Why does the Channel Tunnel enter the ground around 10 km from the coast?

Question

I know that digging tunnels is always much more costly than building ways or train above ground.

Why doesn't the Channel Tunnel start around the coastline? Why does it have an around 10 km long portion under land on the British side?

Answer 1

The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia.

_{Full-sized image here.}

Some of the limiting factors for the Channel Tunnel are:

• Railways don't like steep gradients
• The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients.
• The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for the tunnels.

If you look at the long section of the tunnel there is some high ground on both coasts. The width of the high ground on the French side is about 2 to 3 km, whereas on the English side the width about 7 to 8 km.

For reasons of maintaining a comfortable gradient for the TBMs and the rail line and to position the tunnels in the lower part of the chalk marl combined with the width of the high ground on the coast and where the chalk marl is located within the high ground and because the chalk marl is inclined on the English side of the tunnel, the portal for the tunnel on the English side had to be positioned some 9 to 10 km from the coast.