also had to adopt different techniques to work around the riprap and debris than we'd earlier anticipated."

As work was performed around the piers, careful checking ensured the piers were stable and would safely carry BNSF trains. But when pier 7 showed movement of three fourths of an inch downstream, all construction stopped and work began on a redesign to ensure a safe infrastructure over the two-year construction period.

Redesigning the renovation was tricky. Marine traffic uses two channels through the swing span of the bridge, and navigation has to remain open while work is being done. Millsap said the most logical approach called for moving piers 7 and 9 (those that will support the new vertical-lift span) and, ultimately, the new vertical-lift span some 75 feet eastward. The bridge renovation is being done in two parts. Part I will replace the swing span of the bridge with a vertical-lift span.

"The change in the type of moveable span will provide additional room for larger barges to navigate through without coming into contact with the bridge," Millsap said.

Part II will involve replacing the remaining bridge, Millsap said. Work on Part II will go out for bid in March 2010.

The Burlington Bridge became the first iron - or "open-hearth steel" - bridge built in the U.S. across the Mississippi River between Iowa and Illinois. Earlier bridges were built of wood. Building the Burlington Bridge offered unique engineering challenges, and the construction techniques of that era - as well as the techniques used in the reconstruction in the late 1800s - are fascinating.

The river is about 2,000 feet wide at the bridge's location. In spring 1867, two powerful steamboats were used, along with a fleet of flat boats, to move the bridge material, pile drivers and other machinery out onto the river. Max Hjortsberg, chief engineer of the Chicago, Burlington & Quincy Railroad Co., oversaw much of the work.

The bridge had six spans of 250 feet each east of the drawbridge, which was a pivot draw 800 feet long. Then the bridge had two additional spans off the Iowa shore, one 200 feet and one 175 feet long. Several piers required timber pile foundations, ranging from 174 to 333 feet under each stone pier.

Much of the riverbed in this area is made up of fine sand with coarser sand beneath it. While shale was located below much of the sand, getting to the shale proved easier in some places than in others. Diving crews (in diving suits of the era) worked underwater to ensure the piles were securely driven and in place. A special boat used steam-driven air pumps to move sand and water out of the way. A 3,000-pound drop-hammer also was used to help make sure the piles were securely in place.

Once the masonry work was complete, large amounts of riprap were thrown around the piers. As the sand washed out, the riprap dropped into its place below the river's bottom.

The last span - the swing bridge - was completed in 1868. Before the bridge opened to train traffic, the bottom of the river was again scoured and checked to ensure the foundation piers were securely in place. Additional riprap was installed at that point.

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