A tunnel is an underground or underwater passage that is primarily horizontal. Relatively small-diameter ones carry utility lines or function as pipelines. Tunnels that transport people by rail or by automobile often comprise two or three large, parallel passages for opposite-direction traffic, service vehicles, and emergency exit routes.

The world's longest tunnel carries water 105 mi (170 km) to New York City from the Delaware River. The lengthiest person-carrying tunnel is the Seikan Railroad Tunnel. It is a 33-mi (53-km) long, 32-ft (9.7-m) diameter railroad connection between Japan's two largest islands, Honshu and Hokkaido.

One of the most anticipated tunnels was the Channel Tunnel. Completed in 1994, this tunnel connects Great Britain to Europe through three, 31-mi (50-km) long tunnels (two one-way and one service tunnel). Twenty-three miles (37 km) of this tunnel are underwater.


Tunnels were hand-dug by several ancient civilizations in the Indian and Mediterranean regions. In addition to digging tools and copper rock saws, fire was sometimes used to heat a rock obstruction before dousing it with water to crack it apart. The cut-and-cover method—digging a deep trench, constructing a roof at an appropriate height within the trench, and covering the trench above the roof (a tunneling technique still employed today)—was used in Babylon 4,000 years ago.

The first advance beyond hand-digging was the use of gunpowder to blast a 515-ft (160-m) long canal tunnel in France in 1681. The next two major advances came about 1850. Nitroglycerine (stabilized in the form of dynamite) replaced the less powerful black powder in tunnel blasting. Steam and compressed air were used to power drills to create holes for the explosive charges. This mechanization eventually replaced the manual process made famous by John Henry, the "steel-driving man," who swung a 10-lb (4.4-kg) sledge hammer with each hand for 12 hours a day, pounding steel chisels as deep as 14 ft (4.2 m) into solid rock.

Between 1820 and 1865, British engineers Marc Brunel and James Greathead developed several models of a tunneling shield that enabled them to construct two tunnels under the Thames River. A rectangular or circular enclosure (the shield) was divided horizontally and vertically into several compartments. A man working in each compartment could remove one plank at a time from the face of the shield, dig ahead a few inches, and replace the plank. When space had been dug away from the entire front surface, the shield was pushed forward, and the digging process was repeated. Workers at the rear of the shield lined the tunnel with bricks or cast iron rings.

In 1873, American tunneler Clinton Haskins kept water from seeping into a railroad tunnel under construction below the Hudson River by filling it with compressed air. The technique is still used today, although it presents several dangers. Workers must spend time in decompression chambers at the end of their shift—a requirement that limits emergency exits from the tunnel. The pressure within the tunnel must be carefully balanced with the surrounding earth and water pressure; an imbalance causes the tunnel either to collapse or burst (which subsequently allows flooding).

Soft soil is prone to collapse and it can clog digging equipment. One way to stabilize the soil is to freeze it by circulating coolant through pipes embedded at intervals throughout the area. This technique has been used in the United States since the early 1900s. Another stabilization and waterproofing technique—widely used since the 1970s—is to inject grout (liquid bonding agent) into soil or fractured rock surrounding the tunnel route.

Shotcrete is a liquid concrete that is sprayed on surfaces. Invented in 1907, it has been used as both a preliminary and a final lining for tunnels since the 1920s.

In 1931, the first drilling jumbos were devised to dig tunnels that would divert the Colorado River around the construction site for Hoover Dam. These jumbos consisted of 24-30 pneumatic drills mounted on a frame welded to the bed of a truck. Modern jumbos allow a single operator to control several drills mounted on hydraulically controlled arms. In 1954, while building diversion tunnels for construction of a dam in South Dakota, James Robbins invented the tunnel boring machine (TBM), a cylindrical device with digging or cutting heads mounted on a rotating front face that grinds away rock and soil as the machine creeps forward. Modern TBMs are customized for each project by matching the types and arrangement of the cutting heads to the site geology; also, the diameter of TBM must be equal to the diameter of the designed tunnel (including its lining).

Raw Materials

Materials used in tunnels vary with the design and construction methods chosen for each project. Grout used to stabilize soil or fill voids behind the tunnel lining may contain various materials, including sodium silicate, lime, silica fume, cement, and bentonite (a highly absorbent volcanic clay). Bentonite-and-water slurry is also used as a suspension and transportation medium for muck (debris excavated from the tunnel) and as a lubricant for objects being pushed through the tunnel (e.g., TBMs, shields). Water is used to control dust during drilling and after blasting, which is often done with a low-freezing gelatine explosive. Water-and-salt brine or liquid nitrogen are common refrigerants for stabilizing soft ground by freezing. The most common modern lining material, concrete reinforced by either steel or fiber, may be sprayed on, cast in place, or prefabricated in panels.

Choice of method

A tunnel's construction method is determined by several factors, including geology, cost, and potential disruption of other activities. Different methods may be used on individual tunnels that are part of the same larger project; for example, four separate methods are being used on portions of Boston's Central Artery/Tunnel project.

The Manufacturing Process



Final lining


Sometimes the earth removed from a tunnel is simply discarded into a landfill. In other cases, however, it becomes raw material for other projects. For example, it may be used to form the base course for an approach roadway or to create roadway embankments for wider shoulders or erosion control.

Quality Control

Besides maintaining ground stability around the tunnel and ensuring structural integrity of the tunnel lining, proper alignment of the excavation path must be achieved. Two valuable tools are global positioning system (GPS) sensors that receive precise locational data via satellite signals and guidance systems that project and detect a laser beam within the tunnel.

The Future

Exploration methods, materials, and machinery are possible areas of improvement. Sound waves transmitted through the earth can now generate a virtual CAT scan of the tunnel path, reducing the need to drill core samples and pilot tunnels. Some examples of materials research involve cutting tools that are more effective and durable, concrete with more precisely controlled hardening rates, and better processes for modifying soil to make it easier to cut, dig, or remove. Recent developments in machine technology include multiple-headed TBMs that can bore two or three parallel tunnels simultaneously and a TBM that can turn a corner up to 90° while cutting. Better remote control capabilities for digging machinery would improve safety by reducing the amount of time people have to be underground during the digging process.

Where to Learn More


Burroughs, Dan, et al. "Depressing Traffic Top-Down." Civil Engineering (January 1994): 62.

Campo, David W., and Donald P. Richards. "Tunneling Beneath Cairo." Civil Engineering (January 2000): 36.

Iseley, Tom. "Microtunneling MARTA." Engineering (December 1991): 50.

O'Connor, Leo. "Tunneling Under the Channel." Mechanical Engineering (December 1993): 60.


The Cumberland Gap Tunnel. http://www.efl.fha.dot.gov/cumgap/tunnel.htm (January 2000).

"A Short History of Tunnelling." http://pisces.sbu.ac.uk/BE/CECM/Civ-eng/tunhist.html (January 2000).

"Tunnel Jacking." Central Artery/Tunnel Project. http://www.bigdig.com (January 2001).

Loretta Hall

Also read article about Tunnel from Wikipedia

User Contributions:

Namir Al-sarraf
Do the consolidation grouting used in the tunnel or only contact grouting?

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