Tunneling is an important technology for modern civilization, as a tunnel is often the only reasonable way to create a direct path between two points. When the Hoosac tunnel was completed in 1875, it turned a difficult, 20-mile railroad route along “precipitous grades” into a direct 5 mile route, connecting Boston with the Upper Hudson Valley. Large infrastructure projects such as hydroelectric dams often require tunnels to function. The Hoover Dam required more than 3 miles of tunnels 56 feet in diameter to divert the Colorado River around the construction site. And a tunnel can be used to create new land beneath dense urban areas, making it possible to build large-scale horizontal infrastructure like sewers or mass transit that wouldn’t be feasible to build above ground.

What is a tunnel boring machine?

There are a variety of different types of machinery used to excavate tunnels. “Tunnel boring machines” use a rotating cutting head that engages with the entire face of the tunnel at once. This contrasts with something like a roadheader, which uses a much smaller cutter that moves back and forth over the face of the tunnel.

Tunnel boring machine, or TBM
Roadheader. Not a TBM

(The terminology around TBMs is somewhat non-standard. In some cases, any sort of mechanized tunnel shield gets referred to as a TBM. Other times TBM is used strictly for rock tunneling machines, and don’t include machines that tunnel through soil.)

All TBMs share several basic features. At the front, a rotating cutting head excavates the face of the tunnel. Excavated material, known as “muck,” passes through openings in the cutting head and is removed by some sort of conveying system. To advance, a TBM uses large hydraulic cylinders to push off against either the excavated sides of the tunnel (in “gripper” TBMs) or against a prefabricated tunnel lining that has been installed around the inside of the excavated tunnel. Gripper TBMs are limited to use in rock, whereas TBMs that push against a segmented tunnel lining can also be used in soft ground. In soft ground, all this equipment is encased in a cylindrical structure known as a “shield,” which supports the sides of the tunnel while it’s being excavated. In rock, this shield may or may not be used.

TBMs can be further divided by whether some kind of support for the face of the tunnel is required. “Closed” TBMs use a fluid or other material behind the rotating cutting head of the TBM to prevent the face of the tunnel from collapsing, or from water intruding as the tunnel is bored. “Open” TBMs, on the other hand, don’t have this support, and can be used where the tunnel face is strong enough to support itself during excavation and where groundwater intrusion isn’t a problem. The chart below shows this breakdown:

TBMs have evolved along two parallel paths of technological development. The first is the development of machines for tunneling through soil and soft ground, and the second is machines for tunneling through rock. Today, machines of both types get categorized as tunnel boring machines, and the lines between them are sometimes blurry. But they evolved separately to solve different sorts of problems. In soft ground, excavation was comparatively easy, and the primary problem was to prevent the tunnel from collapsing or flooding as it was dug. In rock, by contrast, the tunnel could often support itself temporarily while it was being excavated, and the primary problem was building a machine that was strong enough to cut through the rock.

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