How was the Eurotunnel built underwater?

Engineers used two systems of tunnel linings – cast iron segments bolted together and precast concrete rings. The TBMs excavated a huge amount of chalk. On the French side, the chalk was crushed, mixed with water, and pumped inland behind a specially built dam 37m high.



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On the British side, the digging began near Shakespeare Cliff outside of Dover; the French side began near the village of Sangatte. The digging was done by huge tunnel boring machines, known as TBMs, which cut through the chalk, collected the debris, and transported the debris behind it using conveyor belts.

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Water is drained from the tunnels by large submersible pumps, and reliable operation is critical - there are more than 200 train movements in each direction every day. Eurotunnel engineers sought a high-quality solution to control all pumping processes during excavation.

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To use this method, builders dig a trench in the riverbed or ocean floor. They then sink pre-made steel or concrete tubes in the trench. After the tubes are covered with a thick layer of rock, workers connect the sections of tubes and pump out any remaining water.

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On 11 September 2008, a France-bound Eurotunnel Shuttle train carrying heavy goods vehicles (HGVs) and their drivers caught fire while travelling through the Channel Tunnel. The fire lasted for sixteen hours and reached temperatures of up to 1,000 °C (1,830 °F).

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Collapse of the tunnel isn't what would make it impassible first. The tunnel itself might well last a century or so, but if there's no electric power for as little as a few weeks the tunnel will be closed by water seepage that can't be pumped out.

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The Channel Tunnel (French: Tunnel sous la Manche), also known as the Chunnel, is a 50.46-kilometre (31.35 mi) underwater railway tunnel that connects Folkestone (Kent, England) with Coquelles (Pas-de-Calais, France) beneath the English Channel at the Strait of Dover.

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At its deepest point there is 75 metres of sea water above the tunnel. Managing water pressure was a key concern for the project engineers. The tunnel is actually designed to leak. As Eurotunnel explain on their website, 'seawater from the rocks above the tunnel drips through and is then pumped away.

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It is actually composed of three tunnels, each 50km long, bored at an average 40m below the sea bed. They link Folkestone (Kent) to Coquelles (Pas-de-Calais). Eurotunnel Shuttles, Eurostar and freight trains runs on two monodirectional single-track tunnels.

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A spokesperson for Eurotunnel Le Shuttle said: A train has broken down in the tunnel and we are in the process of transferring customers to a separate passenger shuttle via the service tunnel, to return to our Folkestone terminal. We apologise sincerely for this inconvenience.

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In cooperation with Rönesans Holding, the Heitkamp Construction Swiss GmbH tunnel and railroad construction specialists constructed the world's longest and deepest tunnel, the Gotthard Base Tunnel. This 57 kilometre-long tunnel lies at a depth of 2,300 metres.

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The Eurostar travels through the Channel Tunnel at a speed of 100 miles per hour (160kph) although when the train is outside the tunnel it reaches speed of 186 miles per hour (300 kph).

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The tunnel does leak, but it is designed to. Seawater from the rocks above the tunnel drips through and is then pumped away.

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