Channel Tunnel: Construction of the World’s Longest Underwater Tunnel

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The Channel Tunnel, also known as Eurotunnel or Chunnel, is the world's longest underwater railway tunnel built to connect the United Kingdom with Europe via France. Traveling through the tunnel is possible either by ordinary rail coach or the passengers' own vehicles, which are loaded onto special railcars.

The Chunnel project consists of two main transportation tunnels of 7.6 m diameter spaced at a distance of 30 m, and one service tunnel of 4.8 m diameter. The main running tunnels and service tunnel are connected with 3.3 m diameter passages for functional and safety reasons in the transverse direction at an interval of 375 m throughout the tunnel.

Besides, ducts with a diameter of 2 m were constructed between the main tunnels at every 250 m span. These ducts act as air pressure relief ducts to dissipate the air pressure in front of the train, thereby diminishing the aerodynamic drag on the moving train. The total length of the Channel Tunnel is 50 km, out of which 38 km is located under the seabed.

In addition, two large underground crossover chambers were constructed at a distance of 27 km and 45 km along the tunnel length. These chambers allow the trains to easily switch between the tracks running in the tunnels, for example, during maintenance works being carried out in a particular tunnel section.

1. Geology of the Channel Tunnel

The following points describe the geology of the site of the Channel Tunnel:

1. The Channel Tunnel was constructed on Dover strait, which includes the anticline folds.
2. The seabed of 38 km comprises the chalk rock and clay. The thickness of the chalk bed and clay is given as: (a) Upper Chalk: 90 m, soft white chalk with flints. (b) Middle chalk: 70 m, hard white chalk with little or no flints. (c) Lower chalk: 68 m, combination of white, grey, and yellow chalk was present. (d) Clayey layer: 15 m, calcareous clay, and mudstone were present.
3. The compressive strength of chalk was around 50 MPa, whereas, in clays, it was only around 5 MPa. Therefore, the chalk marl was considered an ideal medium for tunneling.
4. The Channel Tunnel was excavated in the lower region of chalk, which had sufficient compressive strength to act as a supporting medium.
5. Before the construction, around 120 marine and 70 land boreholes were drilled along the tunnel alignment level. Around 4000 line-km of marine geophysical survey was conducted.
6. Chalk marl was mostly classified as blocky. Rock-quality designation (RQD) values of chalk marl were reported around 90%, which was deemed to be fit for construction. The Quality-index for chalk marl was in the fair to good category.

2. Tunnel Construction

To construct a 50 km long tunnel at a depth of 50 m below the sea was a challenging task. It tested the imagination and skills of the top minds in the British and French construction industry. The construction works for the Channel Tunnel started in 1987 and most of the tunneling work was finished by 1991. The various machinery and advanced technologies used in the tunneling of the Channel Tunnel are discussed below:

1. The construction of terminal stations at either ends of the Channel Tunnel was a gigantic construction project on its own.
2. To construct two main tunnels and one service tunnel of 50 km length between the terminals, 11 massive Tunnel Boring Machines (TBM) were used on 12 separate working faces of the tunnel.
3. Out of which, six numbers of open-mode full-face TBMs were used. Cast-iron and concrete segments were used as a lining material during tunnel construction using TBM.
4. The New Austrian Tunneling Method (NATM) was used for making castle hill tunnels, portals, shafts, and pumping stations.
5. Road-headers were used as an excavation tool for large chambers.
6. Hand excavation tools, the cast-iron segmental lining of cross passages, piston relief ducts, pumping stations, and ancillary structures were used to construct the tunnel.
7. The two main tunnels and one service tunnel ran into two vast undersea chambers that were 160 m long, 11 m high, and 18 m wide. The construction of these crossover chambers tested the nerves and skills of the finest engineers in the world.

2.1 Probing

The service tunnels act as pilot tunnels for the main running tunnel drives. Extensive probing was carried out ahead of the tunnel face to locate areas of potential high-water ingress.

Additional downward vertical probing was regularly carried out in the invert, at the rear of the TBM. Probing on the UK side accounted for 7% of the TBM’s downtime, with almost the full width of the Channel Tunnel being probed. Sideways probing was also carried out from the service tunnels to the crown area of both the running tunnels before starting the drive of running tunnels.

