TUNNEL VISION.

Dig your way through three of the world's most earth-shattering tunnels and the technology that made them.

Have you ever sped through a long, dark tunnel and wondered how it was built or why it doesn't collapse? Tunnels are among the world's most astounding engineering feats. Not only do these underground passages provide fast, safe transportation paths, they also help shuttle water, electricity, and gas to and from your community. No wonder runnels can be found nearly everywhere there's dirt!

But tunnels are tough to build. To hollow out a tunnel two hundred years ago, it took workers armed with picks and chisels a week to plow through just three feet of rock and soil. Now with the help of high-tech tools, a tunnel builder can shred through 244 meters (800 feet) of Earth in the same amount of time--without touching a shovel.

Turn the page to learn more about the hidden world of tunnels.

How do you build a tunnel? It depends on the type of rock and soil through which you plow. Boring through a rocky mountain requires different construction methods than scooping out muddy soil beneath an ocean or compacted sand under a large city.

City Tunnel No. 3, New York City 

If you think the streets of New York City are endless, imagine the vast system of tunnels that meander 122 to 244 meters (400 to 800 feet) underground. They're part of City Tunnel No.3, an ambitious project begun in 1970 to help shuttle 1 billion gallons of water a day from mountain reservoirs to the city's residents. When completed in 2020, the massive tube will wind through 97 kilometers (60 miles) of tough bedrock (solid rock layer that supports surface soil) to become the country's largest water tunnel.

One huge obstacle: digging through building-size boulders. Solution? The Tunnel Boring Machine (TBM), a 450-ton rock-chewing tool nicknamed "the Mole." It's armed with 500pound titanium cutting blades that spin like giant drills (see photo, p. 15). New York's custom-built TBM grinds through 15 m (50 ft) of rock and rubble per day, creating a tunnel 7 m (23 ft) in diameter. Following the roaring TBM, workers coat rough tunnel walls with a smooth layer of concrete.

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To carve through sandy spots between boulders, workers use a tunnel shield instead--a razor-edged iron cylinder that funnels loose dirt through its hollow center. Workers then dump dirt onto conveyor belts that haul it to elevator shafts, where the dirt is lifted out of the tunnel. The shield also stops loose sand from caving in on workers while they line the tunnel walls with even thicker concrete.

St. Gotthard Base Tunnel, Swiss Alps 

The mighty Alps are the largest mountain range in Europe. Sixty percent of Switzerland's 15,900 square-mile surface is covered with snow-capped peaks--and loads of noisy mountain traffic.

To reduce the gridlock, workers are boring the 57-kilometer (36 mile)long Gotthard Base Tunnel through the base of the Swiss Alps. When the project is completed in 2012, electric-powered, 250 km/hour (150 mph) trains will glide through the world's longest railway tunnel.

Railway tunnels through the Alps already exist, but their paths follow the rise and fall of the mountains, making for slow journeys. The Gotthard will blaze a straight, efficient path as low as 2,313 m (7,590 ft) below the range's peaks.

Engineers use different tools and techniques to cut through diverse mountainous rock layers, which vary from hard granite (coarse-grained volcanic rock) to softer slate. Stretches of rock will be either blasted with dynamite or bored with a TBM, but workers must immediately support the crumbly walls with thick layers of concrete and steel arches to prevent their collapse.

So far, engineers have begun tunneling work entrances in the mountain. Workers track their progress using the Global Positioning System (see "Eyes in the Sky," page 17) to ensure the tunnels connect in the middle.

Seikan Tunnel, Japan

One of the world's most dangerous tunnel projects has been Japan's underwater Seikan railroad tunnel, a 25-year undertaking begin in 1964. Dug 244 m (800 ft) under the sandy seabed of the Tsugaru Strait, the 53 kin- (33 mi)-long tunnel links the two islands of Hokkaido and Honshu. Although built to avoid treacherous seas, which often sank ferries shuttling between the islands, the tunnel claimed the lives of 33 workers during construction!

