John Sherbrick Jr., age 7, of Allentown, Penn., for his question:
HOW ARE TUNNELS BUILT?
The world's longest tunnel of any type is the New York City West Delaware water supply tunnel that runs for 105 miles from the Rondout Reservoir in the Catskill
Mountains into the Hill View Reservoir, near the northern city limits of New York City. Construction on the tunnel
that is 13 feet six inches in diameter was started in 1937 and was completed in 1944.
Construction of a tunnel under a great natural barrier ranks among the world's great engineering feats.
Engineers classify tunnels according to the conditions under which they are built. Some are driven through hard rock while others pass through loose gravel and soft earth.
While rock gives the greatest resistance to tunneling, it provides the advantage of not needing supports for the top and sides of the passage. Small pockets are drilled into the rock and filled with charges of high explosives. The charges are then exploded and pieces of rock are carted out as the tunnel deepens.
Five of the world's greatest rock tunnels are located in the Alps. Great care must be taken in digging tunnels through soft earth, silt, clay or in the mud of a river bed since there is always a danger of caveins. A sheath or shell of timbers or steel must be used as a roof until a permanent lining can be built. Most modern earth tunnels have concrete linings.
Digging machines called moles are used in the construction of earth tunnels. The face of a mole is a revolving disk that cuts into the earth. A conveyor belt then carries the dirt to waiting muck cars.
When building underwater tunnels, either the compressed air method of holding back the water is used, or a method called the shield system. Both are designed to hold back the water.
Tunnels are built to provide a flow of water or to permit the movement of vehicles.
The first major railroad tunnel in North America was cut through the Hoosac Mountains in Massachusetts from 1855 to 1873. The Hoosac Tunnel is four miles long and wide enough for double sets of tracks.
The longest U.S. railroad tunnel is the Cascade Tunnel in Washington that extends 7.8 miles.
The world's longest sub acquerous railroad tunnel will be the Seikan Rail Tunnel beneath the sea bed of the Tsugaru Strait between Tappi Saki, Honshu and Fukushima, Hokkaido, Japan. Once due to be completed in 1979 at a cost of $552 million, the date has now been set back to at least 1982 with a much higher projected final construction cost. The tunnel was started in June, 1972.