Verb Noun Adjective

Differ

Operator

Additional

Protect

Acceptability

Locative

Alter

Variability

Selective

Text 2: HOW TUNNELS ARE BUILT

People dig some tunnels through rock or soft earth. Other tunnels, known as submerged tunnels, lie in trenches dug into the bottoms of rivers or other bodies of water.

Rock tunnels. The construction of most rock tunnels involves blasting. To blast rock, workers first move a scaffold called a jumbo next to the tunnel face (front). Mounted on the jumbo are several drills, which bore holes into the rock. The holes are usually about 10 feet (3 meters) long, but may be longer or shorter depending on the rock. The holes measure only a few inches or centimeters in diameter. Workers pack explosives into the holes. After these charges are exploded and the fumes sucked out, carts carry away the pieces of rock, called muck.

If the tunnel is strong, solid rock, it may not require extra support for its roof and walls. But most rock tunnels are built through rock that is naturally broken into large blocks or contains pockets of fractured rock. To prevent this fragmented rock from falling, workers usually insert long bolts through the rock or spray it with concrete. Sometimes they apply a steel mesh first to help hold broken rock. Workers using an older method erect rings of steel beams or timbers. In most cases, they add a permanent lining of concrete later.

Tunnel-boring machines dig tunnels in soft, but firm rock such as limestone or shale and in hard rock such as granite. A circular plate covered with disk cutters is attached to the front of these machines. As the plate rotates slowly, the disk cutters slice into the rock. Scoops on the machine carry the muck to a conveyor that removes it to the rear. To cut weaker rock such as sandstone, workers use road header's and other machinery.

Earth tunnels include tunnels that are dug through clay, silt, sand, or gravel, or in muddy riverbeds. Tunneling through such soft earth is especially dangerous because of the threat of cave-ins. In most cases, the roof and walls of a section of tunnel dug through these materials are held up by a steel cylinder called a shield. Workers leave the shield in place while they remove the earth inside it and install a permanent lining of cast iron or precast concrete. After this work is completed, jacks push the shield into the earth ahead of the tunnel, and the process is repeated. Some tunnel-boring machines have a shield attached to them and are able to position sections of concrete tunnel lining into place as they dig. Such a machine dug part of the London subway system

If the soil is strong enough to stand by itself for at least a few hours, workers may not use concrete sections. Instead, they would hold the soil in place with bolts, steel ribs, and sprayed concrete.

Tunneling through the earth beneath bodies of water adds the danger of flooding to that of cave-ins. Engineers generally prevent water from entering a tunnel during construction by compressing the air in the end of the tunnel where the work is going on. When the air pressure inside the tunnel exceeds the pressure of the water outside, the water is kept out. This method was used to build the subway tunnels under the East River in New York City and the River Thames in London.

Submerged tunnels are built across the bottoms of rivers, bays, and other bodies of water. Submerged tunnels are generally less expensive to build than those dug by the shield or compressed-air methods. Construction begins by dredging a trench for the tunnel. Closed-ended steel or concrete tunnel sections are then floated over the trench and sunk into place. Next, divers connect the sections and remove the ends, and any water in the tunnel is pumped out. In most cases, the tunnels are then covered with earth. Submerged tunnels include the railroad tunnel under the Detroit River and the rapid transit tunnel under San Francisco Bay.

How а tunnel is constructed

А tunnel-boring machine digs into rock with attachments called disk cutters. The broken rock, called muck then is removed bу conveyor and rail саг and brought to the surface in аn elevator. Meanwhile, concrete sections of the tunnel lining аге lowered through а shaft. А shield оп the tunnel-boring machine holds uр the roof until workers can erect а new section of tunnel lining.

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