|by Paul Holden, PRO|
20 April 2004
HOW THE CHANNEL TUNNEL WAS BUILT
Before Margaret Thatcher and Francois Mitterand agreed in 1985 that a tunnel could be built, there was a long history of proposals dating back to 1802 which had always been thwarted by concerns regarding Defence of the Realm. In 1881 a Col. Beaumont was actually commissioned to build about 2000 yards of experimental tunnel on the English side, and a similar experiment on the French side - but defence concerns prevented further progress. (Col. Beaumont’s original tunnel was encountered when work finally began.)
The route of the tunnel is not a straight line, because geographically, there is an 80 metre cliff on the English side, and geologically, they needed to keep the tunnel in a layer of chalk marl to minimise leakage and drainage problems. In addition to the curving route thus determined, there are two low points, with a rise in the middle of the Channel.
Actually there are three tunnels - two railway tunnels 8 metres in diameter, and one central 5 metre tunnel to provide an emergency evacuation route, and to supply fresh air to the train tunnels. (This central tunnel was actually driven first, in case there were geological problems.) It is connected to the main tunnels at 375 metre intervals to assist passenger evacuation. The railway tunnels are also connected by 2 metre diameter cross tunnels, to dissipate the pressure caused by speeding trains.
During the construction there were up to 1,500 men working underground 24 hours a day, moving chalk out of the tunnels at the rate of 2,400 tons per hour every 24 hours. On the English side, environmental concerns about the disposal of this chalk caused them to have to spend an unscheduled £44 Million to build a sea wall to stop the chalk falling into the sea.
The main tunnels were dug by laser-guided machines costing £16M each, with two crossover tunnels, at the ‘third points’ under the channel. These are the largest underwater caverns in the world in soft rock. To ensure the unbroken supply and quality of the concrete tunnel linings, the company had to build its own factories to produce them. They designed to progress the tunnels at the rate of 250 metres per week, but the actual record was 428m. As Project Director, he was responsible for signing off £90M every month, at the rate of £3M per day! The original planned timescale envisaged a construction period of 8½ years - but they paid additional money to the contractors to reduce this to 7 years.
Still, 13 years later, Colin admits to a feeling of frisson every time he sees a Eurostar train passing, which reflects his immense satisfaction of helping to turn a dream into a practical reality. He is still happy to give talks to interested groups, for a contribution to his favourite charity, the Guildford Cathedral, of which he is now Chief Steward, and the first lay member of the Cathedral Chapter, to which he was elected in 2002.