How do the Chunnel and Seikan Tunnel work?

Question

I looked them up on Wikipedia, but I don't really understand their explanations. How did they build the underwater tunnels? How do they make sure it doesn't all collapse?

Answer 1

The answer is boring. Tunnel boring to be precise.

The Chunnel was built using Tunnel Boring Machines. These are like giant drills that chomp their way through the chalk layers under the seabed under the Channel. As they moved forward through the geology, the tunnel lining would be constructed behind it using interlocking concrete segments to create rings. The machine would then push against the last ring to move itself forward again. Once it had gone far enough to put in another ring the machine would be stopped, the hydraulic rams used to propel it forward would be withdrawn and the next ring put in place.

The tunnel didn't collapse primarily because of the chalk through which the tunnel was being driven, but mostly because the sides of the tunnelling machines held it up. The only unsupported sections of wall were those directly behind the machine as it moved forward, but these were never much more than the width of the concrete rings that formed the tunnel wall itself.

The clever bit was making sure that the tunnels met where they should, as construction was done simultaneously from the English and French ends. To make sure they met where they should, the machines had to be kept on track using lasers. If memory serves me right, the two tunnelling teams were less than an inch out by the time the tunnels met.