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Tunnel Basics
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With more than six million kilometers of highways and 240,000 kilometers of railways snaking across the United States, life above ground has become increasingly congested. Tunnels provide some of the last available space for cars and trains, water and sewage, even power and communication lines. Today, it's safe to bore through mountains and burrow beneath oceans -- but it was not always this way. In fact, it took engineers thousands of years to perfect the art of digging tunnels.

Image of a Roman Aqueduct
Ancient Roman aqueduct
Before cars and trains, tunnels carried only water.
Roman engineers created the most extensive network of tunnels in the ancient world. They built sloping structures, called aqueducts, to carry water from mountain springs to cities and villages. They carved underground chambers and built elegant arch structures not only to carry fresh water into the city, but to carry wastewater out.

Image of Worsley Underground Canal Tunnel
Worsley Underground
Canal Tunnel
By the 17th century, tunnels were being constructed for canals.
Without roads or railways to transport raw materials from the country to the city, watery highways became the best way to haul freight over great distances.

Image of the Holland Tunnel
Holland Tunnel
With trains and cars came a tremendous expansion in tunnel construction.
During the 19th and 20th centuries, the development of railroad and motor vehicle transportation led to bigger, better, and longer tunnels.

Image of a tunnel boring machine
Tunnel boring machine
Today, not even mountains and oceans stand in the way.
With the latest tunnel construction technology, engineers can bore through mountains, under rivers, and beneath bustling cities. Before carving a tunnel, engineers investigate ground conditions by analyzing soil and rock samples and drilling test holes.

There are three steps to a tunnel's success.
Today, engineers know that there are three basic steps to building a stable tunnel. The first step is excavation: engineers dig through the earth with a reliable tool or technique. The second step is support: engineers must support any unstable ground around them while they dig. The final step is lining: engineers add the final touches, like the roadway and lights, when the tunnel is structurally sound.

Based on the setting, tunnels can be divided into three major types:

Image of Brunel's shield
Section of Brunel's
tunnel shield
Soft-ground tunnels...
are typically shallow and are often used as subways, water-supply systems, and sewers. Because the ground is soft, a support structure, called a tunnel shield, must be used at the head of the tunnel to prevent it from collapsing.

Check out the forces that act on soft-ground tunnels!

Image of the Hoosac Tunnel
Hoosac Tunnel interior
Rock tunnels...
require little or no extra support during construction and are often used as railways or roadways through mountains. Years ago, engineers were forced to blast through mountains with dynamite. Today they rely on enormous rock-chewing contraptions called tunnel boring machines.

Check out the forces that act on rock tunnels!

Ted Williams Tunnel
Tunnel segment,
Boston Harbor
Underwater tunnels...
are particularly tricky to construct, as water must be held back while the tunnel is being built. Early engineers used pressurized excavation chambers to prevent water from gushing into tunnels. Today, prefabricated tunnel segments can be floated into position, sunk, and attached to other sections.

Check out the forces that act on underwater tunnels!

Now that you know more about the history of tunnels, try digging your own in the Tunnel Challenge!