April Parks, age 15, of Chattanooga, Tenn., for her question:
HOW IS A TUNNEL CONSTRUCTED?
A tunnel is a passage or roadway beneath the ground or water. The construction of a tunnel is known as driving a tunnel and involves advancing the passageway by blasting or boring and excavating.
Tunnels through mountains or under the water are usually worked from the two opposite ends, or faces, of the passage. In the construction of a very long tunnel, vertical shafts may be dug at convenient intervals to excavate from more than two points.
Improved boring and drilling machinery now allow a tunnel to be driven four or five times faster than was possible with older techniques.
The rock drill that is driven by compressed air has helped most in reducing the time of tunneling in recent years. A number of these drills may be positioned on wheeled vehicles, called jumbos, and rolled to the face of the tunnel. Many holes are then drilled concurrently in predetermined places on the rock face. Blasting material is inserted into the holes, the area is cleared and the explosives are detonated. Broken rock is then removed and the process is repeated.
Also used for tunneling is a machine called the mole. It has a circular cutting head that rotates against the face of the tunnel. Attached to the cutting head is a series of steel disk cutters that gouge out the rock on the face as the machine rotates and is pushed forward by hydraulic power.
With the mole, the tunnel can be bored to the exact size desired, with smooth walls, thus eliminating the condition called overbreak, which results when explosives tear away too much rock. The use of moles also eliminates blasting accidents, noise and earth shocks.
A mole can advance about 250 feet a day, depending on the diameter of the tunnel and the type of rock being bored.
A disadvantage of the mole is that it is a very expensive piece of equipment.
In soft earth or mud, a large diameter pipelike device can be driven through the ground by jacks or compressed air. Workers remove the earth as the pipe moves forward, its edge :cutting into the earth. This method was used in driving the Lincoln Tunnel through the muddy bottom of the Hudson River between New York and New Jersey.
Underwater sunken tubed tunnels, such as the Baltimore Harbor Tunnel, have been built by fabricating short tunnel sections in a trench in the riverbed or sea floor. Each section, after sinking, is attached by oversized bolts to the previously sunk section in line. Heavy, thick concrete walls prevent the tunnel from floating.
Another method of underwater tunnel construction uses a caisson, or watertight chamber made of wood, concrete or steel. The caisson acts as a shell for the building of a foundation. The choice of one of three types of caissons the box caisson, the open caisson or the pneumatic caisson depends on the consistency of the earth and the circumstances of construction.
Difficult conditions generally require the use of the neumatic caisson, in which compressed air is used to force water out of the working chamber.