What is the Largest Tunnel Boring Machine?

What is the largest tunnel boring machine?

The largest of them is called Bertha, and it’s attempting to dig a tunnel under downtown Seattle that will connect a new freeway to an old one. The state Department of Transportation says the tunnel is on schedule to open this spring. It’s part of a $3.1 billion underground highway project that will replace the aging Alaskan Way Viaduct, which could buckle under an earthquake and could lead to increased traffic on city streets.

What’s more, the tunnel will have a lot of traffic: It’s slated to carry four lanes of traffic under Seattle, which will help alleviate traffic in some areas. It’s a complicated project, involving civil engineering, geology, politics, luck, and a 7,000-ton giant digging through rock.

TBMs, or “tunnel boring machines,” are a type of large drill that dates back to the 19th century, when they were first used for digging tunnels. They’re designed to bore through tougher rock than other types of excavators.

To do so, TBMs use a system of grippers that power disc cutters that are pressed against the tunnel’s face. This forces the rock into a slurry, which can be pumped onto a conveyor belt to be removed from the tunnel and transported out of its entrance.

But that slurry, in addition to being a challenge, also makes it more difficult to control the machine’s progress and to keep the cutting head from overheating and damaging seals inside. It’s a delicate balance: The cutters need to be powerful enough to get through tougher material without heating up too much, but they need to stay cool so the muck doesn’t just churn ahead of the machine and infiltrate the front face of the tunnel.

In a typical TBM, the front of the cutterhead is lined with a “slurry shield” made up of a large circular metal plate that encircles the tunnel’s entrance. The shield has 56 hydraulic jacks inside it that form a ring and push the cutting head forward like a worm, inches at a time.

The jacks press against the front of the tunnel’s wall, which is made up of 2-ft (0.6-m) concrete panels or segments. Then they pump grout around them, forming the foundation for the new tunnel’s walls and floors.

Once the tunnel’s walls are reinforced, TBMs are able to drive the entire length of the tunnel in just a day or two. This is because the machine’s grippers work by continuously guiding the tunnel’s sidewalls as it drives, while the cutter head’s buckets scoop up and deposit muck to be transferred along a belt toward the main beam and removed from the tunnel.

This process is repeated throughout the drilling, ensuring that the rock is chipped into smaller pieces that can be easily extracted and carried out of the tunnel. TBMs are used on some of the world’s most complex and high-profile tunnel projects, including the Channel Tunnel across the English Channel, Japan’s Seikan Tunnel from Honshu to Hokkaido, Switzerland’s Gotthard Base Tunnel, and the Crossrail Tunnel under London.