What it takes to dig a new tunnel beneath the Hudson

The bottom of the river is said to be like toothpaste. Here's what goes into infrastructure projects like the planned new train tunnel under the Hudson River.
The new tunnel would be another connection between New Jersey and Manhattan, traveling beneath the Hudson River. Photo by Florian Wehde on Unsplash

If you’ve ever taken an Amtrak train in the Northeast Corridor into New York City from New Jersey or further north, then you went through a tunnel beneath the Hudson River. There’s also a chance that while traveling through this tunnel your train was met with delays or even derailments.  

For hardened New York commuters, this dysfunction has long been part of daily life, but a historic request by the US Transportation Department could help change that. In late March, the US Transportation Department made a call to allocate $100 million in 2023 for a New Jersey and New York area rail project. Specifically, it proposes to help fund the building of a new tunnel under the Hudson River for commuters while simultaneously repairing the old tunnel damaged by seawater during Superstorm Sandy. In total, the project is estimated to cost $12.3 billion.

First introduced by Amtrak, NY and NJ Transit in 2011, the Hudson Tunnel Project, which was formed to address this new construction, has been underway for more than a decade. If the latest funding request is signed into the 2023 Appropriations Bill, it would mark the first time it has received federal funding. So far, the project has received $800 million since 2012 from Amtrak, and the states of New York and New Jersey, and has begun building two sections of the tunnel from Penn Station to 10th street; it has also purchased space in Manhattan for a new vent shaft and fan plant.  

This tunnel project is part of a larger effort to improve Amtrak service across the country. It’s one part of a broader initiative called the Gateway Program that aims to beef up rail infrastructure across the Northeast Corridor from Washington D.C. to Boston. Right now, delays going through the Hudson Tunnels affect 800,000 passengers up and down this rail line.

“The Hudson Tunnel Project will allow for the closure and comprehensive rehabilitation of both tubes [in the existing tunnel] without interrupting today’s Northeast Corridor service,” the Chief Spokesman for the Gateway Program, Stephen Sigmund, said in an email. 

“The project will generate 72,000 new jobs and $19 billion in economic activity,” he added. “[It will] reduce emissions, and most importantly, finally deliver a faster, more reliable, more resilient 21st Century rail connection into and out of New York and modernize the busiest section of the Northeast Corridor.”

Despite the benefits of this project, its price tag has faced scrutiny in recent years. New Jersey’s former governor Chris Christie canceled a predecessor of the project in 2010 and the Trump Administration pushed back on budgets related to the project, Sigmund says. 

For the time being, the project seems poised to proceed full steam ahead. This means that civil engineers will be gearing up in coming years to burrow into the muddy bottom of the Hudson River, which is rumored to have the consistency of toothpaste, Chris Barkan, a professor of railway engineering at the University of Illinois at Urbana-Champaign, says in an email. 

Barkan adds that while underwater railway tunnels are not a new engineering feat, they do differ in some significant ways from above ground tunnels. “Large tunnels in mountainous areas are nearly always burrowed through rock,” Barkan says. While he says that a Hudson tunnel would have “substantial bodies of rock on either side of the river,” they would be suspended in “viscous, river-bottom mud” under the Hudson. 

“This is not unique but poses different engineering challenges in their construction and design compared to burrowing through rock,” Barkan says.

To build underwater tunnels, engineers have three main building options. They can go with the tunnel shielding approach, and use a wooden and iron frame to hold the shape of the tunnel in place while miners chisel away at the earth to deepen the tunnel. They could alternatively use a tunnel-burrowing machine to drill into rock and earth to mechanically remove it. Or, they could pre-make sections of the tunnel on land and then sink these tubes to the bottom of the river, where they can be attached manually by divers. 

Regardless of technique, underwater tunnels have to be built from concrete and steel to withstand pressure from the water above. Such engineering accomplishments are not without their potential risk, but for a city like New York, Barkan says that tunnels are a much better option than any above-ground solutions.

“An important benefit of railway tunneling in urban areas such as NYC is the ability to gain access to central urban areas without consuming valuable real estate that would be better used for commercial, residential, manufacturing, or other purposes,” Barkan says. “Such access enables placement of railway stations in locations most convenient to passengers near places of residence, employment, retail.” 

Even if they get approved, the newly proposed federal funds for this project won’t become available until 2023 at the earliest. Sigmund says that construction work on the tunnel is already well underway: two sections of the tunnel’s concrete casings were completed in 2015. It will be a long haul to achieve all the goals outlined in this project, but at the end it will hopefully result in infrastructure that will last for centuries to come.

“All of the regulatory approvals have been obtained and the funding commitments from our states and Amtrak are in place,” Sigmund adds via email. “Our 2021 Financial Plan has an August 2023 date for start of construction, and we are looking at some potential projects, such as the Tonnelle Ave bridge over the portal to the new tunnel, for potential early work.”