Tunnels

November 24, 2012 1:40 pm

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Tunnels
A tunnel is an underground passageway. Tunnels save us time. In cities, tunnels let trains move quickly under busy streets without stopping for red lights or slowing for traffic. Tunnels through mountains and under bodies of water save us from going a long way around. Some tunnels are built for trains, some for cars, and some for both.
REALLY LONG TUNNELS
You’re whizzing along in a train with sunshine pouring in the window. Then, whoosh, everything outside goes black. You know that you’ve entered a tunnel.
Some tunnels are so long that you’ll be traveling through the dark for many minutes before coming to the end. The Channel Tunnel, for example, connects France and the United Kingdom. It goes under the waters of the English Channel. The Chunnel, as it’s called, is 31 miles (50 kilometers) long! You would be in the dark for 20 minutes on a train traveling 100 miles (160 kilometers) per hour through the Chunnel.
Car tunnels are usually shorter than train tunnels, and from a car you can see what’s ahead and behind along the tunnel. During the day, you can often see light at the end of the tunnel. Some of the longest car tunnels in the United States are in New York City. The Brooklyn-Battery Tunnel goes under the East River. It’s almost 2 miles (3 kilometers) long. The Holland Tunnel is just a bit shorter. It connects New York to New Jersey under the Hudson River.
HOW ARE TUNNELS DUG?
Tunnels through soft rocks, such as chalk and limestone, are usually dug with a huge tunneling machine, or mole. A mole has a rotating cutting head at the front. The machine gradually creeps forwards as it cuts into the rock. Machines carry the dug-out rock from the tunnel. Behind the cutting head is another machine that puts a steel or concrete lining in place around the newly made tunnel. Tunnelers often dig from both ends to speed up digging. They carefully line up their tunnels to meet in the middle.
Tunnels through hard rock are blasted open with explosives. Tunnelers drill holes in the rock. They put explosives into the holes and set them off. The blast breaks up the rock, forming a new section of tunnel. The remains are cleared away, and the whole process is repeated farther inside the rock.
Tunnels across rivers are often built from sunken tubes. Sections of tunnel are sunk in the river and joined together underwater. The tunnel is covered with heavy concrete to protect it and to keep it from floating back to the surface.
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