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 tunnelsinclude 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 tunnelsare 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.
The Mersey Tunnel
3. What are the purposes of underwater tunnels?
The cities of Liverpool and Birkenhead are joined by a tunnel which goes under the river Mersey. It is the famous Mersey Tunnel, one of the biggest underwater tunnels in the world. Its total length is over two and a half miles. During the year 1956 more than 10 million vehicles used the tunnel. Its construction has been a great engineering achievement. The work started in December 1925 on the Liverpool side and a few months later on the Birkenhead side. It had been decided to approach the work by driving from each bank of the river two pilot headings, an upper and a lower one, which would meet under the middle part of the river. Vertical shafts were sunk on both sides of the river and the excavation work began. At first the working face of the heading was broken up by compressed air drills, later explosives were used. The headings met on the 3rd of April, 1928, twenty-seven months after the work had begun. The divergences in line and level were found to be a fraction of an inch, showing how accurately and correctly the survey work and the determination of working levels had been done. The next stage of the work was the enlarging of the pilot headings into the full-sized tunnel. Steel, cast iron and concrete were used in lining the tunnel. From the very start it was realised that the ventilation of a tunnel of such length, which was to be used by vehicles propelled by internal combustion, would be a very difficult problem. Finally, a system of ventilation was adopted in which air is blown into the tunnel through ducts at roadway level and drawn off along the roof through exhausts. The Mersey Tunnel was completed in 1934. It was opened on the 18th of July, 1934. At the time it seemed a complete solution of the communication difficulties that had existed between Liverpool and Birkenhead. Today it is obvious that the solution has been only temporary. The ever-increasing exports from the port of Liverpool and the rapid development of Merseyside as an industrial centre have resulted in a great increase in motor traffic through the tunnel. Plans are now being made for the use of the space between the walls supporting the reinforced concrete roadway at the lower level of the tunnel (1840).
Central Artery/Tunnel Project (Big Dig)
Some call the Central Artery/Tunnel Project in Boston, Massachusetts, the "largest, most complex and technologically challenging highway project in American history." Others consider it one of the most expensive engineering projects of all time. Locals simply call it the "Big Dig." The tunnel is eight lanes wide, 3.5 miles long, and completely buried beneath a major highway and dozens of glass-and-steel skyscrapers in Boston’s bustling financial district. What does it take to dig a tunnel like this? A lot of hard work and a handful of engineering tricks.
Today, engineers use special excavating equipment, called "clamshell excavators," that work well in confined spaces like downtown Boston. These special machines carve narrow trenches -- about three feet wide and up to 120 feet deep -- down to bedrock. In Boston, engineers are pumping liquid slurry (clay mixed with water) into the trenches to keep the surrounding dirt from caving in.
Huge reinforcing steel beams are lowered into the soupy trenches, and concrete is pumped into the mix. Concrete is heavier than slurry, so it displaces the clay-water mix. The side-by-side concrete-and-steel panels form the walls of the tunnel, which will allow workers to remove more than three miles of dirt beneath the city. As if tunneling beneath a city isn’t hard enough, the soil beneath Boston is actually landfill -- it’s very loose and soggy. Engineers had to devise a few tricks to keep the soggy soil from collapsing. Their solution: freezing the soil!
Engineers pump very cold saltwater through a web of pipes beneath the city streets. The cold pipes draw heat out of the soil little by little. Once frozen, the soil can be excavated without sinking. Engineers also inject glue, or grout, into pores in the ground to make the soil stronger and less spongy during tunnel construction.
Fast Facts:
1. The project will excavate a total of 15 million cubic yards of dirt, enough to fill Foxboro Stadium -- where the New England Patriots football and Revolution soccer teams play -- 15 times.
2. Reinforcing steel used in the project would make a one-inch steel bar long enough to wrap once around the Earth at the equator.
3. Moving all the dirt in the tunnel will take more than 541,000 truckloads. If all those trucks were lined up end to end, they'd stretch 4,612 miles. That's the same distance from Boston, Massachusetts, to Brasilia, the capital of Brazil.
4. The tunnel will emerge next to the FleetCenter, home of the Boston Bruins hockey team, and will cross the Charles River under the widest cable-stayed bridge in the world, the Charles River Bridge.
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