TUNNELS

With more than six million kilometers of highways and 240,000 kilometers of railways snaking across the United States, life above ground has become increasingly congested. Tunnels provide some of the last available space for cars and trains, water and sewage, even power and communication lines. Today, it's safe to bore through mountains and burrow beneath oceans -- but it was not always this way. In fact, it took engineers thousands of years to perfect the art of digging tunnels.

Before cars and trains, tunnels carried only water.Roman engineers created the most extensive network of tunnels in the ancient world. They built sloping structures, called aqueducts, to carry water from mountain springs to cities and villages. They carved underground chambers and built elegant arch structures not only to carry fresh water into the city, but to carry wastewater out.

By the 17th century, tunnels were being constructed for canals.Without roads or railways to transport raw materials from the country to the city, watery highways became the best way to haul freight over great distances.

With trains and cars came a tremendous expansion in tunnel construction.During the 19th and 20th centuries, the development of railroad and motor vehicle transportation led to bigger, better, and longer tunnels.

Today, not even mountains and oceans stand in the way.With the latest tunnel construction technology, engineers can bore through

mountains, under rivers, and beneath bustling cities. Before carving a tunnel, engineers investigate ground conditions by analyzing soil and rock samples and drilling test holes.

There are three steps to a tunnel's success.Today, engineers know that there are three basic steps to building a stable tunnel. The first step is excavation: engineers dig through the earth with a reliable tool or technique. The second step is support: engineers must support any unstable ground around them while they dig. The final step is lining: engineers add the final touches, like the roadway and lights, when the tunnel is structurally sound.

Based on the setting, tunnels can be divided into three major types:

Soft-ground tunnels...are typically shallow and are often used as subways, water-supply systems, and sewers. Because the ground is soft, a support structure, called a tunnel shield, must be used at the head of the tunnel to prevent it from collapsing.

Rock tunnels...require little or no extra support during construction and are often used as railways or roadways through mountains. Years ago, engineers were forced to blast through mountains with dynamite. Today they rely on enormous rock-chewing contraptions called tunnel boring machines.

Underwater tunnels... are particularly tricky to construct, as water must be held back while the tunnel is being built. Early engineers used pressurized excavation chambers to prevent water from gushing into tunnels. Today, prefabricated tunnel segments can be floated into position, sunk, and

attached to other sections.

Soft-Ground Tunnel: ForcesThe heavy, wet ground pushes on all sides of the tunnel. The tunnel's walls are squeezed by the ground.

Rock Tunnel: ForcesThe rock walls are very dense and can support themselves. Some sections of the rock are less dense than other sections.

These loose chunks of rock push on the sides of the tunnel.

Ancient Roman aqueduct

Worsley Underground Canal Tunnel

Holland Tunnel

Tunnel boring machine

Hoosac Tunnel interior

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Underwater Tunnel: ForcesWater pushes on the sides of the tunnel. The tunnel's walls are squeezed by the water.

London Underground

Vital Statistics:Location: London, EnglandCompletion Date: 1863 (first line)Length: 19,800 feet (3.75 miles)Purpose: SubwaySetting: Soft groundMaterials: Cast iron, brickEngineer(s): Sir John FowlerShortly after the opening of the Thames Tunnel, Parliament authorized construction of the first subway system in the world, the London Underground. Work began in 1860 on the first stretch of the underground subway, the Metropolitan Railway. By all accounts, it was a royal mess. Tunnel diggers used the cut and cover method: they carved huge trenches in the streets, lined the trenches with brick, covered the trenches with arch roofs, and then restored the street above. This sloppy method paralyzed traffic and made canyons out of city avenues, but it was a huge success. The new subway carried more than nine million people in its first year! Soon, Londoners were craving more, and they got it. This time, with the help of James Henry Greathead's tunnel shield, London engineers could tunnel under the city without completely destroying the streets above. Greathead's round iron shield supported the soft soil as it moved forward and carved a perfectly round hole hundreds of feet below London's bustling city streets. Inside the shield, tunnel workers laid cast-iron segments end to end. These segments eventually formed a stiff, waterproof tube, perfect for subways. Following London's lead, New York, Boston, Budapest, and Paris soon boasted subways of their own.Fast Facts:

The earliest lines on the London Underground follow the direction of major streets and rarely pass under buildings. This is because many Londoners feared that the tunnel would undermine the foundations of the city's buildings.

The trains in the London Underground were the first to be powered by electric engines. During World Wars I and II, the London Underground subway stations were used as air-raid shelters.

