The Channel Tunnel

Twin rail lines plunge into the Channel Tunnel near Coquelles, France for their 31 mile underground/underwater run. (Photograph Holger Weinandt licensed under the Creative Commons Attribution-Share Alike 3.0 Germany license)

In 1802 a French mining engineer named Albert Mathieu had a wonderful idea. After years of war between France and England the two European powers were finally at peace. Suppose one took advantage of this reconciliation to forge a permanent tie involving the two nations? Not just an effort to create goodwill between the two, longtime rivals, but a project that would physically link the two countries together. Something that would give them a way to move people and goods across the 20 miles of water of the English Channel without relying on the small, erratic ferries that made passengers seasick as they were knocked about by the unpredictable waves.What if one connected the two great nations together using an underground tunnel?

Mathieu presented his idea to the then current ruler of France, Napoleon. Unfortunately for Mathieu the "Peace of Amiens" lasted only a year and his idea was soon forgotten, but only after being ridiculed by the British Press in a cartoon showing French troops invading England via balloons and a tunnel.

Seven Quick Facts
-Length: 31.35 miles (50.45 km)
-Maximum depth under sea level: 490 feet (150m)
-Finished: May 6th 1994
-Cost: $21 billion
-Location: Folkestone,Britain, to Calais, France
-Made of: Two parallel rail tunnels and a smaller service tunnel constucted of steel and concrete.
-Other: Could have been constructed a century earlier if not for political squabbles between Britain and France.

It is probably for the best that Mathieu's idea was put on the shelf. His early designs showed a tunnel propped up by wood beams and illuminated by candles so that horse drawn carriages could make the twenty mile trip between the nations. While such passageways were practical for the mines of that day, it is doubtful that such a scheme would have succeeded under the Channel. At that time nobody even knew what the geology was like under the sea floor. Were there cracks that might let the seawater disastrously rush in?

The Proposed Victorian Tunnel

The idea did not die, however, and was revived by another French engineer named Thomé de Gamond in 1857. By then railway engine had been invented and tracks crossed the length and breadth of both continental Europe and Britain. The train, instead of Mathieu's carriages, seemed a much more practical way of moving people and goods through a long tunnel. That same year, in fact, a similar project was already underway in the Alps. The Fréjus Rail Tunnel was being dug under the mountains between France and Italy: a distance of 8.5 miles. By then tunnels had even been built underwater. In 1843 a walkway had been completed under the Thames River in London.

An 1802 tunnel design by the Albert Mathieu depended on horse drawn carriages to make the 20 mile trip.

De Gamond reasoned that as long as the geology permitted, a tunnel could be constructed under the channel. To check the channel floor he made a number of risky solo dives to the sea bed using the primitive diving equipment of the day. His explorations led him to believe that a layer of chalk ran under the channel from Cap Blanc Nez, in France to Dover, in England. The chalk was soft enough to dig, yet self supporting, waterproof and would provide a good medium for a tunnel. De Gamond's design had the passageway surface at an artificial island on the Varne sandbank about midway between the nations. The island would serve as an international port and provide ventilation for the tunnel.

It wasn't until the mid 1870's that politics made further work on the tunnel possibe. In the wake of the Franco-Prussian war of 1870-71 the French and British became much more friendly because they saw a potential common enemy in the Germans. The French Channel Tunnel Company was founded and given permission to do some more digging to check the geology. By 1881 serious tunneling got under way from both sides of the channel using new tunnel boring machines invented in 1875. It was estimated a 7 foot diameter pilot tunnel would be completed within 5 years.

Because steam engines would pollute the tunnel air with smoke, it was decided that special trains driven by compressed air would be used to move cars through the tunnel. The invention of the electric train would probably had made this design obsolete, however, before the tunnel even opened.

A political cartoon showing a French invasion using balloons, boats and a tunnel. Fear of this caused the British to canel the project.

