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Transit U: Tunneling Under Downtown Nashville


By: Lee Rennick

One part of the Transit Proposal to be voted on in Nashville on May 1 is the creation of a tunnel that will run under the city of Nashville from SoBro to Music City Central. It could potentially decrease traffic in downtown Nashville by thousands of cars.

Tunnel Building Methods

Three methods are used when building tunnels: Cut and Cover, Sequential Excavation Method (SEM), and Tunnel Boring Machines (TBM).

Cut and Cover is just what it sounds like. A big hole is cut into the ground, the tunnel structure is built, and then the hole is covered. This method has the biggest surface impact.

Sequential Excavation Method was originally developed in Austria in the 1960s, and it has caught on in the United States because it is both economical and it takes advantage of the inherent strength of the rock mass it is being built into. This allows for the construction of large and complex caverns with very little surface settlement. Earthquake zones use this method because it is so resistant and stable.

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Tunnel Boring Machines look like something from Arnold Schwarzenegger’s Total Recall. They can bore through anything from hard rock to sand. While the Sequential Excavation Method uses drilling and blasting, Tunnel Boring Machines can run under a street or a building with minimal disruption above ground. As the boring machine cuts into the rock, it spits out the crumbled rock, which is constantly analyzed for changes in geology. Design changes can be made quickly if the environment is not conducive to continued tunneling

Urban Tunneling

Keeping life flowing as usual and protecting historic buildings is essential when building in a city. That’s why Tunnel Boring Machines are an important element to the building process. They cover a lot of ground with very little noise and vibration.

While large openings have to be dug at the start and end of the intended tunnel, what happens in the middle can be planned to not interfere with daily business.

Recently a new set of railroad tunnels have been built under Park Avenue in New York by WSP, who have been working with a Nashville team.

Frank Pepe, Senior Vice President and Director of the WSP Tunneling and Geotechnical Group, notes that the tunnels they build can have high style, or they can be more minimalistic. “We design the tunnels and the openings to the surface to fit with their external environment.”

More and more cities are tunneling to move transportation away from the surface, and allow it to become a place for foot traffic and bicycles. There is no more room in downtown Nashville to build roads, so tunneling is an opportunity that has been, or is currently being embraced by many of the cities Nashville competes with for economic development.

Want to learn more about building a tunnel? Click Here.

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