Boring Made Interesting
A local tunneling project to extend a subway line created interest in the large Tunnel Boring Machine (TBM) being used. Just how does this 23-foot-diameter machine with a length of 200 feet and a weight of 1.2 million pounds cut 35 feet of tunnel every day? And how does thermal processing play a role?
Historically, tunnels were hand-dug by several ancient civilizations in the Indian and Mediterranean regions. Fire was sometimes used to heat a rock obstruction before dousing it with water to crack it apart. The cut-and-cover method – digging a deep trench, constructing a roof at an appropriate height and covering the trench above the roof (a tunneling technique still employed today) – was used in Babylon 4,000 years ago.
Gunpowder advanced the hand-digging process when it was used to blast a 515-foot-long canal tunnel in France in 1681. The next two major advances came nearly 200 years later. Nitroglycerine – stabilized in the form of dynamite – replaced the less powerful black powder in tunnel blasting. Steam and compressed air were used to power drills to create holes for the explosive charges.
The technology continued to advance in relative baby steps for the next century until 1954 when James Robbins invented the TBM. The front face of the TBM (cutter head) contains numerous cutting wheels or disc cutters that roll against the rock face breaking it into small pieces as the cutter head rotates at 0.1-10 revolutions per minute. Hydraulic cylinders thrust the face and disc cutters into the rock wall. As the rock is crushed, the slurry (typically referred to as muck) is passed through holes in the face and conveyed to the rear of the TBM where it drops into carts that transport it out of the tunnel. Some TBMs are equipped at the rear with robotic arms that position and attach segments of tunnel lining as soon as the machine has moved sufficiently forward.
Modern TBMs are customized for each project by matching the types and arrangement of the disc cutters to the site geology. These cutters are positioned to cut concentric tracks in the rock face – generally 2-4 inches apart. The diameter of the TBM must be equal to the diameter of the designed tunnel (including its lining). With so many different TBM arrangements, there are also a wide variety of cutting discs. These discs are typically about 17 inches in diameter and are made from hardened alloy steels specifically developed for maximum efficiency in rock-tunneling applications. The steels – typically high-alloy tool steels – are heat treated to provide the ideal balance between hardness (for wear resistance) and toughness (to prevent premature breakage). This ideal balance, and consequently the heat treatment, will vary depending on the site geology. Cemented carbides and cutters with carbide insert rings are used for the most demanding hard-rock applications.
The cutters are the most obvious material challenge for the TBM, but materials and their thermal processes are involved in almost every design aspect of the TBM. The disc cutters – there might be 40 or more on a single TBM – rotate in bearing assemblies. The design and heat treatment of these bearings is another critical design element of the TBM. The TBM cutterhead is attached to the main body by a specially designed and hardened two-row, high-capacity tapered roller bearing.
Today there are around 120 TBMs at work throughout the world. In 2005, the world’s longest overland tunnel was completed in the Swiss Alps. At 21 miles long, it surpassed the previous overland record of 16 miles. Another Swiss tunnel project – expected to be competed by 2012 – is currently under way that, at 35 miles long, will surpass the current record. The Seikan Tunnel – Japan’s tunnel between the islands of Honshu and Hokkaido, which passes part of the way underneath the seabed – is the longest railway tunnel in the world at nearly 33 miles long. The Channel Tunnel, once nicknamed the “Chunnel,” is the second longest at 31 miles, extending from Kent, England, under the English Channel to northern France.
Hopefully, now you know just a little more about how these large tunnel borers work and how thermal processing plays a role. IH
Photo courtesy of Herrenknecht AG © 2008