Other Consultants: Gannett Fleming, DB Sterlin, Inc., Wang Engineering, Atlas Engineering, Huff & Huff, ICC Group Inc., Alderidge Electric, Inc., Area Equipment, Westbrook Associated Engineers, Inc.
Description:The MDW Bridge Z-100 project eliminates an existing Metra bottleneck at the Fox River, increases the reliability of the Milwaukee West Line service, accommodates future passenger ridership and freight operations, and allows better operational flexibility with a second mainline track. The second mainline track at the bridge crossing eliminates the previously existing switches, which further restricted train speed at this location. The MDW Z-100 project provides a new railroad bridge for the Metra Milwaukee West Line crossing of the Fox River, replacing the existing bridge and adding a second mainline crossing. The new structure yields immediate benefits to Metra and the community by improving efficiency, flexibility, and system continuity.
There were three major challenges with the Z-100 bridge:
The Existing US 20 Bridge
In 1961, the Illinois Department of Transportation constructed a bridge to carry US Route 20 across the Fox River. This five-span structure is supported on spread footings and spans above the existing Bridge Z-100 directly above Pier 2. Overhead clearance from top of rail on existing Bridge Z-100 to the low steel of the US Route 20 bridge was 26’-5”. Because of scour concerns, the new Bridge Z-100 required deep foundations at each of the substructure units. The design team evaluated the complexity of constructing a deep foundation beneath the existing roadway structure. The engineers determined Span 2 should be positioned so the deep foundation elements of Piers 1 and 2 could be constructed without concern for a heavily travelled roadway structure overhead. This decision resulted in a minimum span length for Span 2 of approximately 135’.
2. The Hydraulics of the Fox River
The new bridge has three new piers in the waterway, two fewer than the existing bridge, presenting less of an obstruction to water flow than the existing bridge, although the shifted pier locations created hydraulic challenges for the interim conditions when both new and existing piers would simultaneously obstruct water flow. The proposed temporary causeway created additional hydraulic challenges during the interim condition, but the causeway was vital to provide construction access flexibility to mitigate the overhead constraints of the US Route 20 bridge and high-voltage power transmission lines. The team developed hydraulic models for multiple interim conditions in addition to the proposed condition and coordinated closely with the Illinois Department of Natural Resources Office of Water Resources (IDNR-OWR) to secure necessary permits for both temporary and permanent construction within the floodway.
3. Maintaining Rail Traffic
Fifty-four Metra trains and up to eight freight trains per day cross the Fox River. Metra required at least one track be available throughout the construction period with minimal outages available for cutovers and other contractor operations. The TranSystems Team evaluated offset alignments that allowed construction activities to occur without impacting rail operations. The preferred alternative maintained the existing Track #2 alignment with Track #2 positioned downstream at 18’ centerline to centerline. This geometry required the contractor to construct the bridges in two stages. First, constructing the substructure and superstructure supporting Track #1. Next, shifting rail operations onto the new bridge and demolishing the existing structure. Lastly, extending the substructure units and erecting the remaining superstructure supporting Track #2.