Penn’s Landing Feasibility Study: ENGINEERING
 
Reconnecting the city to the waterfront is more than simply capping over I-95; the project area has a challenging geometry that is not consistent from south to north, making a complete understanding of the conditions key to testing the master plan concept. These types of investigations could not be performed during the Master Plan process but they were the first step in this study in order to confirm the Master Plan’s concept for Penn’s Landing.
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While there are existing bridges and caps over I-95, the current configuration is incomplete. While there are limited ways to cross over I-95 and Columbus Boulevard from Old City, the design of the bridges blocks views of the waterfront from Front Street and the experience is unpleasant. In the photograph below, you can see the changing elevations across the project area. Notice that the Chestnut and Walnut bridges are at two different grades. Also note that Columbus Boulevard and I-95 are at different heights as well and that those heights change as you move from south to north.
 
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At Walnut Street, Columbus Boulevard is higher than I-95 but at Chestnut Street I-95 is higher. This condition means that, in order to achieve the mandated clearances over the two roads for trucks and future light rail on Columbus Boulevard, the Walnut Street and Chestnut Street bridges will still have to be at slightly different heights, though the goal with this new plan is to minimize those differences to create as seamless a transition from Chestnut to Walnut as possible. The image below shows how new technology will allow us to rebuild the bridges at a thinner width than they are today. In particular, notice how much lower and flatter the new Chestnut Street Bridge will be, allowing a clear view to the waterfront for the first time. Opening up this view means that, once you’ve arrived at Front and Chestnut, you can feel as though you’ve arrived at the waterfront.
 
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To know whether we could incorporate the existing I-95 caps into the new plan or if the project would be best served by constructing all new structures, the team performed visual assessments of all structures in the Penn’s Landing study area using PennDOT criteria. Based on the evaluation of the structures’ current conditions and estimated lifespan as well as the desire to reduce the “humps” in the Chestnut and Walnut Street bridges to achieve an improved line of sight to the Delaware River, the design team determined that removing all existing structures and building new would be the best and most efficient course of action. While there may be cost savings in the short term by attaching new structure to the existing cap and bridges, upcoming necessary repair and replacement of the structures would cause complications. Constructing all new structures now makes the most sense.

Once the team determined that all new structures would be needed, they proceeded to consider a design of the new bridges that would minimize their structural depth. Minimizing their depth would allow maximized views to the water while achieving an elevation at the east side of Columbus Boulevard that would allow for the desirable slopes of the tilted park as tilts toward the river. However, the depth of any new structure must also allow sufficient depth for plantings. The current I-95 caps have trees planted within raised planters, which block views and create a less inviting park space. Advances in technology mean that we can build much thinner bridges and caps that still allow for trees and other plantings.
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The original concept for the tilted park included a structured parking garage underneath. Further investigation into this idea proved that it was cost prohibitive. Building the park on fill would be just as effective and would mean significant cost savings. Additionally, we are working with the Philadelphia Water Department to understand if some space underneath the tilted portion of the park could have a stormwater management function.

Our engineers also performed geotechnical investigations and soil-testing to better understand the soil conditions underneath the existing Great Plaza at Penn’s Landing. These tests allowed engineers to make recommendations on the type of fills and retaining walls that would be necessary to hold up the tilted portion of Penn’s Landing Park. While we initially looked at a variety of fill options, the geotechnical testing helps us understand that some of the lightweight fill options available today, such as Geofoam, will not actually work for the conditions at Penn’s Landing. This knowledge also helped us determine a much more accurate cost estimate for the project.