The Illinois Department of Transportation knew the bridge over the Wabash River in Mount Carmel, Ill., needed to be replaced. The original 2,800-ft, 12-span steel through-truss structure was too narrow to accommodate modern traffic concerns.
Finding a suitable alternative, however, was problematic due to four factors:
- Uneven bedrock: Levels were close to the surface on the Illinois side and dramatically lower—up to 100 ft deep—beneath the riverbed and over to Indiana;
- Seismic activity: The bridge is located near both the Wabash Valley and New Madrid seismic zones;
- Flooding and scour: Water levels can change upwards of 30 ft in a short period. This can also carry fallen trees; and
- Thermal effects: Temperature variations could affect the integrity of the piers.
“Every time we designed for, say, seismic events and got a foundation that worked, when you go to check it for scour, you find out it doesn’t work for that,” Benesch project manager David Morrill told Roads & Bridges. “When we got that right, we checked it for thermal and it wouldn’t work for that.”
Piers four and five were especially a nuisance, as they were in the water over one of the areas of shallow bedrock. “The rock was so close in those areas that we just couldn’t drive the piles because there was no way we could get a cofferdam around those piles and get the water to build the footing,” Morrill said.
Benesch considered several options to address all four issues, ultimately deciding to build piles beneath the bridge footings to support them. To account for seismic and thermal forces, the piles were driven at an angle instead of vertically. Pile groups were arranged differently for each foundation, addressing the challenges of piers four and five without compromising the rest of the structure.
“Again, we designed it to be economical, but then in field conditions, you find you didn’t quite get the resistance your soil borings and geotechnical analysis would have indicated,” Morrill said.
Access to the Illinois side of the jobsite required navigating around a levee protecting Mount Carmel.
Once the bridge was completed, crews realized the lowest girders—a combination of hybrid steel and precast, prestressed concrete—left little room above the top of the levee, meaning sunlight couldn’t penetrate to nurture grass needed to prevent soil erosion. Instead, Benesch planted several large boulders to keep the soil in place.
Other features of the new bridge include extrawide shoulders for bicyclists. R&B
