1994

Channel Tunnel

Before the Channel Tunnel opened in 1994, the only way to get from Great Britain to France was by boat. Airplanes have been a possibility during the last century, but they don’t really provide any advantages over boats. Engineers could build a bridge, but a bridge of that length at that location is problematic.

And so engineers chose a tunnel—the longest undersea tunnel ever at 31 miles (50 km) long. And it is not just one tunnel—it is actually three. There are two train tunnels to allow bi-directional travel and a service tunnel between them.

The original proposal for the construction was put forward by French mining engineer Albert Mathieu in 1899, but a detailed geological survey wasn’t carried out until 1964. Construction commenced that year. As a result of this government-funded effort, engineers picked a chalk layer that offered strength but relatively easy digging. They used tunnel-boring machines, and started with the service tunnel because it is smaller: 16 feet (4.8 meters) in diameter. The two train tunnels are 25 feet (7.6 meters).

To speed up the drilling process, tunnel-boring machines started at both ends and headed toward the middle. This raises two obvious questions: How did engineers make sure they would exactly align, and what happened when the boring machines ran into each other? The first problem is particularly complicated. The tunnel changes angles both horizontally and vertically along its run because of the geology. The primary solution was old-fashioned surveying. The boring machines also used laser-controlled guidance systems. A large laser was bolted to the ground in the existing tunnel and aimed at a target at the back of the boring machine. In this way, the machine could stay aligned with its exact path. After a period of progress, the laser would move forward in the tunnel.

To keep the boring machines from colliding, the machines that started on the English side were pointed down into the earth as they approached the midpoint. That got them out of the way and entombed them. The French machines advanced to finish the job, and then they were dismantled.

Brilliant engineering solved a huge transportation problem. Twenty million people and twenty million tons of freight use it every year.

SEE ALSO Tunnel Boring Machine (1845), The Wright Brothers’ Airplane (1903), Laser (1917), Kinsol Trestle Bridge (1920).

Boring machine, used to carve out rock to construct the Channel Tunnel.