Main Street Bridge: Naperville, Illinois
Mike Eichten, P.E., Project Manager, Earth Tech, Inc., Chicago, Illinois
Andy Hynes, P.E., Project Manager, City of Naperville, Illinois
Located outside of Chicago, Naperville, Ill. is a well-established city with a strong sense of community. The bustling suburb is home to many businesses and boasts a historic downtown area with shopping and restaurants. Central to the downtown community is the Main Street Bridge, which serves as a gateway to the area. Constructed in 1931, the historic three-span concrete T-beam bridge began showing signs of significant deterioration in the 1990s and it became clear that a strategy was needed to address the situation.
In 1996, the City of Naperville, in conjunction with the Illinois Department of Transportation (IDOT), hired the consulting engineering firm, Earth Tech, Inc. to provide the preliminary engineering studies, final design and construction management for the construction of a new bridge at Main Street. A detailed analysis of the existing bridge concluded that none of the structure could be salvaged and a full reconstruction was necessary. Design for a new two-span, post-tensioned concrete structure was approved in 1999. Adding to the challenges of this project was IDOT's designation of the bridge as a historic structure. As such, a bridge of similar design had to be found within the state to replace it on the list of historic bridges before construction could begin. After three years of searching, the historic status of the Main Street Bridge was officially transferred to a different structure and the preliminary engineering phase of the project was complete. The approval came at the right time as a regularly scheduled inspection indicated continued deterioration of the beams that would require weight limits to be posted.
The age of the structure presented another challenge as no engineering or as-built plans existed for the bridge structure to be demolished or the building located immediately adjacent to its northeast corner. During the bridge design phase, borings were taken to determine the depth of the existing footing of the north abutment and rock coring was performed to determine the integrity of the existing rock below the foundation. The building immediately adjacent to the bridge was believed to be constructed in the mid-1800s and the stone foundation for the structure was tied into the concrete from the north abutment of the existing bridge foundation. Since no documents about the construction of these structures were available, it became clear to the design team that some amount of exploratory digging and design would have to be done during construction. As a precaution, the contractor, Lakes and Rivers Contracting, Inc. of Lemont, Ill., was required to take daily measurements on the adjacent building to ensure any movements would be immediately detected.
Demolition of the existing structure
Making the removal of the bridge more difficult was the lack of engineering or as-built documents. Completed in one week, the contractor conducted careful inspections and exploratory digging before the demolition of the bridge.
"The demolition of the bridge and construction of the north abutment adjacent to the historical buildings presented several challenges," said John Stevens, P.E., Construction Manager for Earth Tech. "During demolition of the north abutment, it was determined the existing abutment was supporting the building slab-on-grade. The design of the abutment was modified and a portion of the old abutment was incorporated into the new abutment to minimize the impact of the building and maintain the structural integrity of the foundation."
The demolition work also provided the Naperville Department of Utilities with the opportunity to make some improvements. An 8-inch diameter water main that was located beneath the existing bridge was removed and replaced with a new 12-inch water main that was installed in a trench in the riverbed. The Department of Utilities' Electric Division also added an electrical duct bank with the project to improve their system reliability.
Bridge deck design
Although the total span was relatively short (120 feet), the design of the bridge deck involved a number of complicated factors including the approximate 30 degree skew of the river to the roadway, a vertical curve in the top of the deck and the two arches along the bottom of the deck. All of these challenges created the need for extremely close coordination between the engineers and contractors to ensure proper layout of the reinforcing steel, post-tensioning ducts, concrete forms and screed elevations.
One innovative idea that the contractor employed in the bridge deck construction was the use of pre-fabricated arch forms. "The geometry of the bridge including the skew of the roadway to the river and the deck arches complicated the layout and the fabrication of the deck forms," said Stevens. "The contractor prefabricated the deck arch forms offsite, which saved time and labor hours."
Along the south riverbank, a gap within the existing low-flow walkway portion of the Riverwalk existed due to clearance issues created by the depth of the existing bridge deck beams. By constructing the new Main Street Bridge with a post-tensioned concrete slab superstructure, adequate clearances were attained and a low-flow walkway constructed to connect the existing portions of the Riverwalk.
Eye to the environment
Prior to beginning construction activities, the City of Naperville had to obtain stormwater permits from DuPage County, the Illinois Environmental Protection Agency and the Army Corps of Engineers. Detailed hydraulic flow models and standard environmental protection measures such as perimeter erosion control fence and stormwater inlet filters were required as part of those permits.
During the stormwater permitting process, wetland specialists from Earth Tech worked closely with the county staff to identify and mitigate the impact caused by the demolition of the existing south abutment and construction of the proposed low-flow walkway. As a result, credits from a wetland bank were acquired. To address major fluctuations in the height of the river level, Earth Tech specialists proposed a variety of landscaping features to provide both stabilization of the riverbank and aesthetics.
Two other unusual environmental issues were encountered during construction. First, the team learned during the design phase that the asphalt on the bridge deck had a presence of asbestos. This required an intricate removal process that had the contractor using a backhoe to mechanically remove the asphalt material while being continually sprayed with water. The material was then transported in covered dumpsters to a site approved to store hazardous material. Second, Earth Tech recommended the use of a turbidity curtain—filter material with a flotation device on the top edge and weight on the bottom edge—while work was being performed on the river to reduce the amount of sediment in the water and reduce the impact on wildlife in the river. When the turbidity curtain arrived, it was recommended that it should only be used in situations where water velocities do not exceed five feet per second and the DuPage River regularly exceeded this. It was proposed to use a concrete barrier wall to create a cofferdam that would allow work on the riverbed. The cofferdam would be moved in two stages. This allowed work to occur on one side of the river while water flowed on the other. The team decided to tie the turbidity curtain to the downstream end of the cofferdam, which would keep the curtain out of the high velocity flow in the river and still accomplish the goal of trapping sediment. This proved to be a very successful solution and a visual inspection showed very little sediment seeping through into the normal flow of the river.
Success on all fronts
Several items were crucial to the success of this project including adherence to the project schedule, creation of a safe environment for workers and pedestrians, an aesthetically pleasing structure that incorporates historic elements, as well as a comprehensive community relations program to keep everyone aware of the project status. The design team incorporated elements into the project based on input from the community. The architectural enhancements included pedestrian overlooks at the center pier and abutments, brick pavers, stone columns with "acorn" lights and planters, benches, entry columns, bridge fa‡ade lighting, as well as use of decorative concrete formliners for the substructure and sides of the superstructure. Downtown businesses expressed a strong desire to have the bridge open to traffic in time for the busy holiday shopping, which the design and construction team met with the opening of the bridge on November 23, 2005. The 10-month schedule also minimized disruption to local residents and ensured that the bridge was open for the City's 175th Anniversary celebration. The attention to aesthetic details and pedestrian amenities make the bridge a focal point to the community for decades to come.