Alternatives Development & Screening
During the initial conceptual engineering phase, the project team developed 18 conceptual alternatives (Alternatives 1A through 9B) based on engineering design factors such as: geometry, design speed, spacing between two bridges, navigational clearances, grades, and relationships to other projects. The project team also evaluated rehabilitation of the existing bridge and several other alternatives (including suggestions from the public and alternatives developed through a value engineering process). A total of 25 alternatives were identified and evaluated.
A two-step screening process was used to evaluate all 25 alternatives.
Step 1: Fatal Flaw Screening – The fatal flaw screening (Step 1) evaluated the alternatives based on their ability to satisfy the following criteria from the project's Purpose and Need Statement:
- Provides rail connectivity
- Meets navigational requirements
- Has logical termini
- Is feasible and constructible
- Avoids critical property impacts
The fatal flaw screening eliminated 15 of the 25 alternatives, including the rehabilitation alternative.
Step 2: Detailed Screening – During the detailed screening (Step 2), the 10 remaining alternatives were compared and contrasted for their ability to meet the project's goals and objectives:
- Optimize existing and planned infrastructure
- Consider operational, design, construction requirements
- Minimize environmental/cultural/socioeconomic/property impacts
The detailed screening eliminated all but two alternates. 9A and 9B have been retained for detailed study in the Environmental Review.
In conjunction with Alternatives 9A and 9B, the project team evaluated four bridge design types:
- Girder approach spans with an arch main span;
- Delta-frame approach spans with an arch main span;
- Deck-truss approach spans with a through-truss main span;
- Girder approach spans with a through-truss main span.
The girder approach spans with an arch main span has been selected for further design.
The project team thoroughly evaluated the rehabilitation alternative, which would rehabilitate the existing bridge without modifying the track alignments. In addition, the project team considered conversion of the existing swing bridge into a lift bridge. The evaluation concluded that a rehabilitated bridge would not be suitable for continued freight rail and/or passenger rail use, due to the current condition of the bridge and the infeasibility of reconstructing the bridge to a state of good repair without significant rail operations disruptions and prohibitive costs. The rehabilitation alternative would not allow for the required level of rail service during the construction period. Rehabilitating the existing bridge for non-rail use also did not pass the fatal flaw screening. The span over the navigation channel would need to be replaced to provide the necessary vertical clearance for mariners, with transition ramps from the existing trusses. The center swing-span pier and several approach spans would need to be removed. Retaining the area occupied by the existing bridge for non-rail use would not meet navigation requirements, and would negatively affect the new rail bridge alignments by increasing right-of-way impacts and/or reducing the achievable speed. The rehabilitation alternative was, therefore, eliminated from further consideration.