Built in the 1930s, the historic IL 89 bridge over the Illinois River served as a critical connector between Putnam and Bureau Counties in Illinois.
Surrounding communities relied on this fracture-critical, narrow structure for commerce, emergency response, and general travel. Despite numerous structural repairs over the years, the existing 19-span, 1,775-ft-long structure had significantly deteriorated and no longer met modern roadway standards or width. It became imperative to find a solution to replace the bridge while maintaining traffic across the river.
Determining the new alignment, including the layout of the new substructure units, was a primary challenge. A detour route was not feasible, so the selected replacement alternative drew upon innovative engineering techniques to build a new nine-span, steel plate girder structure on an offset alignment. The close proximity of the new alignment to the existing bridge, as well as limited space available to line up the navigational span piers with the existing piers, required research of multiple layouts. Ultimately, offsetting the proposed new piers from the existing ones was the best option. However, offsetting them to create a larger navigational span would have created a deeper structure and thus increase the roadway profile raise. Therefore, the decision was made to maintain the length of the navigational span, but shift it slightly north in the longitudinal direction of the bridge. This allowed construction of the substructure footings without interfering with the existing ones. This navigational span shift while maintaining the width of the channel would wind up saving approximately $7 million.
The new alignment of the IL 89 bridge required several special design considerations, including new embankments behind the new bridge abutments. Settlements of more than 20 in. were anticipated on this structure, which would typically take years to occur. To accelerate this process, wick drains, as well as a surcharge on top of the new embankment, were used to consolidate and settle the soil in a matter of days.
Settlement also would have induced significant down-drag forces that the existing bridge abutment piles, which were in use during construction, could not handle, impacting the adjacent roadway. This prompted the use of separation sheet piles down to rock to prevent settlement issues on the existing piles.
Another key design consideration, particularly regarding piers and foundations, was vessel collision. The Illinois River sees major vessel traffic around the IL 89 bridge location, so the size, number, and configuration of barge tows, as well as the frequency of barge traffic, were the main drivers behind how to best apply the use of drilled shafts to build those piers and foundations.
The area in the vicinity of the IL 89 bridge was formerly a large coal mine with very poor soil conditions including rock and shale with coal seams. The type of shale present, the variation in calculated drilled shaft capacities, and where to end the drilled shafts in the rock prompted full-size testing during the design phase in order to accurately determine shaft lengths and avoid construction delays.
Drilled shaft design was highly complex. During initial boring, rock quality designation was very low and the subsurface strata was problematically soft. Therefore, SPT testing was performed without rock cores, and a full-scale load testing was warranted to provide the necessary information needed to successfully design the shafts and minimize risks during construction.
Another project complexity involved the hydraulics of the river, which played an important role in determining the number of substructure units needed during design. The area of the river around the IL 89 bridge flooded often, affecting a private boat club, a key project stakeholder. A model to study the hydraulics of the river was analyzed to ensure the river elevation during a flood event, when all substructure units were active, was manageable and that backwater elevation would not flood the boat club.
The complex hydraulics also played into the use of a causeway for construction. A viable construction layout of the causeway as well as other means of construction were presented to the U.S. Army Corps of Engineers in order to obtain the construction permit. In the past, a flooding event would submerge part of the existing bridge approaches, disrupting traffic for days. To resolve this issue, the design of the new bridge included raising the approaches to limit the risk of future roadway flooding. This required the relocation of the entrances to two project stakeholders—a major grain facility and a nearby boat landing—while maintaining access at all times. It also required the use of sheet piling at different stages in order to manage the difference in elevations during construction.
Taking the long view
Numerous social, economic, and sustainable design considerations were made to ensure the success of the new IL 89 bridge. One of those was that the new bridge alignment was set 46 ft away from the existing alignment, with about 10 ft of clear distance. By ensuring this distance, impacts to nearby wetlands and endangered plant life were minimized. Potentially negative right-of-way acquisition impacts to adjacent property owners were also avoided.
The width and overall geometry of the new structure was a major safety concern for the surrounding communities. The existing bridge was narrow and in an advanced state of deterioration. Occasional bicyclists shared the two-lane roadway with heavy truck traffic. Safety was drastically improved through the creation of a wider roadway with 8-ft shoulders to accommodate bicycles.
The unique traffic plan implemented during this project also had several safety and economic benefits. After an origin and destination study was conducted, a Traffic Management Plan (TMP) was developed to maintain traffic on this busy highway and the waterway beneath. Keeping the existing bridge open during construction using temporary pavement and embankment, versus implementing a detour route, minimized impacts to local businesses and stakeholders. It also maintained fast access routes for emergency vehicles, an important consideration because emergency response agencies on both sides of the Illinois River share equipment and resources during emergencies.
Designing the new IL 89 bridge on an offset alignment proved to be the most cost-effective solution that met the needs of surrounding communities. Shifting the navigational channel to maintain the same navigational span, as well as testing drilled shafts during the design phase, prevented project delays and potential claims. After overcoming several complex design challenges, this innovative bridge replacement solution provided a safer, more efficient river crossing for vehicular, pedestrian, and waterway traffic.