About Tunnels and Bridges
Tunnels and bridges are underground and aboveground passageways for cars, trucks, trains, and pedestrians. There are three basic types of tunnels: soft-ground, rock, and underwater. Soft-ground tunnels are used for subways, water-supply systems, and sewers. Because they are shallow and set into soft ground, a support structure called a tunnel shield must installed at the head of the tunnel to prevent collapse. Rock tunnels are often used as roadways or railways through mountains. Builders blast the rock with dynamite, or use tunnel-boring machines. Typically, rock tunnels do not require much extra support. Underwater tunnels are more difficult to build, however, as engineers must hold back the water during construction. With underwater tunnels, prefabricated tunnel segments are floated into position, sunk, and then attached to other sections.
Engineering Web: Tunnels and Bridges - Machine Design
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