II. Read the text. Choose 8 sentences to give the main idea of the text
Tunneling
I. Tunneling is difficult, expensive and dangerous engineering work. Tunnels are built to provide direct automobile or railway routes through mountain ranges, under or over rivers. They can also provide underground channels for water, sewage or oil. Before the 19th century men had not acquired enough skill in engineering to carry out extensive tunneling. Tunnels, however, were known in ancient times. They were, for instance, driven into the rock under the Pyramids of Egypt, and the Romans built one in Rome for their chief drain, parts of which still remain. One of the earliest tunnels known was made in Babylon. It passed under the Euphrates river, and was built of arched brickwork being 12 feet high and 15 feet wide.
Other ancient tunnels were built for water supply and for drainage.
Modern tunnels are often very long and deep. The Simplon Tunnel on the France-to-Italy railway, for example, is 12 miles long and in one place the peaks of the Alps rise over 6,000 feet above it. Some tunnels are over 50 feet in diameter. Many are circular in cross-section. Others are horseshoe-shaped, with a level floor on which it is easy to lay permanent roads and railways.
II.Driving a tunnel is a venture into the unknown. The engineer can seldom be certain what awaits him except one thing—trouble. Ahead may lie soft and treacherous ground, fissures and faults, holes and cavities; icy subterranean rivers may freeze him, hot springs may scald him, high temperatures may roast him, rock falls and ground squeeze may crush him, or riverbeds above may collapse and drown him. These, and many more, are the dangers he must face. The crews are set driving toward each other blindly, often from miles apart; yet they must "meet on a dime." In tunnel mythology, there is the tale of two crews who failed to meet and passed each other, and now must keep on forever in the vain hope of meeting one day. Maybe this was because one of the crews allowed a woman visitor: tunnel miners believe it unlucky to let women set foot in a heading.
The engineers begin by making a surface survey between two ends (called "portals" or "adits"), to determine the line of a drive. Usually, they dig intermediate shafts along the line of the tunnel to open up more faces and thus speed the work. At the portals it is easy to transfer the surface line into the heading and so keep the crews on line, since the engineer can sight his instruments directly from outside. But for headings starting from the bottom of a shaft, fixing the direction is much more difficult.
Two steel wires about l/50th of an inch in diameter, with heavy weights at the bottom, are suspended in the shaft. The distance between the wires, or "base," may be some 10 feet, but in small and crowded shafts it might well be less. On top, by means of his theodolite or "transit," the engineer aligns the two wires with the surface line of the tunnel. Below, another engineer does the reverse, setting his transit dead in line with the two wires. He then projects this line into the heading. With a 10-foot base and a 2-mile drive, any error in lining up on the wires is multiplied over 1000 times.
Drives of up to five or six miles from each end of a tunnel under mountains are not uncommon. The engineer must therefore work with accuracy and repeat the aligning many times. The job takes lime, skill, and patience, and is usually hampered by tough conditions underground. But there is a feeling of triumph when the crews hole through, shake hands at the junction, and cheer when the check across shows they have met to a "gnat's whisker".
III. Answer the questions:
1. What is the purpose of tunnels?
2. Why is it a dangerous engineering work?
3. What are the ancient examples of tunnels?
4. What prejudices do tunnel crews have?
5. What do engineers start by?
6. What mistakes can be fatal?
IV. Choose the word from the list to complete a phrase: collapse, drown, crew, miner, survey, portals, wires, aligning
1. The_________begin the fight of their lives when the water pours into the tunnel.
2. A preliminary___________is carried out before construction begins.
3._________should be done several times.
4. But for the brave man passing by, a little boy, playing near the old bridge, might have
5. Bricks can be made hollow to allow_________ to be put inside.
6. Bridge is an example of a structure that_________after being in use for only 4 months.
7. The ends of a tunnel are also called the__________.
8. The___________worked hard to prevent an accident.