Iowa Demonstration Project: Improvements to the 24th Street–I-29/80 Interchange in Council Bluffs
Project Overview and Lessons Learned
The Iowa DOT, Nebraska Department of Roads, and FHWA, in coordination with the city of Council Bluffs and the Metropolitan Area Planning Agency, proposed improvements to the Council Bluffs Interstate System (CBIS) around Council Bluffs, IA, with improvements extending across the Missouri River on I–80 into Omaha, NE. The proposed improvements were intended to upgrade mobility through the I–80, I–29, and I–480 corridors; improve the condition of the roadways; reduce traffic congestion and crashes; strengthen system linkages by making transitions between interstates easier; correct functional design issues; and accommodate planned development. These improvements, once implemented, were designed to bring the interstate segments up to current engineering standards and modernize the existing roadway to accommodate future traffic needs.
The 24th Street interchange reconstruction was an initial component of the CBIS. The interchange serves major businesses, such as a large outdoor retailer, a convention and event center, and several casinos, hotels, and semitruck service centers. Access to these businesses and attractions was a major concern during the construction period when access from the interstate to 24th Street was restricted.
The primary component of this project was to replace the existing four–span concrete bridge with a wider and longer two–span steel girder bridge. The city of Council Bluffs recently completed roadway improvements on 24th Street south of the bridge consisting of a five–lane roadway with a raised median. Similar improvements to the north of this interchange were built concurrent with the bridge reconstruction.
As part of the 24th Street interchange improvement project, the existing bridge was replaced in two phases. Each construction phase included one through lane in each direction and a third lane to accommodate left turns. The project widened westbound I–29/80 to the median side in preparation for the ultimate CBIS 12–lane reconstruction project scheduled for 2011. Traffic was shifted onto this widened section to allow the new single bridge pier to be constructed. The new vertical profile on 24th Street was raised about 5 feet (ft) to gain the necessary vertical clearance over the interstate. This grade raise required reconstruction of about 1,340 ft of 24th Street and portions of all four ramps of this rural diamond interchange. New interchange signals and lighting were incorporated to handle the design geometry and anticipated traffic volumes.
Normally, construction time for a project of this magnitude would span two consecutive construction seasons. This project was completed in only one season under an accelerated construction schedule using contract and construction innovations that included the following:
- Use of cost–plus–time bidding to reduce the time required to deliver the project
- Use of full–depth, precast bridge deck panels made with self–consolidating, high– performance concrete (HPC) to ensure quality, increase speed of construction, and improve safety
- Use of HPC throughout the bridge and high–performance steel (HPS) welded plate girders to increase quality of the completed bridge
- Incorporation of a structural health monitoring system to evaluate and document the performance of the in–service materials after project completion
- Fully contained flooded granular backfill installed behind the abutments to mitigate settlement that inevitably occurs with conventionally compacted backfill
- Use of intelligent transportation system (ITS) technology to optimize traffic control during construction
A key innovation was reconstructing the bridge with full–depth precast bridge deck panels. The Iowa DOT uses partial–depth panels for low–volume bridges, but full–depth panels are still a new concept for high–volume corridors. These panels are cast offsite in a controlled environment, steam cured, and made with an innovative self–consolidating HPC to improve consolidation around the complicated arrangement of reinforcing bars and post–tension ducts. The use of prefabricated precast deck panels not only shortened construction time, but is also expected to improve long–term performance of the bridge because the panels were produced under controlled conditions in strict compliance with quality control measures.
HfL Performance Goals
Safety, construction congestion, quality, and user satisfaction data were collected before, during, and after construction to demonstrate that innovations can be deployed while simultaneously meeting the HfL performance goals in these areas.
- Work zone safety during construction—No motorist incidents were reported during construction, which means the Iowa DOT exceeded the HfL requirements for worker safety. A key feature of this project was accelerating the construction schedule to only one April–to–October construction season, eliminating hazardous winter driving conditions through the work zone.
- Worker safety during construction—No worker injuries occurred during construction, which exceeded the goal of less than a 4.0 rating on the OSHA 300 form. Postconstruction facility safety will be checked in future years.
