Introduction
Eurotunnel has other many names which are Channel Tunnel and Channel. The name Eurotunnel came from the company which controls it and who won the tender. This is a 50.450 km rail that connects France with England. Its starts from Strait of Dover and ends in Coquelles in France and passes through Folkestone and Kent. Its the second largest rail tunnel in the world and the longest undersea tunnel with a distance of 37.9 km.
Brief history
In 1751 this period Napolitic engineers looked for new ways of crossing the sea to the other side which led to the competition which was supposed to show the best initiative and creativity. In 1752 , Albert Mathieu proposed a horse drawn traffic, this was after the signing of the Treaty of Armenians. 1974 saw a tunnel in the UK which was started and after construction of 250 meters it was abandoned due to financial constraints.
Also within this period France tried but also they stopped. In 1984 the idea was supported by both Margaret Thatcher and Francois Mitterrand and it was put under consideration. Eurotunnel Group which was later to manage the project was formed in 1986. In 1987 the excavation began and early 1991 a breakthrough was achieved and they begun equipping and finishing the line. Late 1994 saw its completion. There was big celebration witnessed by both head of states; UK and France.
Structure of Eurotunnel
The structural data of Eurotunnel was considered important for the stability of the excavations and against external factors like the earth quakes. Since these region had various kinds of rocks like the Mesozoic and Paleogene it was considered well documentation was necessary both in terms of seabed topography and its geology.
The seabed of Strait was a result of Quaternary erosion after several periods of emergence. The seabed reaches a maximum of 60m along the tunnel alignment. The Strait can be divided into two parts that is the UK part consisting of the undisturbed structure and the French part which is deformed.
The construction of the tunnels was mostly done in the Chalk Marl, this was the basal unit of the Chalk. Chalk Marl is a marine deposit that is mainly a mixture of small fossils which because of its high clay content is relatively impervious to water. Chalk is porous hence its risky for the tunnel. More than 85% of the tunnels were constructed within the Chalk Marl. The Chalk is of the Cretaceous age while the Chalk Marl is about 100 million years old. This means that dinosaurs were still wandering on the landmasses when the Chalk Marl was deposited.
The re activation of faults that were there in Jurassic and basement rocks caused the fracturing of the Chalk formations in which the tunnels were driven in. The tunnel is situated in the areas that has most highly tectonised zone. This is so because in the French side the route intersects only a few major faults compared to the UK side no major fault with a throw was noticed during tunneling.
Between Folkestone and Dover the discontinuities dip steeply with degrees of between 60 and 70.
Mott MacDonald who ere the design management employed 500 technical personel. The collective design input was over 1.3 million man hours.
Running tunnels and caverns
he design of Channel Tunnel comprised of three tunnels; two of which were 7.6m in diameter bored running tunnels, a central tunnel which was 4.8m diameter used for service, this were connected by cross passages at regular intervals. This service tunnel acts as an emergency escape route with its own transportation system provides access for maintenance and carries the ventilation system. The aerodynamic drag from the speedy trains is reduced by 2m diameter pressure relief ducts which are connected to the running tunnel.
There are two vast undersea caverns each in UK section and the other in France which are scissor cross-overs linking the main running tunnels which acts has a precaution measure if a section of tunnel should be closed. The New Austrian Tunneling Method was used to construct the UK cross-over cavern which is the world’s largest subsea excavation with dimensions of 156m in length by 18m breath and 10m in height internally. With the help of some 200 stations was used to monitor the movement of the cavern excavation.
UK cut-and-cover
The UK land sections of the Channel Tunnel involved various cut-and-cover construction techniques which included; multi-propped walls, embedded cantilever diaphragm walls, deep open cut and piled chambers. Near the portal the Castle Hill, an extensive section of cut-and-cover had to be constructed fully within an existing landslip, which called for close control of groundwater movements and levels plus a carefully phased sequence of top-down construction, which included use of heave reducing piles.
Folkestone terminal
Where the tunnel emerges to the west of Castle Hill high-speed turnouts connect the tunnel tracks to the new terminal at Folkestone which houses all the services and interchange facilities for the Channel Tunnel trains.
To prevent interference between incoming and outgoing trains the railway layout takes the form of a loop within the terminal, with the assistance of three arrival lines and two in departure lines which earlier served ten parallel platform tracks where vehicles drive on and off the shuttles. This layout is such that it allows for future extensions of the platforms to 16. A total of 54 km of both ballasted and non-ballasted track is provided with turnouts of 100 which includes maintenance sidings and stabling. The UK tunnel had concrete expanded segmental lining while the voids were grounted to control water ingress.
Road infrastructure beside the terminal includes new links with the M20 London-Dover motorway and the re-routed A20 trunk road which was also designed by Mott MacDonald. Seven new bridges and viaducts were constructed with the intention of keeping motorway traffic flows moving during construction.
Since the use cooling of the tunnel through circulation of air from one end to the other was not possible they decided to use two chiller plants. These chiller plants were put at each end. The chiller plants cool 54 million gallons of water in a 150 mile pipe with a diameter of 24 inch. The tunnel is usually kept dry by five pumping stations and sumps. Three were built under the sea and one on each shore.
In France, construction began at Sangatte on Calais coastline around 3 km from Coquelles. A concrete wall was built after the excavation of a hole of 70 meters deep and 55 meters in diameter. A shed was built to shelter the shaft in the middle of the large construction site which housed offices and a lining segment manufacturing factory. The workers, materials and equipments were lowered through the shaft down to a distance of 47 meters. This place was dry and it made possible the assemble of boring machines. Three machines were used to excavate the three tunnels and two machines began to drive inland towards the terminal site.
