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How Tunnels Work

Tunnel Construction: Soft Rock and Underwater

Boring head
 Photo courtesy City and County of Denver
A TBM boring head showing the disk cutters

Tunneling through soft rock and tunneling underground require different approaches. Blasting in soft, firm rock such as shale or limestone is difficult to control. Instead, engineers use tunnel-boring machines (TBMs), or moles, to create the tunnel. TBMs are enormous, multimillion-dollar pieces of equipment with a circular plate on one end. The circular plate is covered with disk cutters -- chisel-shaped cutting teeth, steel disks or a combination of the two. As the circular plate slowly rotates, the disk cutters slice into the rock, which falls through spaces in the cutting head onto a conveyor system. The conveyor system carries the muck to the rear of the machine. Hydraulic cylinders attached to the spine of the TBM propel it forward a few feet at a time.

TBMs don't just bore the tunnels -- they also provide support. As the machine excavates, two drills just behind the cutters bore into the rock. Then workers pump grout into the holes and attach bolts to hold everything in place until the permanent lining can be installed. The TBM accomplishes this with a massive erector arm that raises segments of the tunnel lining into place.

Tunnel-boring machine
Photo courtesy Department of Energy
A TBM used in the construction of Yucca Mountain Repository, a U.S. Department of Energy terminal storage facility

Tunnels built across the bottoms of rivers, bays and other bodies of water use the cut-and-cover method, which involves immersing a tube in a trench and covering it with material to keep the tube in place.

Construction begins by dredging a trench in the riverbed or ocean floor. Long, prefabricated tube sections, made of steel or concrete and sealed to keep out water, are floated to the site and sunk in the prepared trench. Then divers connect the sections and remove the seals. Any excess water is pumped out, and the entire tunnel is covered with backfill.

Channel Tunnel British terminal
Photo courtesy Stephen Dawson/Creative Commons Attribution Share-alike License
The British end of the Channel Tunnel at Cheriton near Folkestone in Kent

The tunnel connecting England and France -- known as the Channel Tunnel, the Euro Tunnel or Chunnel -- runs beneath the English Channel through 32 miles of soft, chalky earth. Although it's one of the longest tunnels in the world, it took just three years to excavate, thanks to state-of-the-art TBMs. Eleven of these massive machines chewed through the seabed that lay beneath the Channel. Why so many? Because the Chunnel actually consists of three parallel tubes, two that carry trains and one that acts as a service tunnel. Two TBMs placed on opposite ends of the tunnel dug each of these tubes. In essence, the three British TBMs raced against the three French TBMs to see who would make it to the middle first. The remaining five TBMs worked inland, creating the portion of the tunnel that lay between the portals and their respective coasts.

Holland Tunnel ventilation tower
  Photo courtesy Eric and Edith Matson Photograph Collection/
Library of Congress Prints and Photographs Division
Inside a Holland Tunnel ventilation tower

Unless the tunnel is short, control of the environment is essential to provide safe working conditions and to ensure the safety of passengers after the tunnel is operational. One of the most important concerns is ventilation -- a problem magnified by waste gases produced by trains and automobiles. Clifford Holland addressed the problem of ventilation when he designed the tunnel that bears his name. His solution was to add two additional layers above and below the main traffic tunnel. The upper layer clears exhaust fumes, while the lower layer pumps in fresh air. Four large ventilation towers, two on each side of the Hudson River, house the fans that move the air in and out. Eighty-four fans, each 80 feet in diameter, can change the air completely every 90 seconds.

We'll look at the "Big Dig" next.