As curious spectators gathered on a frosty December morning, construction crews worked methodically to demolish the historic Black Hawk Bridge. Implosion of the longtime structure, which for decades connected Lansing, Iowa, to rural Crawford County, Wisconsin, clears the way for further construction on a new, wider crossing over the Mississippi River.
"The contractors worked 60 to more than 100 feet above the river in freezing, windy conditions to prepare the bridge and install the explosive charges," said Iowa Department of Transportation (Iowa DOT) district engineer Clayton Burke. "We had visitors from all over show up. There were many people involved with the development of the project, family members and thousands of others that packed local businesses and lined the highway to watch."
Almost 50 tradesmen and a number of engineering and management staff were on hand to safely execute the implosion.
Iowa DOT photo
"There are many dangers to this work, and it needs to be carried out by experienced professionals," said Burke. "The very cold weather was a challenge for everyone outside. Connecting the wires to the explosives took almost two extra hours, but it was better done safely than quickly."
Burke, who serves as project manager for the new Mississippi River bridge, stressed that keeping everyone out of harm's way was the main concern.
"People want to get close to the bridge for a good view. It was critical that no one was within the restricted zone when the charges were detonated. This bridge also poses a unique challenge in that the Iowa section of the truss needs to stay standing due to its proximity to houses, streets and the railroad. Ensuring this section remained stable after the implosion was a concern as well."
Burke noted that implosion was the safest and most efficient way to begin the demolition of the existing bridge. The contractor, Kraemer North America, prepared the structure for the explosive charges, and subcontractor Veit set and detonated them. A separate subcontractor, Energetic Services, performed some of the pre-cutting of the steel truss.
"The first step was to remove the steel grate bridge deck and traffic railings from the existing structure," said Burke. "To do this, the contractor used cutting blades and torches to cut the deck and railing into pieces. They used a telehandler to transport the pieces away from the bridge. Kraemer used torches to pre-cut the steel truss at each location where a charge was to be placed. After the bridge was prepared, Veit placed copper shape charges at the pre-cut locations, wired the charges and then detonated them."
Iowa DOT photo
Once most of the steel truss is in the river, crews use cranes, lifts and torches to cut the top cord of the truss and lift manageable pieces out of the water onto barges, with the steel transported to shore to be cut into smaller pieces by excavators with large shears. Removing the remaining Iowa portion of the truss involves placing temporary shoring towers under the joints of the remaining truss. That's followed by carefully cutting the truss into smaller panels and doing away with them, section by section.
To remove the concrete piers, barges and excavators with large hammers are required to carefully chip them down. The pier in the river is chipped down to just above the river level. Charges must then be drilled into the pier so they are well below the water level. The charges are detonated below the water level to break the pier into small pieces. All of the pieces of the pier must be removed using long reach excavators on barges. Finally, the protection cells are removed by breaking up the concrete caps with large hammers on excavators floating on barges.
The steel sheet pile is pulled by a crane using a vibratory hammer. The final cleanup of the river bottom is performed by the long reach excavators on barges, and a boat with bathymetric survey equipment is used to map the bottom of the river and make sure everything was properly demolished and removed.
As for traffic prior to the detonation, the streets were closed to make sure all cars were out of the restricted zone.
"No parking ordinances were enforced along these streets," said Burke. "The state highways remained open until just before the detonation and reopened after the ‘all clear' was given. The streets under the existing bridge and next to the railroad tracks stayed closed until the remaining Iowa span of the steel truss was inspected and the tracks were cleared for trains."
Iowa DOT photo
Removal of the truss from the river means Kraemer can then work on installing temporary shoring towers for the Wisconsin span of the new bridge. The shoring towers allow Kraemer to continue erecting the new truss on both the Iowa and Wisconsin sides of the river.
According to Iowa DOT, the 1,700-ft. cantilever bridge has served the area since 1931, with a closure from 1945 to 1957 when it was damaged by ice dams. Depreciation over time and changing vehicle size and traffic patterns require a wider, safer crossing.
The $3 million demolition of the old bridge is considered a critical step in moving forward with the replacement structure, which is expected to open in 2027. Burke said he's glad this major hurdle has been cleared.
"It's a huge relief. Even with the best plan, many things can still go wrong. I'm just glad the truss fell the way it was planned to and, most importantly, everyone went home safe." CEG