When excavation began in France the TBMs encountered water inflows. Since the cutter head was sealed the tail shield were fitted with multiple rows of wire brush which pressed the outside diameter of the concrete lining. In the space between the metallic brushes and the lining of the tunnel grease was injected. Between the tunnel lining and the ground, grout cement was injected after grout lines had being fitted in the tail shield. This method sealed the lining from water.
Five machines were used in the French side while six machines were used in the UK side. The end part of France was reclaimed with an outcome of 480 hectares for the terminal complex. The French tunnels which were undersea had water tight bolted and gasketed segmental concrete lining.
69 km of tunnel were constructed in the France side while 84km was constructed in the English side. In France the construction took an average of 110 meters per week while in the English side was it took 150 meters per week. The best performance was 426 m a week in UK and 322 meters in France.
It took 13,000 workers to complete the digging in seven years. Tunneling operations began from both ends. The contractors were the Anglo-French Transminche Link (TML) and a group of ten companies and five banks from the two countries. This companies used heavy machines know has tunnel boring machines (TBMs). The TBMs were 200m long and were telescopic, this allowed the construction of the concrete segmental lining.
Five TBMs were constructed with each dedicated to a certain region in terms of geology. The high pressures of water also dictated the conditions of the machines. The French side required three Earth Pressure Balance machines (EPBMs). The characteristics of these machines were sealed cutter chambers to withstand water pressure and conveyors to carry the cut material away. Two EPBMs were built for each rail tunnel with a diameter of 8.8 m and 1100 tonnes in weight. The head had a thrust of 39,227 kN and torque of 12,748.645 Nm.
The EPBM for the service part were 5.6m in diameter cutter head, a thrust of 39,227kN and a torque of 3,510,781Nm. The water flow in the UK was predicted to be small hence two double shielded TBMs were built. These machines were constructed such that it could be able to withstand unstable and faulted rock conditions. These TBMs had a diameter of 5.6m cutterhead, a thrust of 65,871kN and torque of 5,727,084Nm.
This companies were divided as follows:
- one French TBM driving the service tunnel from Sangatte cofferdam to the French portal,
- three French TBMs driving from Sangatte to under the Channel,
- one French TBM driving one running tunnel from Sangatte cofferdam to the French portal, then the
- three British TBMs driving from Shakespeare Cliff to the British portal,
- other running tunnel from the French portal back to Sangatte cofferdam,
- three British TBMs driving from Shakespeare Cliff to under the Channel.
December 1, 1990 the services tunnel was drilled through. While the rail tunnels met on June 28, 1991 and May 22, 1991.This was witnessed by many people, the media and jubilation. The British TBMs were diverted to the rocks and it was sealed with concrete while the TBMs of France were dismantled. Between 1991 and 1994 there was refining, putting of equipments such as electricals and finishing. In the British side about 5 million cubic yards of chalk were excavated and much of it was dumped In the Shakespeare Cliff to reclaim 90 acres of land from sea which was then called the Samphire Hoe.
The two rail tunnels are 30m apart each measuring 7.6 meters in diameter which carry trains north west and south east. The service tunnel is 4.8 meter in diameter and is normally connected to the rail tunnels at intervals of 375 meters. This service tunnel is served by narrow rubber tyred vehicles which are used for minimal access of facilities. The service tunnel is used for access to workers for maintenance and emergencies exit.
There are 2m diameter pressure relief ducts (PRDs) that are normally above the service tunnel but they don’t connect. This PRDs are connected to the two rail tunnels at an interval of 250 meters. PRDs are used to alleviate piston effect of trains to allow free movement of the air to flow into the other running tunnel.
Public Transportation
It was opened in 1994 after seven years of construction. This railway network offered three kinds of services. That are freight trains, shuttle for vehicles and Eurostar passenger.
Eurostar passenger: this is a train service that connects London and Kent in Britain, Lille and Paris in France and Brussels in Belgium. There are limited or seasonal services to Disneyland and other major cities in France. Its accomplished by eighteen carriage class 373 trains with speeds of 300 km/h. There as being development and construction of new lines to join the LGV Nord which are of high speeds. This kinds of speed enabled the change of terminus from Waterloo International to St. Pancras International Station. It takes between 20 minutes to 35 minutes to go through the tunnel.
Eurotunnel Shuttle: this is a shuttle service that is provided between Calais in France and Folkestone in the UK. It transports vehicles and passenger vehicles in enclosed wagons. Cars and other low vehicles in double deck wagons are carried in the rear rake. The first and last two carriages contains the access ramps. In front of the train heavy and high vehicles are carried such as the coaches and buses. If its busy sometimes cars can be carried in this section. Sanitary facilities are provided in every third carriage in the double deck and the first and last carriages in the single deck. This carriages are normally fire proof and are pressurized.
Eurotunnel freight shuttle trains carry lorries on open rail cars, and the drivers traveling in the passenger coaches. Eurotunnel rail freight service normally carry convectional rail loads between stations with capabilities of moving the containers.
Reference
Colin J. Kirkland, Engineering the Channel Tunnel, 1999.
Anderson G. at el. The Channel Tunnel Story, 1994.
Darian-Smith E. Bridging Dividens: The Channel Tunnel.
Bent F. Megaprojects and Risk: An Anotomy of Ambition. Cambridge University Press, 2003.