Channel Tunnel (Chunnel)

Vital Statistics:Location: Folkestone, England, and Sangatte, FranceCompletion Date: 1994Cost: $21 billionLength: 163,680 feet (31 miles)Purpose: RailwaySetting: UnderwaterMaterials: Steel, concreteEngineer(s): Transmanche Link Engineering Firm

When England and France decided to link their two countries with a 32-mile rail tunnel beneath the English Channel, engineers were faced with a huge challenge. Not only would they have to build one of the longest tunnels in the world; they would have to convince the public that passengers would be safe in a tunnel this size. Tunnel fires, like the Holland Tunnel disaster, were common at this time. How did the engineers resolve this problem? They built an escape route. The Channel Tunnel, also called the Euro Tunnel or Chunnel, actually consists of three tunnels. Two of the tubes are full sized and accommodate rail traffic. In between the two train tunnels is a smaller service tunnel that serves as an emergency escape route. There are also several "cross-over" passages that allow trains to switch from one track to another. Just one year after the Chunnel opened, this engineering design was put to the test. Thirty-one people were trapped in a fire that broke out in a

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train coming from France. The design worked. Everyone was able to escape through the service tunnel. It took just three years for tunnel boring machines from France and England to chew through the chalky earth and meet hundreds of feet below the surface of the English Channel. Today, trains roar through the tunnel at speeds up to 100 miles per hour and it's possible to get from one end to the other in only 20 minutes!Fast Facts:

At the time it was being built, the Chunnel was the most expensive construction project ever conceived. It took $21 billion to complete the tunnel. That's 700 times more expensive than the cost to build the Golden Gate Bridge!

Many of the tunnel boring machines used on the Chunnel were as long as two football fields and capable of boring 250 feet a day.

When construction began in 1988, British and French tunnel workers raced to reach the middle of the tunnel first. The British won.

In the first five years of operation, trains carried 28 million passengers and 12 million tons of freight through the tunnel.

New York Third Water Tunnel

Vital Statistics:Location: New York, New York, USACompletion Date: 2020Cost: $6 billionLength: 316,800 feet (60 miles)Purpose: Water supplySetting: RockMaterials: ConcreteEngineer(s): Grow, Perini & Skanska; Lehiavone & Shea

Six hundred feet below the busy streets of New York City, engineers are boring a 60-mile-long tunnel -- the largest tunnel in America. This tunnel won’t carry cars, trains, or even people, but it will deliver 1.3 billion gallons of water daily to nine million area residents. New York City’s $6 billion Third Water Tunnel is one of the nation’s largest and most complex public works projects ever attempted. In 1954, New York City recognized the need for a new tunnel to meet the growing demand on its 150-year-old water supply system. Construction began in 1970 on the Third Water Tunnel, a tunnel designed to improve the dependability of New York City’s entire water supply system. The majority of the tunnel is being carved with a 450-ton, 19-foot diameter rock-chewing device called a tunnel boring machine. Unlike the older water supply tunnels in New York City, water control valves in the Third Water Tunnel will be housed in large underground chambers, making them accessible for maintenance and repair. When completed in 2020, the size and length of the Third Water Tunnel, its sophisticated valve chambers, and its depth of excavation will represent the latest in state-of-the-art tunnel technology.

Fast Facts: The equipment used to dig the Third Water Tunnel is the same that was used to dig the underwater Channel Tunnel,

or "Chunnel," that connects mainland France to England. The largest valve chamber in the tunnel, the Van Cortlandt Park Valve Chamber, is 620 feet long (longer than two

football fields placed end to end), 42.5 feet wide, and 41 feet high. The tunnel boring machine, which had to be lowered into the tunnel in pieces and assembled at the bottom, is

capable of excavating 50 feet of rock per day at a diameter of 23 feet -- more than twice the rate previously achieved in tunnel construction through drilling and blasting methods.

Seikan Tunnel

Vital Statistics:Location: Honshu and Hokkaido, JapanCompletion Date: 1988Cost: $7 billionLength: 174,240 feet (33 miles)Purpose: Railway

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Setting: UnderwaterMaterials: Steel, concreteEngineer(s): Japan Railway Construction CorporationIn 1954, a typhoon sank five ferry boats in Japan's Tsugaru Strait and killed 1,430 people. In response to public outrage, the Japanese government searched for a safer way to cross the dangerous strait. With such unpredictable weather conditions, engineers agreed that a bridge would be too risky to build. A tunnel seemed a perfect solution. Ten years later, work began on what would be the longest and hardest underwater dig ever attempted. Engineers couldn't use a tunnel boring machine to carve the Seikan Tunnel because the rock and soil beneath the Tsugaru Strait was random and unpredictable. Instead, tunnel workers painstakingly drilled and blasted 33 miles through a major earthquake zone to link the main Japanese island of Honshu with the northern island of Hokkaido. Today, the Seikan Tunnel is the longest railroad tunnel in the world at 33.4 miles in length, 14.3 miles of which lie under the Tsugaru Strait. Three stories high and 800 feet below the sea, the main tunnel was designed to serve the Shinkansen, Japan's high-speed bullet train. Unfortunately, the cost of extending the Shinkansen service through the new tunnel proved to be too expensive. In fact, air travel today between Honshu and Hokkaido is quicker and almost as cheap as rail travel through the tunnel. Despite its limited use, the Seikan Tunnel remains one of the greatest engineering feats of the 20th century.

Fast Facts:

More than 2,800 tons of explosives were used in the construction of the tunnel. One hundred sixty-eight thousand tons of steel was used in the construction of the tunnel. That's enough steel to

build four Petronas Towers!

The railway track runs 787 feet below the surface of the sea, making it the deepest railway line in the world.

During construction in 1976, tunnel workers hit a patch of soft rock with disastrous results. Water gushed into the tunnel at a whopping rate of 80 tons per minute. It took more than two months to control the flood. Luckily, no lives were lost.

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