In 2016, the Southeastern Pennsylvania Transportation Authority began construction to improve the pedestrian tunnel serving its station on the grounds of Villanova University. The tunnel allows SEPTA passengers to cross underneath the railroad tracks to access the campus but was small, deteriorating and didn’t meet Americans with Disabilities Act requirements.
Replacing the tunnel involved boring a 15-foot diameter tunnel under four active railroad tracks (SEPTA and AMTRAK), in addition to constructing new ADA ramps and stairs. LDG provided construction engineering services to Road-Con, including designing a dewatering system to be used during the construction period, temporary support of excavation, performing a geotechnical review, analyzing the formwork and supporting Road-Con during the tunnel boring operation.
Quentin D. Rissler, P.E., bridge engineering program manager in LDG’s Lititz office, provides a look into the know-how required to keep a project like this safe and on schedule.
Did boring the new tunnel cause delays in service?
No; Road-Con kept the old tunnel and railroad tracks open and bored the new tunnel alongside the old one. This allowed us to make improvements without creating delays in service for SEPTA users.
What did the temporary support of excavation entail?
We needed to dig a 20-foot-deep pit for the boring machine so it could excavate the new tunnel. This pit was only 10 feet away from the active tracks. We designed a series of sheet piles to hold the soil up while the pit was being dug. Otherwise, the pressure from the embankment could have caused the tracks to collapse.
What’s a sheet pile?
It’s a steel structure that keeps the soil in place to prevent the tracks from collapsing.
What’s a dewatering system?
It’s similar to a sump pump system you might use in your basement. Natural groundwater is higher than the bottom of the tunnel, so our water/wastewater engineers strategically installed pumps to lower the groundwater level and keep the tunnel dry.
Did the geotechnical review turn up anything interesting?
We weren’t expecting to hit rock during the placement of the sheet piling and subsequent excavation for the ramps and stairways, however we did encounter rock in one excavation close to the tracks. The test borings were conducted by the original design engineer on the project, and they didn’t show that there should be rock at the site of the new tunnel. We looked further and realized the borings weren’t taken at the dig site, but about 10 feet away to avoid overhead electrical wires.
The construction took longer because of the rock the contractor encountered, so they had to do additional work to put everything in place. That was an unexpected condition, and the contractor had to work out additional payment with the owner. But everyone ended up satisfied.
During a more than 25-year career, Rissler’s design experience has ranged from rehabilitation of existing bridges, new stream crossings and culverts to major interstate bridges on curved alignments. His work on the innovative Pennsylvania Turnpike bridge slide for Road-Con has garnered a 2017-2018 award from the Association for Bridge Construction and Design (ABCD), Susquehanna Chapter for “Outstanding New or Reconstructed Medium Span Bridge.”