Tunnels
Last updated: September 12, 2008.
Tunnels are long underground
passageways that carry highways,
railroads, and pipelines under mountains, seas, and
rivers.
Digging a tunnel can be difficult and dangerous, but the benefits of being able
to transport things in a straight line usually outweight the costs of
the massive engineering operation. Tunnels were once dug entirely by
hand. Today, gigantic tunnel-boring machines make tunneling both
quicker and safer.
Photo: A tunnel at Yucca Mountain, the proposed
nuclear waste site in Nye County, United States.
Photo courtesy of US Department of Energy.
Digging tunnels
Different types of tunnels are used in different types of terrains.
Shallow tunnels are often dug by the "cut-and-cover" method, which
involves excavating a long trench, adding reinforced sides, roof, and
floor or preformed pipe-like tunnel sections, and then covering the
whole thing over with spoil (the original excavated material). Where
shallow cut-and-cover tunnels are not possible, deep tunnels must be
bored using drills, explosives, or boring machines.
Tunneling can be very hazardous. Apart from the risk of earth
collapsing before the roof has been properly reinforced, difficulties
include water pouring into the tunnel from aquifers (natural
underground water reservoirs), dust from blasting, trapped gas, and the
heat and humidity of working deep beneath Earth's surface. All these
things can kill construction workers.
The dangerous nature of tunneling has not prevented the construction
of many remarkable tunnels. As long ago as 36 BCE, Romans built a road
tunnel 4800 ft (1463 m) long between Naples and Pozzuoli entirely by
hand. Today's tunnels are much longer. The world's longest rail tunnel,
the Seikan Tonneru between mainland Japan and the island of Hokkaido,
is 33.5 miles (53.6 km) long, and took 22 years to excavate. 3000
people helped construct the tunnel, 34 of whom were killed before it
was completed in 1988.
The Channel Tunnel
Since 1991, England and France have been linked by an underground
tunnel 31 miles (50 km) long. The tunnel actually consists of two
parallel railroad tunnels and a smaller service tunnel running between
them approximately 148 ft (45 m) below the sea. Cross-tunnels between
the main tunnels provide maintenance and emergency access and
ventilation. Work began in 1987 and tunnel-boring machines working
simultaneously outward from Folkestone, England, and Calais, France,
finally met in the middle in 1990.
The tunnel-boring machines (also known as TBMs or "moles") used to
dig the Channel Tunnel
advanced up to 246 ft (75 m) a day. How do machines like this work? A rotating cutter chips away spoil,
which is carried backward on a conveyor into railroad trucks. Powerful
hydraulic rams shift the position of the cutter and a laser guidance
system keeps it moving straight. Giant gripper pads hold up the tunnel
as it is being excavated and, just behind them, reinforced concrete
tunnel supports are lifted into place as the machine moves along.
The cutting head of each Channel Tunnel boring machine is 28.2 ft
(8.6 m) in diameter and rotates 2-3 times per minute. As the cutter is
pressed against the tunnel face, spoil is removed through the slots in
the cutting head. Some 11 different moles were used, with cutting heads
hard enough to excavate soft chalk marl and wide enough to cut a tunnel
of exactly the right diameter.
Photo: Tunnel boring machines ("moles") like
this were used to dig the Channel Tunnel.
Photo courtesy of US Department of Energy.
Further reading
- The Channel Tunnel: A great guide to the history and future of the Channel Tunnel from BBC News, with some nice diagrams showing where it runs and how it works.