The frequency of such side probes depended on the potential problems. Probing was conducted at closer spacing if found that the ground conditions are deteriorating or when there was a possibility of increased water ingress adjacent to the service tunnels. During probing, normal cored and packer permeability tests were conducted to gather information on rock quality and likely water ingress.

2.2 Underground Technologies Used in the Construction of the Channel Tunnel

For just 1 km stretch of land, the project demanded four different methods of tunneling. It was just a reflection of the challenges that the engineers had to face while construction. The following underground technologies were used in the construction of the channel tunnel:

1. The New Austrian Tunneling Method: This method was used for the construction of crossovers chambers.  
2. Tunnel Boring Machines: A total of 11 TBMs were used to construct the Channel Tunnel.
3. Cut and Cover Construction Method: This method was used for excavating the area and building the tunnel out of reinforced concrete boxes for making the route through the geologically challenging castle hill.
4. Top-Down Construction Method: This method was used where space was limited. It was used to construct the tunnel and terminal's roof at the end of the Channel Tunnel.

2.3 Tunnel Boring Machines Specifications

The following points describe the specifications of TBMs used in the tunneling of the Channel Tunnel:

1. The TBMs were about 250 m long and had a circular boring face.
2. TBM was backed up by an ingenious collection of rams, lining erectors, conveyors, and handling cranes.
3. The TBM machines were working on a relatively simple principle, i.e., the soil was excavated by the rotating circular cutting head, and the four hydraulic rams provided the pressure to move forward.
4. Further, the excavated soils were sent to a conveyor belt connected to the TBM. The conveyor belt carried the spoil along the constructed part of the tunnel and dumped it into the articulated vehicles waiting to take it to the terminal for disposal.
5. The length of the circular cutting head was about 12 m. A control center with closed-circuit television and laser guidance system was used to track the cutting head movement. 
6. The back of the TBM had four main rams. By pushing the rams against the completed concrete lining, the cutting head was forced forward to apply pressure, allowing the head's circular action to excavate spoil.

2.4 Concrete Lining Segments 

Most of the parts comprised 380 mm thick concrete lining, but this varied according to the ground conditions of the site. The following points describe the specifications of concrete lining segments used in the Channel Tunnel:

1. In the tunnels running in the UK side, 1.5 m long lining ring was formed of eight lining segments plus a key lining segment. Whereas on the French side, six 1.4 to 1.6 m wide lining segments plus a key lining segment were used.
2. Concrete lining segments were cast outside the tunnel and delivered to the machine on wagons. More than 700,000 segments were cast and different linings were used for varying ground conditions. Due to this, the segments could weigh anything from 0.75 to 9 tons.
3. On the French side, 250,000 segments were cast at a hill situated directly above the entrance portal.
4. On the UK side, things were not that easy. There was barely enough room to hold a concrete mixer at the base of the Shakespeare cliff. Therefore, engineers had to search for a suitable location for casting the remaining 450,000 segments on the UK side. The correct spot was found eventually at the Isle of Grain in Kent, between the River Medway and the Thames Estuary.
5. A very efficient concrete segment casting production line was set up. These concrete segments were taken up to a distance of 100 km by rail, from the Isle of Grain to Shakespeare Cliff and stockpiled on the site. 

3. UK Side Tunnel Drive 

In the UK, the main site was at the Shakespeare Cliff, from where, the six British tunneling machines started their journeys. Three were sent seaward to meet their French equivalents in the tunnel and three landward to break through 8 km away from the terminal site. The following points describe the various technologies and methods used for tunnel drive in the UK side:

1. At the cliff location, two existing tunnels were used for lowering TBMs.
2. A 110 m vertical shaft was constructed from the top of the cliff down to these existing tunnels to provide access to the machines and the workforce.
3. A large space was created for the first TBM by widening the old stretch of the existing tunnel towards the sea end.
4. This started the journey for the first TBM towards France in November 1987, backed up by the site rail transportation system and the tunnel linings.
5. High-speed gantry cranes were used to transport the concrete lining segments onto the delivery train wagons for shipping to the working face of TBM.
6. While supporting the roof with rock bolts and shotcrete, significant space was created under the cliff site using a road header. This gave enough room to assemble the other five tunneling machines.
7. In addition, one of the world's longest sheet piling wall was constructed in the sea around the lower site of the cliff. This was done to create a platform for the storage of millions of cubic meters of soil arising from the excavation of the tunnel.
8. Later on, this platform was also used by batching plants and workshops. Although many objections were raised at the formation of the platform by certain environmental groups, but it did allow an easy way of getting rid of the excavated soil.