How did workers bore a tunnel beneath an ocean? First, geologists surveyed (measured and assessed) soil composition to determine the best strategy for the project. To gather soil samples, Japanese National Railways  engineers drilled two small passageways or shafts on each of the islands. Soil specimens revealed cracked, porous (holey) layers of volcanic rock--likely to collapse under the weight of ocean water.

So workers grouted the tunnel's path before carving it by pumping a high-pressure cement mixture into the rock to seal any leaky cracks and holes. Then they blasted 2,860 tons of explosives to mine their way through the reinforced rock.

Despite these efforts, four major floods inundated the tunnel during its construction. In 1976, workers struck a soft patch of rock, and 80 tons of water per minute pounded into the tunnel. It took two months to control the deluge! Today, four separate pumping systems work around the clock to keep water out.

3-D Rock Vision

This seismic tomograph (computerized map of earth's composition) of a future tunnel shaft uses sound waves to pinpoint hard, soft, and weakened (by blasting) rock layers. The image, created by Rockvision[TM], sends sound waves through earth, then analyzes their return speed. Sound travels fastest through hard rock (blue). Purple indicates regions of softer rock, where sound travels slower

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Pipe Dreams

How do you deliver I billion gallons of water per day to more than 9 million people? Ask tunnel engineers in New York City.

For nearly 25 years, they have been constructing the country's largest water tunnel: a 60-mile-long tube that burrows the distance of two Empire State Buildings deep beneath the city. It will replace two older tunnels.

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Big Bore

At 2,000 meters (6,562 feet) deep in the Swiss Alps, temperatures soar to 45 [degrees] C (113 [degrees] F). To withstand extreme conditions, engineers custom built a 10 m (33 ft)-wide tunnel boring machine (TBM). The 925-ton rock-chewing beast will bore the world's longest railway tunnel: the $5.8 billion Gotthard Base Tunnel.

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Seikan Tunnel

Japan's Seikan Tunnel is the longest undersea railroad tunnel in the world. At 53 km (33 mi), it connects the islands of Hokkaido and Honshu. During construction, the speedy Tunnel Boring Machine proved too overpowering for the unstable volcanic rock beneath the Tsugaru Strait. Instead, engineers carefully drilled and blasted out the tunnel a few feet at a time to avoid flooding.

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RELATED ARTICLE: Cross-Curricular Connection

History: Pick a period in history and choose one of its engineering achievements. Describe the process of building it in a paragraph or two.

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Did You Know?

* The first tunnels were probably extensions made to caves in prehistoric times. Tunnels carrying drinking water were built thousand of years ago in ancient Persia and India.

* High-speed trains could heat tunnels to 50 [degrees] C (122 [degrees] F) because of friction created by wheels rubbing on tracks. Pipes filled with cold water are often used to keep these tunnels cool.

* Construction of the Seikan Tunnel required enough steel to build over 40 skyscrapers (168,000 tons), 61,422,000 cubic feet of concrete, and 13,800,000 workers--about one-quarter of the working population of Japan.

National Science Education Standards 

Grades 5-8: structure of the earth system * abilities of technological design * science and technology in society

Grades 9-12: understanding about, science and technology * abilities of technological design * science as a human endeavor

Resources

Tunnels by Andrew Dunn, Thomson Learning, 1993

Structures: The Way Things Are Built by Nigel Hawkes, Macmillan Publishing Co., 1990

For more on the Seikan Railroad Tunnel, log onto: www.discovery.com/stories/technology/buildings/tu nl_longest.html

Directions: Read "Tunnel Vision" and then circle the correct answer.

1. A century ago, miners with picks and axes tunneled through (3 feet, 30 feet, 300 feet) of rock a week.

2. High-tech tools now excavate (80 feet, 800 feet, 8,000 feet) in a week.

3. A seismic tomograph shows how fast (light, sound, X-rays) travel through rock.

4. To carve through sandy spots, workers use a (big bore, TBM, tunnel shield).

5. The TBM creates a tunnel (28 feet, 23 feet, 54 feet) in diameter.

ANSWERS

1. 3 ft 2. 800 ft 3. sound 4. tunnel shield 5. 23 ft