The Channel Tunnel

Feted as one of the seven wonders of the modern world by the American Society of Civil Engineers in 1996, the Channel Tunnel is the longest undersea tunnel in the world. Connecting Folkestone, Britain to Calais, France the 31.35 mile tunnel took more than five years to complete and cost billions of dollars. One of the largest construction projects of the 20th century, the channel allows passengers to travel between the British and French capitals in just two-hours and fifteen minutes by train. It consists of three parallel tunnels running under the sea: two main rail tunnels to carry trains from the north and from the south and a Channel Service Tunnel, which allows maintenance workers to access the rail tunnels at regular intervals. Engineers who worked on this project had to overcome various challenges: working in two languages, using two sets of national construction, safety and legal codes as well as integrating with a diverse group of stakeholders.


In 1802, Albert Mathieu, a French mining engineer came up with the idea of linking France and England using an underground highway for horse drawn carriages as a sign of peace between the two great nations. While the idea was never pursued it planted a vital seed for future attempts. In 1856, surveyor Aimé Thomé de Gamond came up with the proposal to build a tunnel that would use railway trains rather than horse carriages. He carried out a number of risky solo dives to the sea bed to check on the channel floor and came up with a design that had the passageway surface at an artificial island about midway between the nations which would serve as an international port and provide ventilation for the tunnel. While he presented the idea to Napoleon it never took off. During the Franco-Prussian War the two nations formed the French Channel Tunnel Company to check on the geology of the sea bed and carry out some digging. Squabbles between the two governments and fears by the British over their security caused the project to be cancelled. Another serious attempt came up again in the 1970s but the British pulled out due to national economic constraints.

Finally, in the 1980s, both governments invited private companies to build the tunnel and present their proposals. There were several designs brought forth from a drive through tunnel and a suspension bridge but because of the extraordinary length, it was felt that access should be limited and they finally settled on one design submitted by the Balfour Beatty Construction Company. Dug under the English Channel, the Tunnel would comprise of two, parallel railway tunnels. Between these two railway tunnels would be a third, smaller tunnel that would be used for maintenance, including drainage pipes and communication cables. Each of the trains running through the tunnel would be able to hold cars and trucks, allowing personal vehicles to pass through without drivers having to endure the long, underground drive.

Inside the Channel Tunnel

Digging Under The Sea

At the time of its construction, the Channel Tunnel was the most expensive engineering project ever undertaken. Satellite data from geophysical surveys helped to determine the alignment and route of the tunnel. Following a single chalk stratum, the French and British dug simultaneously from each end, using laser surveying methods to meet in the middle. Previous investigations revealed that tunneling through the lower chalk stratum layer was feasible. This was because it was resistant to penetration by groundwater because of the presence of clay, and it provided an adequate medium for tunnel boring machines (TBM), as it was more solid and less prone to fracturing and collapsing. The top and middle layers were unsuitable for digging as they contained sand and gravel that made drilling difficult and had high levels of porosity. Tunnel Boring Machines (TBMs) were used to speed up the digging process. TBMs resemble large cylinders and are as long as two football fields. The cutters fixed at the front side of the machine have tungsten teeth. These teeth would cut a round passage on the earth and break up to 250 feet of rock in a day. All the crushed material created by digging is carried out by conveyor belts to the rear of the machine where it was transported out of the tunnel by rail cars or other vehicles. Used as far back as 1825, TBMs had become the standard way of constructing tunnels at that time and were able to reinforce the interior of the tunnel surface by erecting concrete walls as they went along. This was essential for the French as their side of the tunnel passed through a more brittle and fractured layer. In total, eleven TBMs were used – six by the British and five by the French. Though the types of TBMs used were different, the overall tunneling technique was identical.

Through the entire process, engineers used large tunnel boring machines (TBMs), mobile excavation factories that combined drilling, material removal, and the process of shoring up the soft and permeable tunnel walls with a concrete liner. The total amount of dirt and debris that was removed from the tunnel exceeded eight million cubic meters. The British built an enclosed seawall for the storage of the loose sediments whereas the French mixed water with the soil and the mixture was pumped outside. The French had to battle with water inflows almost as soon as they started and had to take extra measures to seal their equipment from the incoming water flow. The British also faced unforeseen water inflows in a 2 mile stretch and had to slow down their progress as each section of tunnel had to be grouted in advance of boring. In December 1990, the French and British TBMs met in the middle and completed the Channel Service Tunnel bore. The French TBM was dismantled while the British one was diverted into the rock and abandoned. However, that was not the end of the construction as crossover tunnels, land tunnels from the coast to the terminals, piston relief ducts, electrical systems, fireproof doors, the ventilation system, and train tracks all had to be added. Also, large train terminals had to be built both in Britain and France. The completed project was opened by Queen Elizabeth II and French President François Mitterrand on May 6 1994.

Safety and Maintenance Features

At intervals of 375 meters, all three tunnels are interconnected by cross passages that provide an opening for ventilation, safety and maintenance. This allows maintenance to be carried out in one tunnel while the other one is still running. There are also piston relief ducts that alleviate pressure within the two running tunnels every 250 meters. Two ventilation systems were also designed –one for everyday use while the other is only for emergencies. 300 miles of cold water piping run alongside the rail tracks to drain off the heat raised by air friction. Each rail tunnel has two walkways; one for maintenance purposes and the other for use in the event of an emergency evacuation and on the side nearest the service tunnel. They are also designed to maintain a shuttle upright and in a straight line of travel in the unlikely event of a derailment. Halfway through the tunnel all three passages meet at an intersection, allowing the trains to move from one tube to another. This also gives the operators greater flexibility should a section of a rail tunnels be shut down. All the trains are electrically powered, eliminating the problem of heavy fumes underground. Service tunnel vehicles were specifically designed for travel in the service tunnel. It is used for maintenance operations and in case of incidents, with the aim of reaching the scene of an incident in minimum time. Furthermore, various fire protection and detection systems are installed at various points along the length of the tunnel.

The Channel Tunnel Today

Since its opening, the tunnel has become very popular and represents a large amount of traffic between France and Britain. Travel time between the two nations has also reduced significantly with a crossing taking about 35 minutes from platform to platform. The Channel Tunnel continues to be a landmark engineering achievement around the world.

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