- Construction Congestion
- Faster construction—Conventional construction methods would have negatively impacted both 24th Street and the interstate with construction–related congestion for an estimated 16 months. Shortened construction time limited construction impact on traffic flow to less than 6 months, surpassing the goal of reducing construction time by half.
- Trip time—A 2–day study was undertaken to measure actual travel times to assess the additional time required to traverse both I–29/80 and 24th Street in the vicinity of the project. The travel time study also included the exiting maneuvers from I–29/80 onto 24th Street. It was found that travel speeds along I–29/80 averaged near or above the posted speed limit in both the eastbound and westbound directions. Neither exit ramp to 24th Street queued back onto the freeway mainline lanes during any of the travel time runs. Consequently, construction had no measurable impact on I–29/80 traffic, which satisfied the goal of no more than a 10 percent increase in travel time. However, traffic on the exit ramps and crossing over the 24th Street bridge experienced 40 percent or more delay time from lane reduction. Researchers calculated a total of 607 vehicle–hours of delay per day while the traffic management plan for the project was in place. This value also represents the daily benefits achieved for motorists from the steps taken to accelerate construction and reduce overall project duration.
- Queue length during construction—Queue lengths on the interstate lanes were nonexistent. Queues on the exit ramps were less than the 0.5–mile maximum goal and were prevented from spilling out onto the interstate mainline through signalization and the use of queue detection with ITS. Travel speed across the bridge dropped to more than 20 mi/h less than the posted speed, resulting in queue lengths that were absorbed onto local roads.
- Smoothness and noise—Smoothness across the 24th Street bridge was dramatically increased. IRI dropped from a preconstruction value of 199 in/mi to a postconstruction 86 in/mi. Although the HfL goal for IRI of 48 in/mi—reasonably attainable on long, open stretches of pavement—was not met on this project, the 113 in/mi drop in IRI value is a reflection of the high quality of construction.
- Noise—Quality was measured in terms of noise (OBSI) and smoothness (IRI) both before and after construction. The sound intensity data showed a substantial 4.8 dB(A) reduction in noise from a preconstruction level of 99.2 to a 94.4 dB(A) postconstruction level, meeting the HfL requirement of 96.0 dB(A) or less.
- User satisfaction—The traveling public and businesses gave the project high marks for overall satisfaction and recognized the importance of keeping traffic flowing during construction. Satisfaction with the finished product is high and meets the HfL user satisfaction criteria of 4–plus on a 7–point Likert scale.
The costs and benefits of this innovative project approach were compared with those of a project of similar size and scope delivered using a more traditional approach. The economic analysis revealed that the Iowa DOT's approach realized a cost savings of about $1 million or 8 percent of the total project over conventional construction practices. A significant amount of the cost savings was from reduced construction time.
Through this project, the Iowa DOT gained valuable insights on the innovative processes deployed—both those that were successful and those that need improvement in future project deliveries.
- It is important to provide plenty of lead time (early letting) for projects of this type with nonstandard details. This project had the extra lead time needed to process submittals on the innovative construction techniques and materials.
- The Iowa DOT implemented a successful preconstruction testing program to evaluate construction details unique to the full–depth panels, such as the shear joint transfer for different roughened surfaces between the panels, the shear stud pocket size for welding the stud to the bridge girders, and the stud bend testing. Also, a mockup of the haunch area behind the abutments was built to study the effectiveness of the fully contained flooded backfill. This program was invaluable in finalizing the bridge design and incorporating innovation.
- Collaboration with the industry to address details related to the innovations before design was very important to the success of this project.
- For projects such as this with new construction techniques, it is advantageous to reduce critical path steps in the construction schedule.
- Consider having a designated design engineer on call for quick resolution of design issues on the critical path.
- Considerable experience was gained with the many innovations introduced.
- Consider using a Web–based project communication system to communicate project information and streamline shop drawings and requests for information.
The Iowa DOT gained considerable experience with the innovations used on this project and, because of the success, is encouraged to include these innovations in future projects. Success was measured in increased safety, quality, and the reality of bringing the project to completion in far less time than with traditional contracting and construction.