Ever-Changing River Conditions Complicate Construction for New Lewis and Clark Bridge
With more traffic and larger vehicles traveling U.S. 85, the North Dakota Department of Transportation (NDDOT) undertook an extensive project to increase capacity by widening 33 miles of roadway to four lanes from Watford City to Williston, North Dakota. The last piece of the U.S. 85 Four-Lane project replaces the historic Lewis and Clark Bridge near Williston with a new steel plate girder superstructure.
As they build the new bridge, NDDOT and their engineering firm, KLJ out of Bismarck, North Dakota, are working with the contractor, Johnson Bros. of Roanoke, Texas, to protect the river and surrounding wildlife while managing construction through constantly changing conditions on the fast-flowing Missouri River.
The Lewis and Clark Bridge traces its roots to a pontoon bridge built in 1916. After a flood demolished that structure, a two-lane steel bridge was built in 1927. Heavier traffic and the weight of larger trucks led to a new bridge in 1973, but now that, too, has exceeded capacity with nearly 18,000 vehicles crossing the Missouri River on U.S. 85 each day.
The old two-lane bridge will be replaced with a new four-lane, structural steel bridge just to the west. Stretching over 1,500-feet-long with six spans founded on steel piling, the bridge under construction features a conventional concrete deck and a substructure with some aesthetic curve treatments.
"The new bridge will have seven girder lines; the old bridge only has two, so it will be a little more robust," said Josh Schroeder, Structural Engineer with KLJ.
Johnson Bros. began construction of the two-year, $78.8 million Lewis and Clark Bridge project in spring 2015.
Taking Water Out of the Equation
"Any time you're doing a marine project, the best thing to do is take the water out of the equation," said Andy Anderson, Senior Project Manager for Johnson Bros. To achieve that, "We built a temporary bridge alongside the new bridge and all the equipment and men work from that trestle out over the water."
Designed to support and stage a large quantity of heavy machinery, the steel and wood temporary structure measures 40-feet-wide and 1,500-feet-long. "Right now we have two 300-ton cranes on it, and occasionally four to five concrete trucks, so it's a substantial structure," Anderson said. "We install it as we need it-and whenever we have the opportunity with the equipment free."
Removing water from the equation also meant building a cofferdam-a routine step except, in this case, for the seal that keeps it dry. With a current up to 40 feet per second at the construction site, the Missouri River moves too fast for plastic tarps, and the team wanted a more environmentally friendly alternative than traditional epoxy.
"We actually used sawdust," said Bill Gathman, Project Engineer for NDDOT's Williston District. "It's an old technique that goes all the way back to the days of whaling ships. When a seam opened up in those ships, they packed it with sawdust; when the water touched it the wood swelled to form the seal."
For this cofferdam, "Although it's steel sheeting, we introduced sawdust all the way down where the sheets come together to create the vertical seams," Gathman explained. "The water pressure trying to get into the cofferdam pulls the sawdust into the seam. We don't really need to do anything other than get the sawdust in front of the seam. Mother Nature and physics take over and do the rest of the work for us. It's fast, easy, and inexpensive."
Looking Out for Wildlife
The biodegradable pine sawdust also provided another benefit. "When it got introduced into the river, there were absolutely no environmental issues as there would have been with a petroleum-based epoxy," Gathman said.
Crews also used biodegradable vegetable-based oil in the hydraulic equipment. To protect local wildlife, the project includes a separate underpass that allows animals to move freely without danger of traffic-related injury, a screen to protect fish, and a high-flow structure to keep the water fresh.
Throughout the project, NDDOT coordinated with the North Dakota Game and Fish Department, U.S. Army Corps of Engineers, and U.S. Coast Guard, maintaining numerous environmental commitments and permits. "We have environmental specialists who keep the agencies up-to-date," Schroeder said. "They're forwarded our weekly agendas and they're free to come out any time they have questions. If there are any changes to the plan, we make sure they're comfortable with them."
Managing Ice and Cold Temperatures
Of course, even with comprehensive planning, the river sometimes presents unexpected challenges. "We have rapidly moving water, ice in the river, and elevation changes as the water rises and falls throughout the year," Gathman said.
"When you're working in a river crossing, the only constant is change," Anderson added.
Crews worked through the winter, benefiting from some mild conditions but preparing for temperatures that can fall as low as 20 degrees below zero. "In a lot of ways, once the river freezes over solid enough, it actually helps us because guys can walk on the ice," Anderson said. "In one regard it's a bit of a challenge, but on the other side it becomes a help to the project."
To ensure the concrete cured properly in the cold, crews used tent-like structures and various heating methods. "If it was a flat concrete pour, like a footer in the bottom of the cofferdam, we put blankets over it and induced heat under the blankets," Anderson said. "If it was tall like a column, we built a structure around it. We always custom-built for the structure we poured at the time. We used indirect heaters whenever possible because we didn't want to introduce fumes where crews were working, but there were times when it made sense to use a standard kerosene heater."
With crews building the substructure across the river before installing beams, work on the new bridge is halfway complete and will finish by late fall. Once traffic transitions to the new route, the old bridge will be demolished. To continue protecting the river, crews will forego explosives and take the old bridge apart in sections, with nets to keep debris out of the waterway. Demolition of the old bridge is scheduled to finish by mid-2017.