Iconic construction projects: The Channel Tunnel

The building of the Channel Tunnel was an extraordinary engineering project. The American Society of Civil Engineers included it in its list of the seven wonders of the modern world, and it is easy to ‘Sea’ why! Constructing such a long tunnel under a major body of water brought huge technical challenges for the engineers who worked on the Channel Tunnel.   

The history of the Channel Tunnel

The 50km long Channel Tunnel was officially opened in 1994, but the idea of a tunnel between England and France goes much further back in time.  

It was first proposed as early as 1802 by French engineer Albert Mathieu-Favier, to allow horse-drawn coaches to pass through a tunnel lit by oil lamps! Over the next two centuries various plans and surveys were made, and digging actually started on several occasions. Concerns about wartime security was a familiar problem, as was expense. Eventually a private finance initiative during the 1980s gave the green light to the project proceeding, and an Anglo-French treaty to build the Channel Tunnel was signed in Canterbury Cathedral in 1986.  

Designing the tunnels

There are actually three tunnels – two for the rail traffic and a service tunnel between them. Each tunnel is 32 miles (51 km) long, and the undersea stretch is 24 miles long – the longest undersea tunnel in the world. The tunnel was dug through the chalky ground below the sea. 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. The water is a mixture of groundwater and seawater, collected at six drainage stations and is continuously monitored to protect the enormous pumps and pipes from corrosion and to make sure that water discharged to the sea is not environmentally harmful.’ 

When was Channel Tunnel construction started?

It was in late 1987 that the Channel Tunnel project finally started with digging on the English side. The French tunnelling began in early 1988.  

How was the Channel Tunnel built?

The tunnels were dug by huge tunnel boring machines (TBMs), and one was actually buried under the tunnel once the digging was finished.  

13,000 engineers, technicians and skilled workers were employed on the project. The TBMs on either side inched towards each other at an average of around 150 metres a week. Debris was collected and transported to ground level, and the walls of the tunnel lined with concrete to deal with the water pressure and as a waterproofing measure. These walls will last for 120 years before they need to be replaced.  

How long did the Channel Tunnel take to build?

Would the two sides of the tunnel meet? That was a real worry for the project’s surveyors, but by using special lasers and electronic measuring technology the big moment did finally arrive.  

In December 1990 English and French engineers met in the middle of the service tunnel, and the two train tunnels were completed in May and June 1991 respectively. There was still lots of work to be done, however. Crossover tunnels, land tunnels from the coast to the terminals, electrical systems, fireproof doors, ventilation systems and train tracks were just some of the things that had to be added.  

Finally in December 1993 a test train was run through the tunnel, and the tunnel officially opened in May 1994, so the project had taken six and a half years.