4. French Side Tunnel Drive

Six open-mode face TBMs were used from the French side to construct the tunnel. The following points describe the various technologies and methods used for tunnel drive from the French side:

1. On the French side, no convenient working site was available to aid the construction activities.
2. Therefore, a 75 m deep shaft with a 55 m diameter was formed to lower the tunnel boring machines into the position to start their excavation journeys towards the UK side.
3. On the UK side, the 12 m long cutting heads of each machine had to be dismantled to allow installation inside the tunnels. Whereas the French shaft was large enough to accommodate them as a whole, lowered into place by cranes with a carrying capacity of 400 tons.
4. The engineers on the French side developed a method for disposing of the excavated soil. At the shaft base, the excavated soil was mixed with water so that the mixture could easily be pumped away from the construction site to a dam for storage.
5. The cutting head of the TBMs was versatile. In poor and fractured ground conditions, the closed mode of cutting head was used to avoid the water ingress into the tunnel. Whereas in good ground conditions, the open-mode was used to enhance the speed of construction.  
6. The linings segments used in the French side were either cast-iron or bolted concrete segments. The cast-iron segments were used where the ground was particularly poor and where a good seal was required. The bolted concrete segments were used in good ground conditions. 

5. Construction of Crossover Chambers

Two mammoth crossover chambers were constructed out of the chalk rock with the help of service tunnel. The main purpose of these chambers was to allow the trains to change their direction.

The chambers were constructed in such a way that they formed three separate tunnel sections making it easy to conduct maintenance works without shutting the complete tunnel. The various approaches used to construct the UK and French side of the crossover chamber is discussed below.

5.1 UK Side Crossover Chamber

The following points describe the UK side crossover chamber used for constructing the Channel Tunnel:

1. The UK side crossover chamber was 7 km away from the coast.
2. The crossover chamber work involved the construction of the largest undersea cavern in soft rock anywhere in the world.
3. The construction of the crossover chamber began first in June 1989.
4. The crossover chamber is of immense size with a length, width, and height of 160 m, 18 m, and 11 m, respectively.
5. The first sequence in the construction of the chamber was the diversion of the middle service tunnel from its path so that the horizontal and transverse boring could be allowed from it. 
6. Therefore, the service tunnel was constructed way before the two main running tunnels. Once the service tunnel reached the site of the chamber, the chamber was excavated by making adits from the service tunnel. 
7. The NATM was used to construct the gigantic chamber. Generally, the NATM is used to construct the underground structures in hard rock. However, the first application of NATM in soft rock was accomplished by constructing the crossover chamber in the UK.

5.2 French Side Crossover Chamber

The following points describe the French side of the crossover chamber used for making Channel Tunnel:

1. The French side crossover chamber was 12 km away from the coast.
2. In the planning phase, it was decided to construct the biggest open arch of 35 m below the sea bed. Later on, this task was considered very risky.
3. More fractured and fissured rocks were encountered on the French side than on the British side. Therefore, the threat of collapse or excessive settlement of the open roof was too great.
4. After that, the engineers decided to construct the chamber in smaller sections. Firstly, the main tunnels were constructed up to the chamber location, as opposed to the UK side method. Further, a series of small tunnels were constructed from the main tunnel to form a roof for the arch of the chamber.
5. The arch was formed by filling the sections of these small tunnels with concrete. Thus, enough area was created beneath the arch roof to allow the excavation of the chamber.
6. Although this technique was very effective, it increased the overall construction time because the chamber was constructed once both the main running tunnels had reached the chamber site, which was 12 km away from the French sea coast.

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