Engineering the Channel Tunnel

Britain and France are separated by sea, yet a

fifty-kilometer railway tunnel connects them thirty-seven point nine

kilometers beneath, beneath the channel.

The tunnel isn't floating in the sea.

It is bored through solid rock, averaging about forty-five

meet, meters below the seabed.

Engineers targeted chalk marl. It resists groundwater,

is relatively easy to excavate, yet remains strong enough for

tunneling.

Beneath the seabed, groundwater can be under high pressure.

If connected fractures provide a pathway, water can be driven toward the

tunnel.

From Britain and France, tunnel boring machines excavated

through the rock toward each other beneath the English Channel.

Digging from opposite sides, they met with astonishing precision,

only thirty centimeters apart horizontally and eight

centimeters vertically.

The Channel Tunnel is actually three tunnels, two for

trains and a central service tunnel connected by

emergency cross passages. A fast train acts like a

piston pushing air ahead.

Pressure relief ducts let air move between the rail tunnels, helping

manage pressure changes.

Without cooling, heat from high speed rail traffic could

raise the tunnel

temperature to about fifty degrees Celsius.

Chilled water carries that heat away.