An Engineering Miracle - How they built the Channel Tunnel

So how do you go about digging a tunnel under the English Channel? The first thing to do was to choose a possible route and obviously the first to be looked at were those linking Dover with Cap Gris Nez (between Calais and Sangatte), where the Channel is at its narrowest. Several specific routes were investigated including one that went right under Dover harbour. In the end, following surveys, the present route was chosen.

It was first necessary to survey the geological conditions under the sea and here the surveyors were lucky. The entire route runs through chalk marl, a 100 million-year-old rock that is pretty impervious to water, is very strong (so strong that a tunnel can be built with no supporting walls or roof – though the Channel Tunnel is lined with concrete and steel !) yet is not too hard to drill through. Take a look at our panel showing the geology of the lower sea-bed between England and France and note how the tunnel dips low at certain points.

Once the engineers were satisfied that the project was feasible and the route had been planned the finance had to put in place and that created some massive problems. The Tunnel was initially costed at a very low price, but with inflation, cost over-runs and the cost of financing, the total had risen to £12,000 million (£12 billion) by the completion date.

Tunnelling began on September 1987, from both the French and English ends, the service tunnel being the first project, though as this progressed out under the sea the other two tunnels were begun as well. The service tunnel always stayed about a mile ahead of the other tunnels, as a safety measure.

What happened to all the chalk and rock excavated from the tunnels – a total of 7 million tons (4 million cubic metres) ? Someone suggested they just dig a hole and bury it! But no, in reality some of it was used as landfill on the English terminal site; and 5 million cubic metres were placed in the sea near Shakespeare Cliff at Folkestone to create an artificial island. Known as Samphire Hoe this has become an important environmental site and has increased the size of the UK by 90 acres (enough for 45 football pitches).

The first major breakthrough came on October 30th 1990 when a two-inch hole was drilled from the English service tunnel into the last remaining rock separating it from the French service tunnel – it was then possible to pass from England to France through a tunnel! In all eleven boring machines (all with girls' names) were used. At its maximum, 15,000 engineers and workers were involved on constructing the Tunnel. The service tunnel was eventually completed to its 4.8 metre (16 feet) diameter and carries modified road vehicles (electric powered) for maintenance and other services.

The two main tunnels have a diameter of 7.6 metres (25 feet) and carry the rail track, with overhead catenary for power. The power supply in the tunnel is 25,000 volts. There are access points from the main tunnels to the service tunnel with fire doors as a safety measure. The main tunnels are lined with huge pipes carrying very cold water pumped through at high pressure to keep the tunnel temperature below 90°F (30°C) – the trains generate a lot of heat as they pass.

There are two huge undersea crossover points within the tunnel, so trains can gain access to either tunnel and these huge caverns were a major engineering feat. The cavern near the French end was modelled on the Mount Baker Ridge freeway tunnel in the USA.

Day by day the boring machines progressed, driven forward by compressed air with rows of chisel blades cutting through the rock. As it was cut it was carried back by conveyors into special trucks which were then hauled out of the tunnel. At the same time precast tunnel segments were hoisted into place and bolted, forming the lining of the tunnel as it is seen today. Some of the tunnelling machines, at the end of their life, burrowed further into the undersea rock well away from the main tunnels and remain buried there to this day, though a few were initially saved, having since been used for scrap.