Fears of a French Invasion

In 1883 tunneling was abruptly put to an end. The French and British governments were squabbling over the Suez Canal and colonies in Africa. The British military complained that if war came the tunnel might provide an easy way for the French to invade. The tunnel company volunteered to build a valve that could be used to flood the tunnel in an emergency and a fortress on the British side to close it with explosives if necessary, but their ideas fell on deaf ears. The British military leaders' vision was shortsighted however. During World War I they sorely wished that the tunnel had been built as it would have made supplying their troops France much easier.

It wasn't until the 1970's that another serious attempt to build a tunnel was again made. Unfortunately a bad economy caused the British to pull out of the project and the digging was abandoned.

The Engineering the Modern Tunnel

In the 1980's permission was given to a company to build the tunnel using private funds. It was decided that the tunnel would be designed to only accommodate trains. The public favored a drive through tunnel, but because of the extraordinary length, it was felt that access should be limited. It would be too easy to have a major traffic accident underground shut down the whole operation. Cars and trucks would be able to use the facility by driving to terminals on either side where they would be loaded onto special extra-large shuttle trains that would carry them through the tunnel.

The "chunnel," as it was nicknamed, was actually designed as three tunnels: Two 25 foot diameter tubes running parallel to carry trains and a 16 foot diameter service tunnel between them. To speed the digging the company used eleven tunnel boring machines (TBMs).

A tunnel boring machine similar to that used on the chunnel. The disc at the front rotates, cutting the rock.

TBMs had first been used as far back as 1825, but became the standard way of constructing tunnels after 1953. The machines cut a round passage through the earth by using a disc-like head with tungsten teeth that rotates to break up the rock. The rock is then sent by conveyer belt to the rear of the machine were it can be transported out of the tunnel by rail cars or other vehicles. Boring machines are also capable of reinforcing the tunnel interior surface by erecting concrete walls as they go along. Such re-enforcement was necessary on the French side of the channel where the geology was less stable.

The project used eleven boring machines each of which were the length of two football feilds and capable of chewing though 250 feet of rock a day. On the British side six were lowered into an excavation near Dover's Shakespeare Cliff. Three were pointed toward the channel to make the underwater portion of the tunnel and three toward the mainland to make the tunnel approaches. In France the same thing was done with five machines at an excavation near Sangatte.

The Tunnel Opens

On December 1st of 1990, the service tunnels, which proceeded in front of main tunnels so they could assess the geology, were connected in a ceremony watched by the press. It took until May 6th 1994, however, before the tunnel was official opened for business in a formal dedication involving British Queen Elizabeth II and French President François Mitterrand.

A cross section of the chunnel showing the rail tubes, service tube and interconnections.

Construction took eight years and cost $21 billion making it the most expensive constrution project in the world up to that point. The result was a tunnel 31.35 miles (50.45 km) long going underground at Folkestone in Britain and emerging again at Calais in France. At the time that it was opened it was the second longest tunnel in the world, but with longer portion of its run underwater (23 miles, 37km) than any other subway. It runs an average of 148 feet (45m), with a maximum depth of 490 feet (150m), below the English Channel. With the addition of a high-speed track from Dover to London in 2006, passengers can travel between the British and French capitals in just two-hours and fifteen minutes by train. New proposed higher-speed trains might shorten that trip to under two hours in the future.

All the trains that run through the tunnel are electric powered helping to eliminate the problem of having fumes underground. However there are several diesel locomotives available for service and emergency work should the power fail. The tunnel has experienced two fires large enough to close down at least one of the two tubes for an extended length of time. Nobody was killed in either incident, however.

Entering the Channel Tunnel on the French side.

The service tube that runs down between the rail tunnels not only allows work men and equipment to move through the tunnel without blocking the trains, but is connected to the rail tunnels at regular intervals to provide a way for the air pressure in front of an oncoming train to be relieved. About halfway through the tunnel all three passages come together at a "canyon." At that location switches allow the trains to move from one tube to another, giving the operators greater flexibility should a section of a rail tunnels be shut down.

The Channel tunnel was one of the largest construction projects of the 20th century and remains a marvel of engineering even today. It was selected by American Society of Civil Engineers as one of the Seven Wonders of the Modern World in 1996.

Copyright Lee Krystek 20011. All Rights Reserved.