The Channel Tunnel
The Engineer posted on October 17, 2006 |

Construction Sites

Sangate - Construction Site In France

In 1987, construction of the Channel Tunnel began in France at Sangatte on the Nord-Pas de Calais coastline at a location just over 3 kilometers away from the French terminal site at Coquelles.

A circular access shaft, 70 meters deep and 55 meters in diameter was excavated and lined with concrete. A hangar -like shed was built to shelter the shaft in the centre of the large construction site, which also contained offices and a lining segment manufacturing factory

All materials, workers and equipment were lowered down the shaft to the working platform at 47 meters, where the tunnel boring machines (TBM) could be assembled in dry conditions.

From this point, three machines excavated the undersea rail tunnels and service tunnel beneath the seabed towards Kent and two boring machines began the drive underground and inland towards the terminal site. One of these machines was then re-assembled to excavate the second running tunnel, thus using only 5 machines on the French side.

Gantry cranes with pulleys were used to lower the TBM sections and subsequently the concrete lining segments.

Once tunneling began, construction trains with a variety of wagons were used to transport the linings, the spoil and the workers. A railway control room at the top of the shaft regulated the movements of all the construction trains.

At the base of the shaft, a spoil treatment plant mixed the waste rock with water before pumping it 500 meter to the disposal site at Fond Pignon.

Once construction work was complete, the Sangatte shaft was transformed into a permanent feature of the tunnel system, housing the ventilation and cooling system installations.

The rest of the construction site has been landscaped and rehabilitation work continues with environmental monitoring of the Fond Pignon site.

Shakespeare Cliff - Construction Site In Great Britain

Construction work began on the British side at a platform located at the foot of Shakespeare Cliff. Situated between Folkestone and Dover on the Kent coast, this had been the site of a previous tunnel attempt in 1974.

The earlier tunnel workings were used as one of the two access shafts to the underground workings, with a rack and pinion railway used to convey equipment and materials to the marshalling area underground. The six TBMs were each assembled in a large cavern area, over 20 meters high and equipped with overhead cranes for lifting the TBM sections, which had first been excavated to accommodate the 8.6 meter diameter machines.

From this point under the platform at Shakespeare Cliff, three undersea tunnels were bored towards France and three underground tunnels towards the terminal site at Folkestone.

The service tunnel machine on both sides bored in advance of the two running tunnel machines. Probes on the service tunnel machine provided advance warning of difficult ground conditions and the data obtained provided data on alignment and conditions for the larger tunnel drives.

Terminal Sites

UK Terminal Construction

Work on the Folkestone terminal started in 1988 with a major landfill engineering project in order to provide the level area needed for the railway transport system.

Constraints were imposed by the geographical features - the 140-hectare site lies at the foot of the North Downs chalk escarpment and to the south, the long narrow site is bounded by the M20 motorway.

The tunnel portals are located at a point to the west of Castle Hill, just north of Cheriton and from this point the shuttle trains make their way on a loop track to the platform area.

The one-kilometer tunnel constructed by cut and cover method at the west end of the site to accommodate the shuttle train loop track has two functions. It increases the amount of space available for the other essential structures and road networks and it provides noise protection for the adjacent villages of Newington and Peene. A second acoustic and visual barrier between the villages and the site was erected at the start of construction work in the form of a 3-meter high earth bund

Major structures include the overbridges, access ramps and the platforms where vehicles load into and unload from the shuttle trains.

Buildings include a control tower, frontier control buildings for French and British authorities, passenger and freight amenity buildings, security and operations staff headquarters.

French Terminal Construction

The site chosen for the terminal at Coquelles, near Calais, required a major drainage exercise and millions of tonnes of earth were spread and compacted to lay the foundations for the 480-hectare terminal complex. With no land constraints, the railway system stretches from the Beussingues trench and loops around the terminal above ground before reaching the platform area.

As large as an airport, the terminal site contains 53 kilometers of railway track and 36 kilometers of road network, in addition to the Eurotunnel head office, an additional control tower, passenger terminal buildings and frontier controls. A separate terminal area contains all the freight facilities.

A striking feature is the viaduct across one of several lakeland areas, which provides a panoramic view for passenger vehicle travelers on their way to the tollbooths.

Tunnel Boring Machines

The tunnel boring machines were specially designed for excavating the chalk marl rock which geological surveys had shown to lie beneath the seabed along the proposed tunnel route.

The ground conditions were expected to be generally favourable in the dry and stable chalk marl, except for the section to be bored in the layer of upper chalk between Sangatte and the terminal site at Coquelles.

However, several factors combined to make this a unique challenge for the machines:

the length of the tunnel to be excavated undersea (in excess of 20 km).>

the high rate of advance required to meet the construction programme requirements.

Some facts and figures serve to illustrate the achievement of these machines and all the teams of workers involved in this remarkable engineering feat.

NUMBER OF DRIVES (tunnels excavated)

  • 12 - 6 undersea, 6 underland

NUMBER OF TBMs

  • 11 - 6 undersea, 5 underland (a French machine bored 2 underland tunnels)

Dimension of UK TBMS

  • Undersea rail tunnels - dia. 8.36m ; length 230.00m
  • Undersea service tunnel - >dia. 5.36m ; length 225.00m
  • Underland rail tunnels - dia. 8.72m ; length 253.00m
  • Underland service tunnel - dia. 5.76m ; length 225.00m
RATE OF ADVANCE - best day - 75.5m ; best week 428m ; best month 1,719.1m

Dimension of French TBMS

  • Undersea rail tunnels - dia. 8.72m ; length 265.00m
  • Undersea service tunnel - dia. 5.72m ; length 318.00m
  • Underland rail tunnels - dia. 8.62m ; length 211.00m
  • Underland service tunnel - dia. 5.59m length 204.00m
RATE OF ADVANCE - best day - 56m ; best week 292.6m ; best month 1,105.7m

Dates of Breakthroughs:

  • Undersea service tunnel - December 1, 1990
  • Undersea rail tunnel north - May 22, 1991
  • Undersea rail tunnel south - June 28, 1991

Date Tunneling Commenced:

  • December 1, 1987

Finished Tunnel Diameter:

  • Rail tunnels - 7.6m
  • Service tunnel - 4.8m
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